Advances in Crop Science and Technology
Open Access

Always, population growth is considerable pressure on available environmental resources. About half of Egypt’s residents live in urban areas, with most people spread across the densely populated centers of greater Cairo, Alexandria and other major cities in the Nile Delta. Egypt’s fertile area totals about 3.3 million ha, about one-quarter of which is land reclaimed from the desert. However, the reclaimed lands only add 7% to the total value of agricultural production [1]. Even though only 3% of the land is arable, it is extremely productive and can be cropped two or even three times annually [2]. Fortunately, Egypt had four seasons during the year, but generally there are only two familiar seasons for Egyptian agriculture that is a mild winter from November to April and a hot summer from May to October. The most differences between the seasons are variations in light intensity, daytime temperatures and prevailing winds. Consequently, there are some strategic crops such as wheat (Triticum aestivum L.), faba bean (Vicia faba L.) and onion (Allium cepa L.) in addition to Egyptian clover (Trifolium alexandrinum L.) during the winter season. In the summer season, there are some other crops such as maize (Zea mays L.), rice (Oryza sativa L.), cotton (Gossypium barbadense L.) and sesame (Sesamum indicum L.) in addition to cowpea (Vigna unguiculata ).

Recently, special attention has been directed towards increasing productivity of Egyptian cotton unit area^-1. Delaying cotton planting date than the suitable date (as a result of the late harvest date of some winter field crops) reducing seed cotton yield and its quality [3-7] and thereby low-income farmers. Cotton classification is the process of describing the quality of cotton in terms of such properties as cleanness, length, smoothness, color, maturity, strength and contamination. It is known that Egyptian cotton is characterized by having the longest, finest and strongest fibers, with fiber lengths exceeding 3.5 cm, some even ≥ 5.0 cm. Its fibers are generally used to manufacture high quality ring-spun yarns with end-uses including sewing thread, lace yarns and high-quality dress and shirt fabrics [8]. In this concern, Long et al. [9] reported that fiber quality is characterized by fiber length, strength and micronaire, and the textile industry has a preference for long and strong fibers of moderate micronaire for producing high-quality yarns. Fiber fineness affects processing performance and the quality of the end product in several ways. Cotton requires slower processing speeds to fibers per crosssection, which in turn produces stronger yarns. Also, color deterioration affects the ability of fibers to absorb and hold dyes and finishes. Unfortunately, the cultivated area of this important crop continuously till it reached about 571 thousand ha in 2016 [10].

Accordingly, late cotton planting date during the summer season is one of the main problems associated with the Egyptian farmers. Consequently, the cropping system adopted by the farmer in soils of the Nile Valley and Delta must be physically viable, sustainable, less exhaustive acceptable to farming community and most important thing is that it should be economical. However, most plant species are capable of influencing the quality of their environment [11] where plants may exert substantial effects on nutrient cycling [12]. On the other hand, Basra and Saha [13] mentioned that fiber maturation included four stages of growth (initiation, primary elongation, secondary wall formation and maturation). It known that cotton plant store substantial amounts of photo-assimilate as starch in stems and roots prior to flowering [14]. Therefore, it is expected that environment surrounding cotton seedling and growth could be have substantial effects on boll formation. Certainly, environmental conditions can be playing a vital role in cotton growth and development stages. The effects of climatic factors such as evaporation, sunshine duration, humidity, surface soil temperature and maximum air temperature are the important factors that affect significantly flower and boll production of cotton [15].

For edaphic factors, the mobility in soil is dependent on the chemical form of the element used. The availability of nitrogen (N), phosphorus (P), potassium (K) and water are the major constraints in cotton production in most cotton producing environments [16]. Therefore, the allelopathic effect of the winter and summer field crops on seed cotton yield and its attributes were differed due to soil N, P, K and ferulic acid contents, as well as, population density of Bacillus sp. in the cotton rhizosphere. Ferulic acid (4-hydroxy-3-methoxycinnamic acid) and Bacillus sp. could affect either negatively or positively cotton growth. Ferulic acid (phenolic compound) is a strong dibasic acid in which the first proton dissociation generates the carboxylate anion, while the second produces a phenolate anion. The anion has a high degree of resonance stabilization, which increases its acidity in comparison with similar phenolic acids [17] and could affect plantmicrobe interactions that play important roles in a number of vital ecosystem processes such as nutrient cycling [18]. According to Gui- Ying et al. [19], ferulic acid inhibited the growth, enzyme activities and root activity of cotton seedlings from long term sole cotton fields.

N fixation by both symbiotic and free-living bacteria is highly sensitive to interference effects (including allelopathy) from certain plant species [20]. There are several free-living N-fixing bacteria that grow in close association with plants such as Bacillus sp. that is one of the predominant genera of plant growth promoting bacteria [21]. Root-colonizing species of Bacillus is well known for the enhancement of plant growth [22] and is responsible for the biocontrol activity in rhizosphere of cotton seedlings [23]. Consequently, if Egyptian agricultural production must be intensified, a cropping system should be followed depending on proper management to offer optimum productivity of cotton crop unit area^-1 with regarded to crop interference effects.

Interference between plants typically refers to either competition for resources (nutrients, light and water) or chemically-mediated interference "allelopathy" [24]. Thus, relay intercropping cotton (seedling and growth stages) with wheat (reproductive stage) is a recent technology in crop intensification field, where, the crops overlap in time, growing as an intercrop, from March till May [25-27]. Moreover, summer intercrops were interfered strongly with some preceded winter crops to overcome the negative allelopathic effects [28]. The objective of this investigation was to evaluate the crop interference of Egyptian clover, faba bean, onion, wheat, cowpea and sesame crops on Egyptian cotton characters for maximizing land usage with economically efficient cropping system and good fiber quality.

A two-year study was carried out at Sids Agricultural Research Station, Beni Sweif governorate (Lat. 29°12' N, Long. 31°01' E, 32 m.a.s.l.), Egypt, during 2015/2016 and 2016/2017 seasons. Chemical analyses of the soil (0-30 cm) were done by Water, Soil and Environment Research Institute, ARC (Table 1). According to Jackson [29] and Chapman and Pratt [30] before growing of the winter crops. Soil texture is clay. Furrow irrigation was the irrigation system in the region. Cultivars of winter field crops were Giza 6 for Egyptian clover, Giza 843 for faba bean, Giza 6 improved for onion and Misr 1 for wheat. Cultivars of summer filed crops were Giza 95 " extra-long staple" for cotton, Cream 1 for cowpea and Shandweel 3 for sesame. Egyptian clover and cowpea seeds were kindly provided by Forage Crops Research Department, Field Crops Research Institute, ARC. Faba bean seeds were kindly provided by Food Legumes Research Department, Field Crops Research Institute, ARC. Onion transplants were kindly provided by Onion Research Department in Sids Agricultural Research Station, Field Crops Research Institute, ARC. Wheat grains were kindly provided by Wheat Research Department, Field Crops Research Institute, ARC. Cotton seeds were kindly provided by Cotton Research Institute, ARC. Sesame seeds were kindly provided by Oil Crops Research Department, Field Crops Research Institute, ARC.

Table 1: Soil chemical properties of Sids location before the winter crops before growing the winer crops in the two seasons.

In the two winter seasons, Egyptian clover and faba bean seeds were inoculated by Rhizobium trifolii and Rhizobium leguminosarum , respectively, before seeding it and Arabic gum was used as a sticking agent. In the two summer seasons, cowpea seeds were inoculated by Rhizobium melitota before seeding it and Arabic gum was used as a sticking agent. Calcium super phosphate (15.5% P₂O₅) was applied at rate of 476 kg ha^-1 during soil preparation in the two winter seasons. Mineral N fertilizer was applied at rate of 35.7 kg N ha^-1 for Egyptian clover, 17.8 and 35.7 kg N ha^-1 for faba bean, 191.3 and 285.6 kg N ha^-1 for onion and 119.5 and 178.5 kg N ha^-1 for wheat under intercropping and sole cultures, respectively. Also, mineral N fertilizer for cotton plants was applied at rate of 142.8 kg N ha^-1 in two equal doses at 45 and 60 days from cotton sowing, meanwhile mineral N fertilizer for cowpea plants was applied at rate of 17.8 and 35.7 kg N ha^-1 under intercropping and sole cultures, respectively, in one equal dose at 25 days from cowpea sowing. Mineral N fertilizer for sesame plants was applied at rate of 35.7 and 71.4 kg N ha^-1 under intercropping and sole cultures, respectively, in two equal doses at 25 and 40 days from sesame sowing. Mineral K fertilizer was applied for all the tested crops as recommended for each crop. Table 2 shows sowing and harvest dates of winter and summer field crops in the two growing seasons.

Table 2: Sowing and harvest dates of all the studied field crops in the two seasons.

The experiment included twelve cropping systems as follows:

• Egyptian clover seeds were broadcasted at the rate of 47.6 kg ha^-1. After the third cutting of Egyptian clover, cotton seeds were grown in two sides of the bed, two plants together distanced at 25 cm. This cropping system was expressed as Egyptian clover/cotton in the winter season and sole cotton in the summer season (conventional cropping system).

• Egyptian clover seeds were broadcasted at the rate of 47.6 kg ha^-1. After the third cutting of Egyptian clover, cotton seeds were grown in two sides of the bed, two plants together distanced at 25 cm. Two rows of cowpea seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as Egyptian clover/cotton in the winter season and cotton+cowpea in the summer season.

• Egyptian clover seeds were broadcasted at the rate of 47.6 kg ha^-1. After the third cutting of Egyptian clover, cotton seeds were grown in two sides of the bed, two plants together distanced at 25 cm. One row of sesame seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as Egyptian clover/cotton in the winter season and cotton+sesame in the summer season.

• Two rows of faba bean seeds were grown in middle of the bed, two plants together distanced at 15 cm. Cotton seeds were grown in two sides of faba bean beds, two plants together distanced at 25 cm. This cropping system was expressed as faba bean+cotton in the winter season. After faba bean harvest, cotton continued alone in the summer season (sole cotton).

• Two rows of faba bean seeds were grown in middle of the bed, two plants together distanced at 15 cm. Cotton seeds were grown in two sides of faba bean beds, two plants together distanced at 25 cm. Two rows of cowpea seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as faba bean+cotton in the winter season and cotton +cowpea in the summer season.

• Two rows of faba bean seeds were grown in middle of the bed, two plants together distanced at 15 cm. Cotton seeds were grown in two sides of faba bean beds, two plants together distanced at 25 cm. One row of sesame seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as faba bean+cotton in the winter season and cotton +sesame in the summer season.

• Four rows of onion transplants were grown in middle of the bed, one plant distanced at 10 cm. Cotton seeds were grown in two sides of onion beds, two plants together distanced at 25 cm. This cropping system was expressed as onion+cotton in the winter season. After onion harvest, cotton continued alone in the summer season (sole cotton).

• Four rows of onion transplants were grown in middle of the bed, one plant distanced at 10 cm. Cotton seeds were grown in two sides of onion beds, two plants together distanced at 25 cm. Two rows of cowpea seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as onion+cotton in the winter season and cotton +cowpea in the summer season.

• Four rows of onion transplants were grown in middle of the bed, one plant distanced at 10 cm. Cotton seeds were grown in two sides of onion beds, two plants together distanced at 25 cm. One row of sesame seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as onion+cotton in the winter season and cotton+sesame in the summer season.

• Four rows of wheat grains were drilled at the rate of 119.0 kg ha^-1 in middle of the bed. Cotton seeds were grown in two sides of wheat beds, two plants together distanced at 25 cm. This cropping system was expressed as wheat+cotton in the winter season. After wheat harvest, cotton continued alone in the summer season (sole cotton).

• Four rows of wheat grains were drilled at the rate of 119.0 kg ha^-1 in middle of the bed. Cotton seeds were grown in two sides of wheat beds, two plants together distanced at 25 cm. Two rows of cowpea seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as wheat+cotton in the winter season and cotton+cowpea in the summer season.

• Four rows of wheat grains were drilled at the rate of 119.0 kg ha^-1 in middle of the bed. Cotton seeds were grown in two sides of wheat beds, two plants together distanced at 25 cm. One row of sesame seeds were grown in middle of cotton beds, two plants together distanced at 20 cm. This cropping system was expressed as wheat+cotton in the winter season and cotton+sesame in the summer season.

In addition to:

• Sole Egyptian clover or wheat by broadcasting Egyptian clover seeds or drilling wheat grains at the rate of 47.6 or 119.0 kg ha^-1, respectively. With respect to Egyptian clover 5 cuts, the fifth cutting was done at first week of May in the two seasons.

• Sole onion or faba bean by growing three or two rows in ridges, one or two plants together distanced at 10 or 25 cm, respectively.

• Sole cowpea or sesame by growing two rows in ridges, two plants together distanced at 20 cm.

The treatments were the combinations between four winter cropping systems and three summer cropping systems. The twelve cropping systems were compared in a split plot design with three replications. Four winter cropping systems were randomly assigned to the main plots, while three summer cropping systems were allocated in subplots. Sub-plot area was 21.6 m². With regarding to intercropping systems and sole cultures of wheat and Egyptian clover, each plot contained six beds, each bed was 3.0 m in length and 1.2 m in width. In case of sole cultures of faba bean, onion, cowpea and sesame, each plot contained twelve ridges, each ridge was 3.0 m in length and 0.6 m in width.

The studied characters

Soil samples were taken from rhizosphere of cotton roots: The following chemical analyses were recorded after 45 days from cotton sowing; N, P and K (ppm), ferulic acid (μg g^-1) and total count of Bacillus sp. (CFU g^-1). These analyses were done in General Organization for Agricultural Equalization Fund, ARC, Giza, Egypt and the Regional Center for Food & Feed, ARC, Giza, Egypt. Soil samples were collected for chemical analysis before adding mineral N fertilizer for cotton plants.

Seed cotton yield and its attributes: The following traits were measured on ten guarded plants from each sub-plot at harvest; intercepted light intensity (lux) at the middle of the plant by Lux-meter apparatus at 12 h and expressed as percentage from light intensity measured above the plant, plant height (cm), nodal position of the first sympodium (cm), numbers of monopodia and sympodia plant^-1, number of open bolls plant^-1, seed cotton yield plant^-1 (g), boll weight (g) and 100-seed weight (g). Lint (%) was calculated as the relative amount of lint in a seed cotton sample, expressed in percentage. Seed cotton yield ha^-1 (t) was recorded on the basis of sub-plot area by harvesting all plants of each sub-plot and converted to yield ha^-1. Lint cotton yield ha^-1 (t) was calculated by multiplying seed cotton yield ha^-1 (t) in lint (%).

Cotton fiber quality: The fiber properties were measured using High Volume Instrument (HVI) methods according to A.S.T.M. [31] by the Cotton Technology Res. Division, Cotton Res. Inst., ARC, Egypt.

Fiber length: Upper half mean (mm); upper half mean is the mean length by the number of fibers in the largest half by weight of fibers in a cotton sample. UHM length is normally equivalent to the staple length.

Uniformity index (%): Uniformity index is the ratio between mean length and upper half quartile length, express as a percentage. Uniformity index is an indicator of how fibers will perform in the spinning of yarn.

Fiber bundle tensile: Strength (g/tex); strength reports the force, in grams, required to break a bundle of fibers one tex unit in size. A tex unit is the weight in grams of 1,000 meters of fiber.

Elongation (%): Elongation at break is the amount of stretch a fiber can take before it breaks.

Fineness: Micronaire reading; micronaire is a measure of the air permeability of compressed cotton fibers. It is often used as an indication of fiber fineness and maturity.

Color: Reflectance 'RD' (%).

Yields of the other crops: Forage yields of Egyptian clover and cowpea ha^-1 (t) were estimated as fresh weight of three and two cuttings, respectively, taken from sub-plot. Seed yields of faba bean and sesame ha^-1 (t) were recorded on the basis of plot area by harvesting all plants of each sub-plot. Bulb yield ha^-1 (t) was estimated as fresh weight of bulbs taken from sub-plot. Grain yield of wheat ha^-1 (t) was recorded on the basis of sub-plot area by harvesting all plants of each sub-plot. All the yields were converted to yield ha^-1.

Competitive relationships

Land equivalent ratio (LER): LER is the ratio of area needed under sole cropping to one of intercropping at the same management level to produce an equivalent yield [32]. LER is calculated as follows: LER=(Y_a/Y_aa)+(Y_b/Y_bb)+(Y_c/Y_cc), where Y_aa=Pure stand yield of crop a (cotton), Ybb=Pure stand yield of crop b (Egyptian clover, faba bean, onion or wheat), Y_cc=Pure stand yield of crop c (cowpea or sesame), Y_a=Intercrop yield of crop a (cotton), Y_b=Intercrop yield of crop b (Egyptian clover, faba bean, onion or wheat) and Yc=Intercrop yield of crop c (cowpea or sesame).

Area Time Equivalent Ratio (ATER): ATER provides more realistic comparison of the yield advantage of intercropping over monocropping in terms of time taken by component crops in the intercropping systems. ATER was calculated by formula: ATER=LER × D_c/D_t [33], Where LER is land equivalent ratio of crop, D_c is time taken by crop, D_t is time taken by whole system.

Financial evaluation

Farmer's benefit was calculated by determining each of total return, costs and net returns of intercropping cultures, as well as, sole cultures.

1) Total return ha^-1 year^-1 (US$)=yield a × price a+yield b × price b +yield c × price c. The prices were presented by Bulletin of Statistical Cost Production and Net Return [10], as well as, market prices where a=cotton, b=winter crop and c=summer crop.

2) Net return ha^-1 year^-1 (US$)=total return-variable costs for the crops in sole and intercropping cultures. The costs were presented by Bulletin of Statistical Cost Production and Net Return [34], as well as, market prices.

Statistical analysis

Analysis of variance of the obtained results of each season was performed. The measured variables were analyzed by ANOVA using MSTATC statistical package [35]. Mean comparisons were performed using the least significant differences (L.S.D) test with a significance level of 5% [36].

Winter crop interference effects on seed cotton yield and yield attributes

According to San Emeterio et al. [24], these results could be divided to interference effects of the tested crops on Egyptian cotton characters that include above and under-ground conditions.

Above-ground conditions: Data in Table 3 show that the winter cropping systems had significant effects on intercepted light intensity within cotton plants, seed cotton yield plant^-1, boll weight, 100-seed weight, lint percentage, seed cotton and lint yields ha^-1 in the two growing seasons, meanwhile plant height, nodal position of the first sympodium, numbers of monopodia and sympodia plant^-1, and number of open bolls plant^-1 were not affected. Egyptian clover/cotton sequential double cropping system was superior to all relay intercropping patterns for seed cotton yield plant^-1, boll weight, 100-seed weight, lint percentage, seed cotton and lint yields ha^-1 followed by relay intercropping patterns; faba bean+cotton and onion+cotton. Egyptian clover/cotton increased seed cotton yield plant^-1 by 11.79 and 22.38%, boll weight by 18.18 and 14.09%, 100-seed weight by 3.27 and 6.33%, seed cotton yield ha^-1 by 32.46 and 24.32%, in the first and second seasons, respectively, compared with wheat+cotton (Table 3).

Table 3: Effect of winter crop interference on seed cotton yield and yield attributes in the first and second seasons.

Also, onion+cotton increased seed cotton yield plant^-1 by 2.07 and 6.77%, boll weight by 3.18 and 4.54%, 100-seed weight by 0.36 and 1.53%, seed cotton yield ha^-1 by 15.21 and 3.27%, in the first and second seasons, respectively, compared with wheat+cotton (Table 3). The positive effect of Egyptian clover/cotton or onion+cotton pattern on seed cotton yield and its attributes could be due to there was nooverlapping between the Egyptian clover and cotton for basic growth resources and these conditions were relatively similar with those of onion+cotton pattern. Leaf canopy of onion formed whole space that is available for cotton plants during the seedling, growth and development stages.

Consequently, cotton plant has the longest period of vegetative growth during available normal climatic conditions from seedling to reproductive stage with high light intensity; 12.79 and 14.61% of full sunlight for Egyptian clover/cotton in the first and second seasons, respectively, and 12.92 and 14.79 % of full sunlight for onion+cotton pattern in the first and second seasons, respectively. Additionally, it is likely that cotton seedlings were benefitted greatly from low day temperatures that play a major role in carbon dioxide assimilation rates and more photosynthates during the spring season. It is known that seed cotton yield and its components decreased with delaying planting date [37,38].

These results reveal that Egyptian clover/cotton or onion+cotton pattern furnished better above-ground conditions for cotton growth and development which could be reflected on high seed germination, the timely appearance of seedling and the optimum development of the root system compared with the other patterns. Similar results were observed by Kamel et al. [4] who found that the preceding winter crops (Egyptian clover, faba bean and wheat) had significant effects on number of open bolls plant^-1 and seed cotton yield unit area^-1. On the other hand, El-Naggar et al. [39] and Babu [40] reported that onion enhanced the seed cotton yield. In another study, Metwally et al. [41] showed that Egyptian clover/cotton recorded the highest seed cotton yield compared with those of relay intercropping patterns. Furthermore, Jayakumar and Surendran [42] proved that cotton +onion resulted in the maximum cotton equivalent yield.

With respect to relay intercropping cotton with faba bean, faba bean +cotton increased seed cotton yield plant^-1 by 6.10 and 8.24%, boll weight by 9.09 and 7.27%, 100-seed weight by 1.33 and 1.96%, seed cotton yield ha^-1 by 24.67 and 7.09 %, in the first and second seasons, respectively, compared with wheat+cotton (Table 3). These results could be due to faba bean+cotton decreased inter-specific competition between the two species for basic growth resources such as solar radiation, air temperature and wind compared with relay intercropping cotton with wheat. It is known that the climate of Egypt in the winter season (December-February) is cold, moist and rainy, in addition to the main feature in the early spring season is the desert or Khamasine winds [43]. Accordingly, it is expected that the genetic make-up of Giza 843 cultivar interacted positively with its plant density (67% of the recommended plant density unit area^-1) to make the surrounding environment with cotton more-lighter than those of wheat+cotton pattern especially from the late winter season to early spring season.

Additionally, the whole space is available for cotton plants that are still in the seedling stage after faba bean harvest, and the gaps that appear after 45 days from cotton planting have to be bridged by the expanding leaf canopy of cotton. Consequently, it is likely that this positive effect will enhance sink capacity of cotton to intercept more solar radiation where resource use efficiency is not likely to be much affected in intercropping systems with component crops that differ in growing period, since competition between component crops is weak [44]. These results reveal that faba bean+cotton pattern furnished better above-ground conditions especially light intensity for increasing cotton above ground biomass accumulation from seedling to development stage than those of wheat+cotton pattern. These results are in accordance with those observed by Hussein and Haikal [45] who revealed that the faba bean+cotton pattern produced the highest seed yields plant^-1 and unit area^-1 compared with the others. Also, Metwally et al. [41] reported that growing cotton with faba bean by relay intercropping system at 20th March gave significant increases in seed cotton yield as compared to the corresponding cropping system of wheat.

With regard to wheat+cotton pattern, light intensity was decreased within intercropped cotton canopy with wheat compared with that of Egyptian clover/cotton, faba bean+cotton and onion+cotton pattern (Table 3). In other words, growing cotton with wheat increased interspecific competition between the intercrops for basic growth resources especially light penetration between them. It is known that shade diminished cotton sink capacity [46] and the fewer cotton bolls plant^-1 of this pattern were produced. According to Du et al. [47], wheat +cotton showed a pronounced delay in early development due to the initial shading from wheat on cotton seedlings. These results are in similar with those observed by Metwally et al. [41] who indicated that Egyptian clover/cotton recorded higher seed cotton yield than those of wheat+cotton.

Under-ground conditions: Egyptian clover, faba bean, onion and wheat were differed significantly for major soil elements, content of ferulic acid and population density of Bacillus sp. in the cotton rhizosphere after 45 days from cotton sowing in the two growing seasons (Table 4). Soil N content of the cotton rhizosphere varied from 14.0 to 32.0 ppm and from 20.0 to 40.0 ppm in the first and second seasons, respectively without significant differences either among Egyptian clover, faba bean and onion or between onion and wheat. Also, P content of the cotton rhizosphere varied from 30.0 to 44.0 ppm and from 36.0 to 48.0 ppm in the first and second seasons, respectively without significant differences between Egyptian clover and faba bean. Moreover, K content of the cotton rhizosphere varied from 262.0 to 330.0 ppm and from 292.0 to 366.0 ppm in the first and second seasons, respectively. On the other hand, ferulic acid content of the cotton rhizosphere varied from 3.0 to 15.1 μg g-1 and from 2.6 to 19.5 μg g-1 in the first and second seasons, respectively without significant differences either among Egyptian clover, faba bean and onion or between onion and wheat. Also, Bacillus sp. content of the cotton rhizosphere varied from 4.7 × 105 to 4.9 × 106 CFU g-1 and from 5.5 × 105 to 5.7 × 106 CFU g-1 in the first and second seasons, respectively without significant differences between Egyptian clover and faba bean. It is known that N and P are among the most limiting nutrients for plant growth. P is generally deficient in most of the soils due to its ready fixation [48].

Table 4: Chemical and biological analyses in rhizosphere of cotton roots after 45 days from cotton sowing.

In general, soil N, P and K availabilities in the cotton rhizosphere were increased by cutting of Egyptian clover or harvesting of faba bean in the experimental soil as compared with wheat+cotton pattern in the two growing seasons (Table 4).

It is likely that deep-growing roots of the legumes could be changed three dimensions in the experimental soil (from the top to the bottom of the soil profile, from North to South and East to West) and thereby accelerated the weathering of the experimental soil by aggregate stability and soil porosity. Accordingly, the fixed N by the legume is released; making it available to other plants and this helps to fertilize the soil [49] and soil bacteria specifically interact with plant roots in the rhizosphere, where bacterial number is generally higher than in the free soil [50]. It is known that PGPR (Rhizobium and Bacillus sp.) require N to decompose crop residues and can get this either from the residue or soil solution. Therefore, it is expected that complementary interactions between the root system of the legume and PGPR such as N transfer or complementary use of different soil nutrients were occurred; leading to an increase in cation exchange capacity (CEC) through PGPR activity that converted organic N to NH₄⁺ in the soil solution or exchangeable pool [51].

On the other hand, P may react strongly with calcium (Ca) in alkaline soils and its solubility was restricted as a result of increasing soil pH above normal range in the two growing seasons (Table 1). Cotton plant takes up only negatively charged primary and secondary orthophosphate ions (H₂PO₄^- and HPO₄^2-) as nutrient. Thus, it may be possible that PGPR secreted some low molecular mass organic acids such as indole acetic acid (IAA) to chelate mineral ions for bring P into soil solution. Consequently, it is likely that the acidification of PGPR cells and their surrounding leads to the release of P-ions by H⁺ substitution for Ca²⁺ [52], and thereby Egyptian clover or faba bean residues contained more P and have lower C/P ratios due to its greater capacity to utilize soil P. So, it is expected that N (released from N₂ fixation) and P were available because of soil pH was reduced due to proton released from roots of the legumes. Consequently, organic P mineralization contributed to the available P pool that mineralized by PGPR, especially there was association between PGPR and plant roots in P nutrition [53]. It is known that legume crop increased the mobilization of P from sparingly soil P compounds [54].

The soil solution must be replenished with K from other sources in the soil and that replenishment may come primarily from readily available, “exchangeable” K. It seems that faba bean as seed crop may be exhausted soil K more than Egyptian clover as forage crop especially dry plant residues of faba bean shoot had concentration of K more than Egyptian clover shoot as mentioned by Tang and Yu [55]. It is known that clay and organic matter act as a buffer, absorbing and releasing mineral ions. Egyptian clover or faba bean residues and living PGPR biomass may be formed organic matter that is important to K fertility with regarded to soil pH that affected the rate of residues decomposition. According to Beegle and Durst [56], organic matter provides many negative charged sites for holding exchangeable soil K. This biological situation facilitated released K into the soil solution in rhizosphere of cotton roots, especially Askegaard and Eriksen [57] reported that there was a residual benefit of the legumes on the growth of the subsequent crop through reducing K leaching.

With respect to ferulic acid, its content was decreased in rhizosphere of cotton roots by cutting of Egyptian clover or harvesting of faba bean compared with wheat+cotton pattern in the two growing seasons (Table 4). It seems that low concentration of ferulic acid in rhizosphere of cotton roots played a major role in under-ground interactions to maintain efficiency of photosynthetic process of cotton plant which reflected positively on the yield attributes. It is known that clover had 2 to 10 times lower concentrations of ferulic acid than various grasses [58] and the lowest content of this acid was found in faba bean among sixteen beans [59]. These results reveal that the legume component Egyptian clover or faba bean improved soil nutrient cycling to the subsequent or intercropped cotton plants, respectively, which reflected positively on boll development.

With respect to onion+cotton pattern, onion is a high-value crop with a shallow root system that is irrigated frequently and fertilized with high N rates to maximize yield. Also, the shallow root system of onion is an important consideration for efficient management of mobile nutrients such as nitrate-N and sulfate-S. Bulbs may be secreted some sulfur (S) compounds in the experimental soil which oxidized by soil Bacillus sp. to sulfate (SO₄^2-) that reduced soil pH and thereby increased soil N availability. Soil N has its greatest solubility between soil pH 4 and soil pH 8. Above or below that range, its solubility is seriously restricted [60].

According to Shanmugham [61], growing cotton with onion absorbed more P and produced more dry matter and seed cotton yield than that of sole cotton. P is taken up as H₂PO₄ ion by the crop from the soil solution. Therefore, onion could be secreted some S compounds in the rhizosphere of intercropped cotton which enhanced growth of Bacillus sp. that produced phytohormones such as IAA [62] and resulted in an increase in P uptake by cotton. In this concern, Narula et al. [63] indicated that Bacillus sp. increased cotton yield. So, it is likely that onion+cotton pattern increased cotton root growth for efficient uptake of the other soil nutrients compared with those of wheat+cotton pattern in the two growing seasons.

Also, soil K availability was increased in rhizosphere of intercropped cotton during the early growth stage of cotton compared with the other winter copping systems. Obviously, onion had positive effects on soil K availability that could be maintained moderate relationship between sources and sink capacity of cotton plant. K plays a vital role where promotes photosynthetic process and consequently more dry matter accumulation in the plant [64] and thereby K deficiency restricted fruit production to a greater extent than vegetative growth [65].

Although content of ferulic acid was found in rhizosphere of intercropped cotton with onion came in the second rank after that of intercropped cotton with wheat but might have restricted root growth and development of cotton with an unclear effect. However, Bacillus sp. may be reduced relatively ferulic acid effects of onion on cotton growth by utilized this acid as sole carbon (C) source [66]. Accordingly, this negative effect seems to be depending on concentration of ferulic acid in the rhizosphere of cotton roots and Bacillus sp. activity that play an important role in plant protection and growth promotion [67] by induced chemical changes in plants [68]. The highest onion yield was observed by Bacillus sp. activity [69]. Accordingly, it is expected that cotton growth and development will be improved by absorbing more soil nutrients which affected positively cotton yield attributes.

With regard to wheat+cotton pattern, growing cotton with wheat during reproductive stage of the cereal component for two months increased approximately inter-specific competition between the two species for absorbing soil N during this period. After wheat harvest, it is expected that wheat residues decomposition probably had higher C to N ratio and immobilized more soil N for the intercropped cotton. Optimal amount of P and K in the soil cannot be utilized efficiently if N is deficient in plant where N mediates the utilization of P, K and other elements in plants [70]. Also, the highest concentration of ferulic acid in rhizosphere of intercropped cotton roots with wheat may be inhibited enzymatic oxidation of indole acetic acid (IAA) which affected negatively cell elongation and division, and thereby subsequent plant growth and development [71]. Thus, once integrated into the soil, phenolics can control nutrient cycling [72] that inhibited significantly the growth and root activity of cotton seedlings [73]. Accordingly, it is likely that intercropped cotton plants with wheat suffered from soil nutrient deficiency especially in the early growth stage which reflected negatively on some physiological functions. These findings imply that wheat+cotton pattern had growth inhibiting on rhizosphere of cotton which reflected negatively on seed cotton yield attributes.

Summer crop interference effects on seed cotton yield and yield attributes

Data in Table 5 show that the summer cropping systems had significant effects on intercepted light intensity within cotton plants, plant height, nodal position of the first sympodium, numbers of monopodia and sympodia plant^-1, number of open bolls plant^-1, seed cotton yield plant^-1, boll weight, 100-seed weight, seed cotton and lint yields ha^-1 in the two growing seasons meanwhile lint percentage was not affected. Sole cotton and cotton+cowpea pattern recorded higher intercepted light intensity within cotton plants, number of sympodia plant^-1, number of open bolls plant^-1, seed cotton yield plant^-1, boll weight, seed index, seed cotton and lint yields ha^-1 than those of cotton +sesame pattern in the two growing seasons without any significant differences between them.

Table 5: Effect of summer crop interference on seed cotton yield and yield attributes in the first and second seasons.

Cotton+sesame pattern recorded the highest values of plant height, nodal position of the first sympodium, number of monopodia plant^-1 in the two seasons compared with the others. Cotton+cowpea pattern achieved higher intercepted light intensity within cotton plants, number of sympodia plant^-1, number of open bolls plant^-1, seed cotton yield plant^-1, boll weight, 100-seed weight, seed cotton and lint yields ha^-1 than those of cotton+sesame pattern in the two growing seasons (Table 5).

Conversely, cotton+sesame pattern decreased significantly intercepted light intensity within cotton plants, number of sympodia plant^-1, number of open bolls plant^-1, seed cotton yield plant^-1, boll weight, 100-seed weight, seed cotton and lint yields ha^-1 than those of cotton+cowpea pattern in the two growing seasons (Table 5). These results could be attributed to cropping interference effects of above and under-ground conditions. These results confirmed by Vaiyapuri et al. [74] who indicated that cowpea did not affect the seed cotton yield, but intercropping sesame with cotton resulted in significant reduction in seed cotton yield.

Above-ground conditions: The positive competitive effect of cotton +cowpea pattern could be due to this cropping system decreased interspecific competition between the intercrops for above-ground conditions especially light intensity (Table 5). Cotton+cowpea pattern increased light intensity by 45.23 % in the first season and 34.57 % in the second season within cotton canopy compared with those of cotton +sesame pattern. Cutting cowpea plants at 45 and 75 days from cowpea sowing led to increase in light penetration within cotton canopy which reflected positively on cotton above ground biomass during boll formation and maturation. These results indicate that there was less inter-specific competition between the two species, especially cotton plants were able to use adequate light and had less competition for growth factors [75].

From the other point, cotton+sesame pattern decreased light intensity by 32.11 % in the first season and 26.30 % in the second season within cotton plants compared with those of sole cotton. These results could be due to shading effect of sesame plants increased interspecific competition between the intercrops for above-ground conditions especially light intensity which reflected negatively on number of open bolls plant^-1 and boll weight. The results are in the same context with Attia and Seif El-Nasr [76] and Khan et al. [77] who found that drastic reduction of cotton plants in cotton+sesame pattern was due to fast growth of sesame at earlier growth stage, which suppressed the growth of companion cotton crop.

Under-ground conditions: The positive allelopathic effect of cotton +cowpea pattern may be attributed to increased inter-specific competition between them for soil N that reflected positively on better nodulation of cowpea and thereby more soil N available for cotton roots. No doubt that cowpea plants absorbing less soil N than cotton plants [78].

These results are in accordance with Rusinamhodzi [79] who found that biological N fixation in cowpea is positively affected by the companion cotton crop in intercropping; some of the N fixed in the intercrop may be transferred from cowpea to cotton crop during the season. On the other hand, deep roots of sesame may be included some allelochemicals that had a negative effect on competitive ability between the two species for soil nutrients.

It is known that sesame is deep rooted which lead to scavenge for fertility below most crops roots zones [80]. The inhibitory effect of cotton+sesame pattern might be due to the higher competitive ability or allelopathic potential of sesame plants [81].`

The interaction between winter and summer crop interference on seed cotton yield and yield attributes

Data in Table 6 show that the interaction between winter and summer cropping systems had significant effects on seed cotton yield plant^-1 and lint percentage in the first season, boll weight and 100-seed weight in the second season, mean while the other studied traits were not affected in the two growing seasons. Cotton+cowpea pattern that preceded by Egyptian clover recorded higher seed cotton yield plant^-1, boll weight, seed index and lint percentage than those of the other cropping systems. On the other hand, the cropping system (wheat +cotton/cotton+sesame) gave the lowest values of seed cotton yield plant^-1, boll weight, 100-seed weight and lint percentage. These results could be attributed to Egyptian clover/cotton interacted positively with intercropping cowpea with cotton to give higher seed cotton yield plant^-1, boll weight, 100-seed weight and lint percentage. In other words, cowpea furnished wet environment to maintain K uptake continuous within different tissues of cotton plants that resultant from residues of Egyptian clover where uptake of K in cotton plant slow down beyond 120 days due to moisture stress in the soil [82]. They added that K resulted in significantly higher seed cotton yield. The inadequate biomass production was a major factor limiting yield formation of intercropped cotton [46]. These data reveal that there was effect of winter cropping system × summer cropping system on seed cotton yield plant^-1 and lint percentage in the first season, as well as, boll weight and seed index in the second one.

Table 6: Effect of the interaction between winter and summer crop interference on seed cotton yield and yield attributes in the two growing seasons.

Winter crop interference effects on cotton fiber quality

In Egypt, expanding the area planted with short and medium staple cotton and reducing the area planted with extra-long staple cotton is a demand that the spinning and textile industry has made for years, the industry relies on short and medium staple cotton to produce the type of yarn required for producing some of the more popular fabrics [83].

Upper half mean, uniformity index, fiber strength, fiber elongation, micronaire reading and color-reflectance were affected significantly by winter cropping systems (Table 7). Egyptian clover/cotton was superior to all relay intercropping patterns for upper half mean, uniformity index and fiber elongation meanwhile the highest fiber strength and color-reflectance, as well as, the lowest micronaire were obtained by wheat+cotton pattern in the two-growing season. These results indicate that Egyptian clover/cotton pattern had good fiber quality compared with the other cropping systems. It is known that fiber length is directly related to yarn fineness, strength, and spinning efficiency [84]. Accordingly, it may be possible that cotton with a low uniformity index is likely to have a high percentage of short fibers. Such cotton may be difficult to process and is likely to produce low quality yarn. Onion+cotton pattern produced high fiber quality (fiber strength and color-reflectance) compared with the other cropping systems. Fine cottons tend to have greater tensile strength than the short and coarse cottons. With respect to relay intercropping cotton with faba bean or wheat, cotton fiber quality was decreased by residues of faba bean or wheat and produced short cotton fibers. It is known that the importance of short-fiber content in determining fiberprocessing success, yarn properties and fabric performance led to reduce the cost of textile processing and to increase the value of the raw fiber [85].

Table 7: Effect of winter crop interference on cotton fiber quality in the first and second seasons.

Above-ground conditions: The positive effect of Egyptian clover/ cotton pattern on cotton fiber quality could be due to this cropping system furnished better above-ground conditions especially light intensity and daytime temperatures before boll formation compared with wheat+cotton pattern. Although different fiber quality properties may be established at earlier stages of fiber development in all the cropping systems, but cotton plant response to nutrient availability might be depended on the availability of other required resources during flowering and boll formation stage.

Therefore, the advantage of Egyptian clover/cotton pattern could be attributed to the last date of Egyptian clover cutting furnished available normal climatic resources for cotton fiber growth compared with the relay intercropping cotton with faba bean or wheat. The light environment surrounding plants affected seedling growth [86]. Moreover, the minimum, optimum, and maximum temperatures for cotton vary depending on growth and developmental processes [87]. These results reveal that Egyptian clover/cotton pattern produced good fiber quality for high upper half mean, uniformity index and fiber elongation compared with the other cropping systems.

Clearly, growing cotton after Egyptian clover received relatively lower solar radiation than those of onion+cotton pattern which reflected on dry matter accumulation of cotton during fiber growth and development. It is expected that canopy structure of cotton plant after Egyptian clover was relatively greater than that of onion+cotton pattern as a result of Egyptian clover residues. In other words, growing cotton plants after Egyptian clover may be received relatively lower solar radiation and higher warmer temperature than those of onion +cotton pattern. In this concern, Pettigrew et al. [88] showed that reduced photosynthetic rates and the modulation of other metabolic factors, in association with lower light intensities, may result in lower micronaire and fiber strength which explained lower fineness and strength for cotton fibers that followed Egyptian clover.

Conversely, higher cotton fiber length (upper half mean, uniformity index and fiber elongation) after Egyptian clover cutting may be due to this copping system furnished relatively warmer night temperature environment that accelerated fiber growth and development compared with onion+cotton pattern. Fiber length (upper-half mean length) was correlated negatively with the difference between maximum and minimum temperature [89]. Fibers grown at 15°C took 3 to 5 d longer to reach 2 mm in length than did control fibers grown at 24°C [90].

Under-ground conditions: Upper half mean, uniformity index and fiber elongation were enhanced as a result of soil N, P and K availabilities after Egyptian clover cutting, faba bean harvest or onion uprooting compared with those after wheat harvest in the two growing seasons (Table 4). The results could be attributed to soil N availability was sufficient to maintain good fiber quality. Soil N availability may be promoted some proteins synthesis such as IAA before boll maturing. According to Gialvalis and Seagull [91], external application of IAA promoted fiber initiation. On the other hand, cotton fiber primary wall contains semi-crystalline cellulose fibrils, which are surrounded by a matrix composed mainly of other polysaccharides including xyloglucan and pectin [92]. The elongating fiber wall must compromise strength and flexibility. Thus, IAA also accelerated entry into the transition phase and the onset of high-rate cellulose synthesis [93]. Moreover, soil N availability maintained good deposition of cellulose layers. Furthermore, N promoted fiber maturity [94]. Accordingly, it is expected that there was an increase in amount of assimilated proteins to carbohydrate available for mature bolls which affected positively fiber initiation, primary and secondary elongation, as well as, maturity.

However, the advantage of onion+cotton pattern probably due to some S compounds, it is expected that bulbs secreted some S compounds which increased soil N and Ca availabilities [60] to enhance some proteins such as Ca dependent protein kinases (GhCDPK1 protein) that is responsible for cotton fiber elongation [95]. It is important to mention that GhCDPK1 protein is localized in the plasma membrane of onion epidermal cells during transient transformation assays [96]. Therefore, it is essential to keep the two companion nutrients (N and S) in balance with each other and to meet adequately balanced supply of both nutrients to plant [97].

With regard to soil P content, residues of the legume crop (Egyptian clover or faba bean) and onion increased availability of soil P (Table 4). These results probably due to soil P availability promoted fiber length [98] through cell division and energy transfer which resulted in early boll development and thereby reflected positively on hastening of maturity [99]. Similar results were observed by Sawan et al. [100] who indicated that P response and availability leading to initiation and development of greater number of fibers per seed.

With respect to soil K content, upper half mean, uniformity index, fiber strength, fiber elongation and color-reflectance were increased as a result of soil K availability of Egyptian clover/cotton, wheat+cotton or onion+cotton pattern compared with those of wheat+cotton pattern in the two growing seasons but the reverse was true for micronaire reading (Table 4). These results could be attributed to soil K availability increased fiber elongation and micronaire by maintaining sufficient water pressure within the boll [99] and cell osmotic pressure to delay fiber cellulose accumulation and carbohydrate acquisition.

With regard to ferulic acid content, upper half mean, uniformity index, fiber length and fiber elongation were increased as a result of decreasing ferulic acid after Egyptian clover cutting, faba bean harvest or onion uprooting compared with those after wheat harvest, but the reverse was true for micronaire reading in the two growing seasons (Table 4). It is known that low content of ferulic acid in rhizosphere of cotton roots did not inhibit major nutrient cycling which reflected positively on enzymatic oxidation of IAA and thereby acceptable rate cellulose synthesis in mature bolls. The phenolic compounds and lignin content are believed to play an important role in cotton fiber development and quality [101] where ferulic acid is part of lignin and flavonoid biosynthetic pathways [102].

With respect to population density of Bacillus sp., it is known that exogenous IAA increased the proportion of epidermal cells that differentiated as fibers [90] where Bacillus sp. improved production of substances with IAA [22]. IAA is required for primary elongation in cotton fiber development [103]. Consequently, these results suggest that Egyptian clover, faba bean and onion residues improved cotton fiber quality compared with residues of wheat crop.

Consequently, it is likely that wheat+cotton pattern led to imbalance in shoot to root growth of cotton during growth and development stages. Accordingly, it is expected that wheat+cotton pattern produced more carbohydrates than necessary to support cotton development and thereby increase in the amount of carbohydrate available to mature bolls. High content of ferulic acid in rhizosphere of cotton roots could be defected in complementary use of major soil nutrients and Bacillus sp which reflected negatively on fiber development and quality. It is known that ferulic acid inhibited enzymatic oxidation of IAA that reflected negatively on cell elongation and division [104] where it is involved in cell wall elongation arrest [105]. These results could be attributed to ferulic acid accelerated cell elongation and division and thereby subsequent the primary and secondary cell wall in a shorter time than the normal growth.

On the other hand, reflectance degree was lowest with the most severe N treatment; this means fibers produced by the most Ndeficient plants appeared less bright than fibers produced by Nsufficient plants [106]. N and K stress during this phase of fiber development was shown to have detrimental effects on cotton fiber quality [107]. Also, Guo et al. [108] indicated that soil P deficiency inhibited completely fiber elongation.

Summer crop interference effects on cotton fiber quality

Upper half mean, uniformity index, fiber strength, fiber elongation, micronaire reading and color-reflectance were affected significantly by summer cropping systems (Table 8). Cotton+cowpea pattern had the highest values of upper half mean, uniformity index, fiber strength, fiber elongation and color-reflectance compared with the other treatments in the two growing seasons. On contrary, the lowest micronaire was observed in cotton+cowpea pattern compared with the others in the two growing seasons.

Above-ground conditions: Cotton+cowpea pattern caused significant increments in cotton fiber quality compared with those of sole cotton (Table 8). The positive competitive effect of cotton+cowpea pattern on cotton fiber quality could be attributed to increase in light penetration within cotton plants during boll formation especially low irradiance decreased fiber strength [109]. Moreover, cutting cowpea plants at 45 and 75 days from cowpea sowing led to increase in solar radiation penetration between intercropped cotton plants than sole cotton. Consequently, it is expected that this biological situation did not reduce leaf area of cotton plant which permitted more transpiration and photosynthesis rate of intercropped cotton. Accordingly, starch content will be increased in the leaves and transferred to the fiber during boll growth and development.

Table 8: Effect of summer crop interference on cotton fiber quality in the first and second seasons.

The negative competitive effect of cotton+sesame pattern on cotton fiber quality could be due to this pattern decreased light penetration within cotton canopy which reflected negatively on the studied traits. It seems that plant density of sesame (50% of the recommended plant density unit area^-1) had adverse effects of shading on cotton plant and affected negatively dry matter accumulation in cotton fiber during boll formation. It is known that long-term shading decreased the strength of cotton fiber [110] and the fiber length of cotton [111] as a result of decreasing sucrose and starch contents of subtending leaves [112].

Cotton+sesame pattern affected negatively light penetration within cotton canopy which may be enhanced endogenous ABA of cotton; fiber elongation was negatively correlated with increasing ABA concentration [113]. It is known that increase in ABA content is positively signal for the beginning of fiber wall thickening [96]. These results are in accordance with those observed by Einhorn and Arrington [114] who revealed that ABA indirectly reduced carbon assimilation via stomatal closure, but may also induce abscission directly through hormone action.

Under-ground conditions: The positive allelopathic effect of cotton +cowpea pattern had on cotton fiber quality could be due cowpea secreted some allelochemicals in rhizosphere of cotton roots where ethylene concentration in different tissues of cotton plant. According to Burg [115], ethylene is produced by roots and it is an endogenous plant hormone that may act as a modulator of the action of other hormones in growth and development [116]. Consequently, ethylene promoted fiber length [96] as a result of enhancing ethylene concentration through plant growth regulators, salicylic acid, gibberellic acid and ethylene, are considered to be allelochemicals [117]. Cotton+cowpea pattern may be affected negatively light penetration within cowpea canopy that inhibited elongation of cowpea epicotyls by endogenous ethylene [118]. The increase in ethylene may be absorbed by intercropped cotton roots and translocated to different tissues of cotton plant through adjacent cowpea plants by making the surrounding environment with cotton moister. Planting cowpea between cotton plants improved soil cover and ultimately reduced moisture loss from the soil [81]. Consequently, these results reveal that cowpea plants furnished better under-ground conditions for soil nutrients availability which reflected positively on enlargement, filling and maturation of boll development. The negative allelopathic effect of cotton+sesame pattern on cotton fiber quality may be due to deep roots of sesame secreted some allelochemicals in the rhizosphere of cotton plants which diminished source and sink capacity and thereby delay in reproductive growth of cotton. Fiber quality is mainly associated with nutritional and environmental conditions during the boll development [112].

The interaction between winter and summer cropping systems

The interaction between winter and summer cropping systems had no significant effects on upper half mean, uniformity index, fiber strength, fiber elongation, micronaire reading and color-reflectance in the two growing seasons. These data show that each of these two factors act independently on cotton fiber quality meaning that winter cropping systems responded similarly to summer cropping systems for these traits.

Competitive relationships

To assess the benefits of growing two or more crops together, or intercropping, is to measure productivity using the LER and ATER. LER compares the yields from growing two or more crops together with yields from growing the same crops in sole culture. ATER provides more realistic comparison of the yield advantage of intercropping over sole cropping in terms of variation in time taken by the component crops of different intercropping systems. Generally, the cropping system Egyptian clover/cotton+cowpea achieved the highest LER and ATER followed by the cropping system onion+cotton/cotton +cowpea compared with the other cropping systems in both seasons (Table 9). Advantage of the cropping system (Egyptian clover/cotton +cowpea) could be due to relative yield of cotton was high as a result of no-overlapping between the Egyptian clover and cotton for basic growth resources, then this advantage was continued by intercropping cowpea with cotton in the summer season. Also, onion+cotton pattern played a major role in increasing seed cotton yield plant^-1 through positive competitive and allelopathic effects of onion and these effects were improved by intercropping cowpea with cotton in the summer season. With respect to the cropping system (wheat+cotton/cotton +sesame), the inhibitory effect of this system could be due to integration between negative interference effects of wheat and sesame on seed cotton yield plant^-1 for above and under-ground conditions during the year. These results are similar to those obtained by Khan et al. [77] who revealed that intercropping cowpea with cotton achieved higher LER than intercropping sesame with cotton, especially negative allelopathic effects of intercropping sesame with cotton were observed by Shah et al. [81].

Table 9: Competitive relationships of winter and summer cropping systems, as well as, their interaction in the two growing seasons.

Egyptian clover 5 cuts: 44.35 and 47.21 t ha^-1, sole faba bean: 3.32 and 3.39 t ha^-1, sole onion: 43.15 and 46.95 t ha^-1, sole wheat: 8.20 and 8.33 t ha^-1, sole cotton: 3.22 and 3.78 t ha^-1, sole cowpea: 50.4 and 49.69 t ha^-1, sole sesame: 1.15 and 1.12 t ha^-1 in the first and second seasons, respectively.

Financial evaluation

The financial return of cotton under different cropping systems compared with sequential double cropping system (Egyptian clover/ cotton) is shown in Table 10. Total return of cotton varied between treatments from US$ 1654 to 2979 ha^-1 as compared with sequential double cropping system (US$ 1780 ha^-1) in the first season. Also, total return of cotton varied between treatments from US$ 1820 to 3265 ha^-1 compared with sequential double cropping system (US$ 2001 ha^-1) in the second one. Net return of cotton varied between treatments from US$ 486 to 1910 ha^-1 as compared with sequential double cropping system (US$ 689 ha^-1) in the first season. Also, net return of cotton varied between treatments from US$ 652 to 2196 ha^-1 compared with sequential double cropping system (US$ 910 ha^-1) in the second one. Net return of the cropping system (onion+cotton/ cotton+cowpea) recorded the highest net return in comparison with the other cropping systems in the two growing seasons. These results reveal that the cropping system (onion+cotton/cotton+cowpea) is more profitable to Egyptian farmers than conventional double cropping system (Egyptian clover/cotton). These results may be due to onion had higher yielding ability compared to the other crops in the cropping systems. These findings are parallel with those obtained by Basu [119] who showed that intercropping cotton with legumes or non-legumes have been found to be profitable in the cotton zones. Also, Metwally et al. [41] indicated that net returns from cropping systems depend on the total production of crop components of cotton, faba bean, wheat and Egyptian clover, as well as, variable and fixed costs.

Table 10: Financial advantages of cotton under different cropping systems in the first and second seasons.

Prices of main products are that of 2016: US$ 395.2 ha^-1 for ton of seed cotton, US$ 615.8 ha^-1 for ton of sesame seeds, US$ 259.6 ha^-1 for ton of faba bean seeds, US$ 169.3 ha^-1 for one cut of Egyptian clover, US$ 137.6 ha^-1 for ton of wheat grains, US$ 52.0 ha^-1 for ton of onion and US$ 10.0 ha^-1 for one ton of cowpea.

Our results reveal that onion had positive crop interference effects on cotton and this effect was improved by intercropping cowpea with cotton in the summer season and can be helpful to understand performance of cotton fiber (quantity and quality). Egyptian farmers achieved an increase in their income when growing cowpea with cotton plants that intercropped with onion. Although the cropping system (Egyptian clover/cotton+cowpea) recorded high lint cotton yield and longer fibers, but economically efficient cropping system (onion+cotton/cotton+cowpea) that produced stronger and finer fiber for the manufacture of higher quality textiles could be recommended.