Effect of varying planting density on weed infestation, crop phenology, yield, and fiber quality of cotton under different sowing methods

The sowing method and planting density of cotton (Gossypium hirsutum L.) are two important tools for enhancing crop productivity and quality while reducing weeds infestation. This study was conducted to explore how to weed infestation, crop morphology, yield-related traits and fiber quality of cotton may be improved through the sowing method and planting density. Cotton line PB-896 was cultivated and hand thinned to maintain 59, 259 plants ha, 44,444 plants ha and 35, 555 plants ha under three sowing methods (flat sowing, ridge sowing, and bed sowing) at Postgraduate Agriculture Research Station (PARS), University of Agriculture, Faisalabad, Punjab during 2017 and 2018. The experiment was laid out in Randomized Complete Block Design (RCBD) with a split-plot arrangement. Obtained results revealed that ridge sowing of cotton reduced the weed density by 25% in comparison to flat sowing. However, bed sowing of cotton increased the sympodial branches, opened bolls plant, lint yield, seed index, and seed cotton yield by 48%, 52%, 38%, 39%, and 50% respectively as compared to flat sowing. Higher planting density found to be more efficient in weed reduction, however, lower planting density improved the monopodial branches (18%), total bolls plant (14%), boll weight (16%), seed index (17%), and seed cotton yield (23%) than higher planting density. In terms of fiber quality, sowing methods only influenced the fiber uniformity that was comparatively higher in bed planting, while planting density only increased the fiber fineness that was better in lower planting density. In conclusion, the bed sowing method along with lower planting density found to be more efficient in improving crop morphology, yield, and yield-related traits.


Introduction
Cotton (Gossypium hirsutum L.) is not only cultivated primarily for fiber but it is also considered as cash crop across the world because it is an important commodity for oil, animal feed, and the fiber industry [1][2][3]. After India, China, and the USA, Pakistan is ranked 4 th in cotton production. Cotton crop is not only providing a major share in Pakistan's export earnings but also contributes to gross domestic product (1%) and agricultural value addition (5.5%). In the year 2018, cotton production increased by 11.8% as compared to the previous year due to the appropriate rainfall interval, better economic return, and subsidized rate of inputs [4]. However, climate change widely affecting the productivity of the agriculture system, especially cotton productivity in Pakistan [5,6]. Changing rainfall patterns, more extreme weather conditions, and high temperature are increasing the vulnerability of cotton growth and production along with other crops [7][8][9][10]. Moreover, cotton productivity is stagnant in Pakistan due to poor management practices: inadequate plant population, traditional sowing method, weed infestation, flare-up insect pest pressure, and diseases [11]. Under the scenario where environmental circumstances are not in control of humans, the only available alternative to enhance crop productivity is to adjust the agronomic practices [12,13]. Among various management practices to enhance cotton productivity, suitable sowing method and planting density are the most important practices toning to the available ecological condition and step toward cotton mechanization [11,14]. In Pakistan, the traditional sowing method (flat sowing) resulted in a patchy plant population due to poor germination. Moreover, the occurrence of rainfall soon after the seeding may result in crust formation and failure of seedling emergence. Under unusual rainfall patterns, the options of raised bed sowing and ridge sowing have the potential to ensure the uniform seedling emergence and plant population that are key factors to harvest profitable yield [15]. As compared to flat sowing, ridge sowing of cotton amended the soil physical indices (soil moisture content=SMC and bulk density), root penetration, and proliferation due to less resistance and enhanced the seedling emergence [16]. Butter et al. [17] and Patel et al. [18] stated that ridge sowing has improved the lint yield in comparison to flat sowing. Previous research findings divulged that the ridge sowing technique has also increased the seed cotton yield [19,20] while reducing the weed infestation and weed biomass production [21]. Maqbool  Pre-sowing saturation irrigation (10 cm depth) was applied each year. Rotavator was operated to manage the previous crop stubbles when soil mellowness was physically appropriate for cultivation. The field was prepared by three cultivation (using cultivator) followed by planking. Tractor-mounted ridger, as well as bed-shaper, were castoff to create ridges and bed (0.75 m wide), respectively. In furrow-ridge and furrow-bed, ridge to ridge and bed to bed distance was 0.75 m. Tractor mounted seed drill having adjustable seed rate was used for flat sowing of cotton in 0.75 m apart rows. At field capacity soil moisture, dry seeds are drilled at 20 kg ha -1 , 24 kg ha -1 and 34 kg ha -1 seed rate to maintain the respective planting densities. In ridge sowing, cotton seeds (5 per hill) were manually dibbled to maintain the specific planting densities, while in bed sowing, on both sides of the bed, 5 seeds per hill were dibbled to maintain the specific planting densities. The dibbled seeds were covered with soil. Irrigation was applied in ridge and bed sowing techniques and seeding was done on the moist edge of the ridge and bed, respectively. Manual thinning was done to maintain respective planting densities at the fourth leaf stage of the crop (BBCF identification code=14) [47]. Postemergence herbicides (Fazelofoop methyl 250 ml in 100-liter water) were applied at 30 days after sowing= DAS to control the weeds using a shoulder-mounted knapsack spray machine with T-Jet nozzle. Nitrogen (N 200 kg ha -1 ), phosphorous ( P 85 kg ha -1 ), potassium (K 95 kg ha -1 ), boron ( B 5 kg ha -1 ), and zinc (Zn 3 kg ha -1 ) were applied to fulfill the fertilizers requirement using urea, di-ammonium phosphate (DAP), murate of potash (MOP), zinc sulphate 33%, and boric acid 17% as the source, respectively. The entire dose of P, K, B, and Zn along with one-third dose of N was applied before sowing. While remaining N was applied in two equal splits at floral bud initiation (BBCF identification code=51) and beginning of boll opening (BBCF identification code=81) stages [47]. Canal irrigation was used to irrigate the crop during the entire growth period. Six irrigations in total were applied to irrigate the crop in each season and the last irrigation was applied on 15 and 6, August of 2017, and 2018. Bollworm and other insects (aphid, jassid, whitefly, thrips, mites, and cotton mealybug) were kept under the economic threshold level by the application of insecticides (Polytrien C= 1250 ml ha -1 , orange= 1875 ml ha -1 , and hathora= 315 ml ha -1 ). In each year crop was harvested in two pickings. First picking was done on 5 th and 9 th October of 2017 and 2018, respectively, while second piking was done 1 st and 6 th November of 2017 and 2018, respectively.

Observation and measurements
In both years of experimentation, individual and total weed density was recorded from 3 randomly selected quadrates (0.5 m × 0.5 m) in each experimental unit at 25 DAS. Weeds were identified in each quadrate and counted carefully and the average of three quadrates is represented for total weed density and individual weed density m -2 .
Ten plants in each plot were tagged to note the plant height (cm), monopodial branches plant -1 , sympodial branches plant -1 , total bolls plant -1 , opened bolls branches plant -1 , and unopened bolls plant -1 and average values were taken. Before commencing 2 nd picking, fifty matured opened bolls from tagged plants of each plot were taken. After sun-drying, boll weight was measured by using a digital electronic weighing balance. Harvested bolls were manually ginned to find out lint yield (g), while the formula given by Singh et al. [48] was used to measure ginning out turn (GOT). Roller type laboratory ginning machine was used to obtain seed cotton from both pickings of each experimental unit. 100-seed cotton were counted from each treatment and weighed to find the 100seed cotton weight, while seed cotton yield (kg ha -1 ) was measured by weighing obtained seed cotton from each experimental unit. To measure fiber length (cm) and strength (g tex -1 ), the method of Moore [49] was followed, while spin lab high volume instrument HVI-900 was used to measure fiber uniformity (%) as well as the fineness (µg inch -1) as explained by Ehsanullah [14].

Data analysis
Recorded data, on all parameters, were analyzed statistically by using Statistics 10 package (student edition) with Fisher's analysis of variance technique [50]. The RCBD with a split-plot arrangement was used for data analysis. Interaction between years, sowing techniques, and planting densities were non-significant for all parameters, except for weed density. Therefore, pooled data (of 2017 and 2018) for all parameters are presented except weed density. Tukey's honestly significant difference test at 5% probability level was used to compare treatment means.

Weed density
Prevailing weed flora in the experimental field consisted of broad leaves weeds, sedges, and grassy weeds. During both years of experimentation, weeds species recorded in all experimental units were Trianthema portulacasterum L. (carpet weed) and Convolvulus arvensis L. (field bindweed) under the grassy weeds category. Cyperus rotundus L. (purple nutsedge) was only sedge weed observed in the experimental field, while Dactyloctenium aegyptium L. (Egyptian finger grass) and Cynodon dactylon L. (barmuda grass) were the existing broad leaves weed (Table 3). Total weed density of grassy weeds, broad leaves weeds, and sedge weed ( Figure 1) as well as individual weed densities were substantially influenced by different sowing approaches and planting densities of cotton in both years of experimentation. However, the interactive effect of the sowing method and planting density did not affect the weed density (total and individual) significantly. Higher weed density (C. rotundus, C. dactylon, and C. arvensis) was recorded in 2018 as compared to the year 2017, while the plant population of D. aegyptium and T. portulacasterum was lower in 2018. The lowest total weed density was recorded in ridge sowing while the highest total weed density in both years of experimentation was recorded in flat sowing (Figure 1 A). Higher planting density (59259 plant ha -1 ) reduced the total weed density, while the highest weed infestation was recorded in lower planting density (35,555 plants ha -1 ) of cotton (Figure 1 B). Amongst individual weed density, C. rotundus, C. dactylon, and T. portulacasterum were the most dominant weed species in all experimental units. It was observed that the ridge sowing method reduced the individual weed density of C. rotundus, C. dactylon, T. portulacasterum, C. arvensis, and D aegyptium by 24%, 21%, 31%, 28%, and 33%, respectively, then flat sowing (Table 3). While weed density in ridge and bed sowing techniques were statistically comparable. Regardless of weed species, individual weed density increased with reducing plant density of cotton, successively (Table 3).

Plant phenology and morphology
Data about plant phenology indicated that sowing methods did not influence any phenological trait significantly, while planting density only influenced the boll maturation period. Similarly, the sowing method influenced the plant morphological traits except for plant height, the number of monopodial branches, and unopened bolls plant -1 , while planting density influenced all morphological attributes of cotton significantly except opened bolls plant -1 (  -1 ). Regarding sowing methods, the bed sowing method gave the highest values for morphological traits (sympodial branches, the total number of bolls plant -1 and opened bolls plant -1 ) that were similar (statistically) to ridge sowing technique, however, these were higher (48%, 47%, and 52%) as compared to flat sowing (Table 4).

Yield related attributes
It was observed that all yield-related traits were substantially influenced by sowing methods and planting density except GOT. Interaction of the sowing method and planting density did not affect any yieldrelated traits (Table 5). Maximum yieldrelated traits (boll weight (g), seed cotton yield plant -1 (g), lint yield (g), seed index (g), and seed cotton yield ha -1 (kg) were noted in bed sowing method that were higher than flat sowing by 16,49,38,29 and 50%, respectively (Table 5). Similarly, cultivation of cotton at 75 cm×37.5 cm plant spacing (35,555 plants ha -1 ) gave the highest single boll weight, seed cotton yield plant -1 , lint yield, seed index, and seed cotton yield ha -1 , while reducing the plant spacing reduced the yield-related traits significantly. As compared to higher planting density of cotton (59,259 plants ha -1 ), cultivation of cotton with lower planting density (35,555 plants ha -1 ) improved the single boll weight, seed cotton yield plant -1 , lint yield, seed index, and seed cotton yield ha -1 by 16,25,27,17, and 23%, respectively (Table 5). Fiber quality Data indicated that sowing methods only influenced the fiber uniformity significantly while planting density only profound effect on fiber fineness. Interaction of sowing methods and planting density was also found to be non-significant for fiber quality (Table 5). It was observed that bed sowing of cotton improved the fiber uniformity by 7% and 4% as compared to flat sowing and ridge sowing, respectively. Likewise, fiber fineness was improved by lowering the plating density of cotton (35,555 plants ha -1 ) as compared to higher planting density (Table 5).

Discussion:
This study showed that sowing methods significantly influenced individual as well as total weed density m -2 , (Table 3 & Fig.  1), some morphological traits (Table 4), almost all yield-related traits, and fiber uniformity (Table 5). Similarly, the variation of planting density also influenced the weed density, morphological attributes, yield-related traits, and fiber fineness (Table 3-5 & Fig. 1).

Weed infestation
In cotton cultivation, weeds infestation reduced the cotton yield by 10-90% [35] depending on the type of weed flora, cultural practices (sowing method, plant spacing, and fertilizer application), and crop varieties. Reduction in cotton yield due to weed infestation can be explained as weed has an extraordinary ability to compete for space, food, and light [29,51]. In 2018, total and individual weed density (except carpet weed and Egyptian finger grass) were comparatively higher than in 2017 that might be associated with more rainfall events and suitable weather conditions soon after the emergence of the crop. Regarding the sowing method, the highest weed population was recorded in flat sowing in comparison to the ridge and bed sowing. This higher weed population in flat sowing could be explained as the weeds seeds are present homogenously at soil surface while in ridge and bed sowing seeds might be confined to the specific area due to alteration of micro-topography in ridge and bed formation [22]. Similarly, Nadeem et al. [21] discussed that the ridge sowing method abridged the density of C. rotundus, T. portulacastrum, C. dactylon and C. arvensis significantly as compared to flat sowing because due to changing land formation from flat to ridges and beds some of the weed seeds exposed. The exposed seeds might be unable to germinate due to weather extremities. Improved sowing techniques (bed and flat sowing) proven to be more efficient because these methods provide suitable soil conditions for crop establishment and root growth and development by reducing the root penetration resistance and improving soil physical properties [16]. Moreover, ridge and bed sowing methods are more efficient as compared to flat sowing in terms of resource use and weed control [52-54]. Increasing planting density reduced the weed infestation that could be explained as the higher population pressure and intercrop competitive ability of Bt cotton reduced the weed persistence. Moreover, the smothering potential of higher planting density reduced weed occurrence [48]. Contrarily, the reduction of plating density promotes weed emergence and density due to less intercrop competition [55]. Sowing methods Amongst three sowing methods, bed and ridge sowing methods proved to be more efficient in terms of resource use, crop establishment, growth, yield, and quality of cotton. Planting methods play an important role not only in establishing cotton stands but also assists the conversion of solar radiation into chemical energy in crop plants but also help in balancing the plant to plant interaction for solar radiation to get maximum yield [56]. Moreover, an effective sowing method also helps to increase the productivity of the crop through improving growth and development [57]. Improved sowing technique (bed or ridge) are efficient methods because of having the potential to increase the soil's physical properties (soil moisture contents), improve root penetration, and the emergence of seedlings [16,26,53,55]. It was observed that improved sowing techniques reduced the boll maturation period as compared to flat sowing but this reduction was nonsignificant (Table 4) [45] concluded that in cotton-wheat rotation, bed sowing of cotton increased the yield by 35% than flat sowing. However, different sowing methods did not influence the number of monopodial branches, unopened bolls plant -1 , and GOT that is also highlighted by Akbar et al. [33] and Ali et al. [15]. In the case of fiber quality, except fiber uniformity, all other quality traits were alike in diverse sowing techniques (Table  5) which were also concluded in previous research studies [14,36].

Planting density
Planting density influenced the boll maturation period, most of the morphological, and yield-related attributes, and fiber fineness substantially (Tables 4 &  5). It was observed that reducing plating density reduced the boll maturation period that might be due to proper utilization of sunlight, aeration, and uptake of nutrients and water and less competition for resources among plants [62]. Amongst morphological attributes, unopened bolls plant -1 were remained statistically similar, while all other traits (sympodial branches, monopodial branches, bolls plant -1 , and opened bolls plant -1 ) were higher in lower planting density treatment. Reduced morphological attributes in higher density can be explained as the excessive vegetative growth under dense population resulted in shading, etiolation, boll rot, abscission of fruit, and delayed maturity [4, 14, 36, 38]. Contrarily, increasing plant to plant spacing resulted in reduced planting density, increased branches plant -1 (sympodial as well as monopodial), and bolls plant -1 ( Table 4) that was due to higher fruit retention on both types of branches [14, 37,38]. Similarly, lower planting density also improved the yield and yield-related traits as compared to a higher density plant population that could be related to improved morphological attributes (higher no. of bolls plant -1 and branches plant -1 ) and better utilization of resources and low competition among plants [36, 40, 62]. While, higher planting density completion among plants for resources (light, water, nutrient, and space) was higher that cause a reduction in final yield [63, 64]. Moreover, a balanced and optimum planting density improved the microenvironment by increasing light interception and soil water evaporation that resulted in higher yield and improved water use efficiency [65-67].
Kerby et al. [68] and Heitholt et al. [69] discussed that alone planting density influenced the photosynthesis capacity, plant height, fruit production, and boll size, location of bolls, numbers of bolls, maturity, and yield. In terms of fiber quality, planting density did not influence the fiber quality except fiber fineness (Table 5). It was observed that lower planting density improved the fiber fineness as compared to the higher density that could be attributed to properly assimilated translocation and improved source-sink relation. Previous research findings of Hussain and Qasim [70] and Ehsanullah et al. [14] also explained that fiber quality remained unaffected under different planting density.

Conclusion
Sowing method and planting density are two key factors that are used to manage the weed infestation, improve the morphological, and yield-related attributes of cotton. The results of this experiment revealed that bed sowing of cotton proved to be more efficient for weed management while enhancing the yield and yield-related traits as compared to flat sowing. Similarly, the performance of the ridge sowing method in terms of weed management, enhancing yield, and morphological traits were comparable to bed sowing. In the case of planting density, where higher planting density ensures better weed management and more plant population per unit area at the same time morphological, yield, and yield-related traits were lower in higher planting density. Contrarily, optimum planting density as in 75 × 30 cm or in 75 × 37.5 cm plant spacing not only improved the crop morphology but also improved the yield, yield-related traits, and quality of cotton. Moreover, GOT and most of the other quality traits of cotton remained statistically similar in different sowing methods and planting geometries. In crux, it can be concluded that the bed sowing method having a planting density of 4-5 plants m -2 is feasible for maintaining higher productivity of cotton and it could be used for cotton mechanization in the cottonwheat cropping zone.