Effect of high density planting and growth retardant spray on growth and yield of Bt cotton variety (Gossypium hirsutum L.)

Introduction

Cotton (Gossypium spp.), is one the important commercial crop of India. It supports the livelihoods of numerous farming communities. It is also known as 'white gold' due to its role in national economy, earning foreign exchange as well as employment generation. It is a subtropical crop with an indeterminate growth habit. It continues both vegetative and reproductive growth simultaneously. Although vegetative growth supports reproductive development, excessive vegetative growth can have negative effects. Cotton yield are stagnated and crop is observed to uneconomical especially under rainfed condition. This necessitates new technologies which will be helpful for reducing cost of cultivation along with sustainable yield. High density planting of cotton is proved to be an alternative technology for improving yield and profitability along with input use efficiency (Nalayini et al., 2018) [8] . The High Density Planting System (HDPS) is envisioned as an alternative cotton production strategy with the potential to enhance both productivity and profitability. It aims to reducing the risks linked to conventional cotton farming practices in India. There is a positive correlation between plant population and seed cotton yield, and hence selecting the most suitable spacing is crucial. Proper spacing allows plants to optimize growth conditions, which is closely related to both root development and shoot growth. Cotton hybrids currently used by farmers often display varied growth behaviors, with many showing a tendency for vigorous vegetative growth. There is risk of overcrowding of plants under HDPS that warrants adoption of varieties having short stature with shorter length of fruiting branches. Mepiquat chloride, a plant growth retardant is been found beneficial to control the vegetative growth of cotton with diverting photosynthates to reproductive parts. (Khetre et al. 2018, Gobi and Valyapuiri 2013 and Shekar et al. 2011) [6,3] . This line of research supports the development of costeffective cotton management practices by integrating key components such as the adoption of straight varieties under high density planting system and strategic use of growth retardants can improve yield with comparatively lower production cost. Considering these facts, it was felt essential to prioritize research and development in cotton cultivation practices. Emphasis was given on improving productivity of straight variety of Bt cotton through optimized planting density, and number of sprays of PGR (Mepiquat chloride) needed to manage canopy of straight variety.

Material and method

The field trial was carried out at Cotton Research Station, Nanded (Vasantrao Naik Marathwada Krishi Vidyapeeth, Parbhani), under rainfed condition during the Kharif seasons 2024 -25. The field was fairly level and uniform. The soil was medium-black cotton soil belonging to Vertisol which was low in nitrogen, medium in phosphorus and having high potassium content. The total rainfall received during Kharif, 2024 season at the location was 871 mm (June -December, 2024) as against average rainfall of 901 mm. Nanded is situated at latitude, longitude and altitude of 19.13 0 N, 77.34 0 E and 274.63 m above mean sea level, respectively. The experiment consisted of twelve treatment combinations, comprising of four different plant densities in main plot and three treatments on number of PGR (Mepiquat chloride) sprays in subplot. These treatments were laid out in Split Plot Design with three replications. Four plant densities i.e., D 1: 55,555 plants ha -1 , D 2: 83,333 plants ha -1 , D 3: 1,11,111 plant ha -1 , D 4: 1,66,666 plants ha -1 were evaluated with spacing 60 cm x 30 cm, 60 cm x 20 cm, 60 cm x 15 cm and 60 cm x 10 cm, respectively. Three PGR sprays treatments i.e., P 1: One spray of PGR, P 2: Two sprays of PGR, P 0: Control (Water spray) was tested in sub plot. Newly released straight variety of Bt cotton, NH 1902 (BG I) was sown by dibbling method on 22 nd June 2024. Size of gross plot was 5.4 m x 5.4 m. Data were recorded on plant height, number of monopodial and sympodial branches plant -1 , drymatter accumulation were recorded at periodical intervals. Data on seed cotton yield, lint yield and stalk yields (kg ha - 1 ) were collected and subjected for statistical analysis. Growth indices were calculated using standard formulae.

The absolute growth rate of plant height refers to the rate at which a growth variable such as plant height (H) increases over time (t). It is represented as the differential of height with respect to time. To calculate this, the formula proposed by Richards (1969) [17] was used, and the results were expressed in units of cm day -1 plant -1 .

(H2 -H1) AGR (cm day -1 plant -1 ) = ---------(t2 -t1)

Where, H 2 and H 1 refer to the plant height of plant in cm at t 2 and t 1 time, respectively.

Absolute growth rate of dry weight (g day -1 plant -1 )

The absolute growth rate of dry weight refers to the rate of increase in a growth variable, namely dry matter accumulation (W), over time (t). It is determined as the differential of 'W' with respect to time 't'. The absolute growth rate of total dry matter was calculated using the formula reported by Richards (1969) [17] and expressed in units of g day -1 plant -1 .

(W2 -W1) AGR (g day -1 plant -1 ) = ---------(t2 -t1)

Where, W 2 and W 1 refer to the total dry matter of plant in g at t 2 and t 1 time, respectively.

Leaf area index (LAI):

It is the ratio of the land area to the leaf area per plant, expressed in the same unit. The formula for calculating the LAI was given by Watson (1952) [22] .

Leaf area plant -1 (dm 2 ) LAI = Land area plant -1 (dm 2 )

Crop growth rate (g g -1 day -1 plant -1 )

The crop growth rate is widely utilized to compare plant stands and communities of different species across various habitats, as well as to assess the production efficiency of a plant stand (Hunt, 1978) [5] . It is calculated in grams per day per square meter (g day⁻¹ m⁻²) using the following formula.

Where, W2 = dry weight of plant at time t 2 (g plant -1 ) W 1 = dry weight of plant at time t 1 (g plant -1 )

Results and discussion I) Plant density

Plant growth characters: Plant height was increased numerically due to increased plant density however was not significant (Table 1). Plant density of 1,66,666 plants ha -1 (D 4 -60 cm x 10 cm) recorded highest plant height at harvest 123.62 cm. Similar findings were reported by Singh et al. (2020) [19] and Thakur (2020) [21] . Gouthami et al.

(2023) [4] reported increased plant height in late season under higher density due to inter plant completion whereas reduction in height under lower density due to suppression of apical dominance. The straight variety of Bt cotton under study might have adjusted in all the spacing under study resulting in non-significant differences due plant height in present study.

The number of monopodial branches plant -1 was significantly increased with decrease in plant density (Table 1). Plant density of 55,555 plants ha -1 (60 cm x 30 cm) (D 1 ) recorded maximum number of monopodial branches than other density. Significant effect of plant density on number of monopodia plant -1 is reported by Thakur (2020) [21] . Wider plant spacing allows achieving a greater number of branches due to more efficiency in the rate of photosynthesis is documented by Singh et al. (2020) [19] . This might be due to better aeration, adequate interception of light and suppression of apical dominance which resulted in to increasing branching in lower plant density. Number of sympodial branches plant -1 were not influenced statistically at harvest. Highest plant density (1,66,666 plants ha -1 ) recorded higher numerical values of sympodial branches (22.87). Dry matter accumulation plant -1 was increased with decrease in plant density (Table 1). At lower plant density of 55,555 plants ha -1 (spacing 60 cm x 30 cm), dry matter plant -1 at 150 DAS (88.12 g) was significantly higher as compared to 1,11,111 plants ha -1 (60 cm x 15 cm -76.11 g) and 1,66,666 plants ha -1 (60 cm x 10 cm -74.87 g). Higher dry matter in lower plant density might be likely due to enhanced canopy development under wider spacing. Kumar et al. (2017) [7] and Parihar et al. (2018) [10] opined that notable improvement in growth and yield-related traits can be attributed to increased availability of solar radiation in wider spacing, which promotes better synthesis and partitioning of assimilates to individual plants. This in turn facilitates efficient translocation of assimilates from source to sink, ultimately resulting in a significant increase in growth parameters such as plant weight and stem girth. Gauthami et al. (2023) [4] also reported higher dry matter per hectare in cotton with high density planting.

Growth indices

Sowing of Bt cotton variety NH 1902 at 60 cm x 10 cm (1,66,666 plants ha -1 ) tended to give higher values of AGR for plant height between 31 -60 days as compared to 60 cm x 15 cm (1,11,111 plants ha -1 ), 60 cm x 20 cm (83,333 plants ha -1 ) and 60 cm x 30 cm (55,555 plants ha -1 ).

Lowest absolute growth rate (AGR) of dry matter values was observed during harvesting period (Table 2). Reduction in AGR of dry matter during the terminal period might be due to reduction in plant dry weight after first picking as well drying of green bolls and shedding of leaves due to senescence during the period. More accumulation of photosynthates with increase in leaf area and other plant growth characters might have resulted in higher values of AGR for dry matter in lower plant density of 55,555 plants ha -1 (60 cm x 30 cm) in comparison with higher densities at all the stages. Plant density with spacing 60 cm x 10 cm (1,66,666 plants ha-1 ) tended to give higher values of crop growth rate (CGR) for plant height between 61 -90 days as compared to other wider spacing and was decreased thereafter (Table 3).

Although AGR for dry weight was lowest in 60 cm x 10 cm spacing, CGR was highest due to increased plant density in unit area. Khetre et al. (2018) [6] and Parihar et al. (2018) [10] recorded highest values of CGR with closer spacing. The mean leaf area index (LAI) was increasing up to 61 -90 DAS stage and gradually decreased thereafter till harvest due to leaf senescence (Table 3). A clear variation in leaf area index was observed due to different plant density and it was noted that sowing of hirsutum cotton at 60 cm x 10 cm (1,66,666 plants ha -1 ) recorded higher LAI over 60 cm x 15 cm (1,11,111 plants ha -1 ) and 60 x 30 cm (55,555 plants ha - 1 ).

Bolls m -2 and seed cotton yield (kg ha -1 ): The plant density of 1.11 lakh ha -1 and 1.66 lakh ha -1 recorded significantly greater number of bolls m -2 over lower densities (Table 3). Higher plant population per unit area has increased the number of bolls picked per square meter. Increase in seed cotton yield was observed with increased plant densities (Table 4). Higher plant density might have utilized maximum natural resources like solar radiation, moisture, nutrients and space more efficiently resulting in greater yield per unit area in closer spacing. Lower plant population is the major cause for its low seed cotton yield in wider spacing. Increase in seed cotton yield with higher plant population was also reported by Pawar et al. (2010) [12] , Parihar et al. (2018) [10] , Pandagale et al. (2020) [9] and Gouthami et al. (2023) [4] .

Harvest index (kg ha -1 ): The plant density influenced the values of harvest index ( [1] . High plant density has produced the greater biological yield but lowest harvest index.

The plant density didn't influence the values of ginning out turn (Table 4). Plant density of 55,555 plants ha -1 (D 1 -60 cm x 30 cm) recorded numerically highest ginning out turn (38.36 per cent) as compared to higher densities.

Effect of number of sprays of PGR (Mepiquat chloride)

Plant growth characters: The plant height was significantly reduced with Mepiquat chloride sprays (Table 1). Two sprays of Mepiquat chloride (P 2 ) had lowest plant height (117.98 cm) over one spray of Mepiquat chloride (P 1 -123.27 cm) and control i.e. water spray (P 0 -129.63 cm). Mepiquat chloride acts as antigibberelline and reduces internodal length which results in reduced plant height (Raut et al. (2019) [15] . Results in present study reducing plant height by Mepiquat chloride sprays are also supported with the findings of Borge and Dalvi (2019) [2] . Numbers of monopodial as well as sympodial branches plant -1 at harvest were not affected due to number of PGR sprays (Table 1). Patel et al. (2021) [11] also reported nonsignificant effect of mepiquat chloride sprays on number of vegetative branches. Two sprays of Mepiquat chloride has found to reduce dry matter accumulation plant -1 significantly over control i.e. water spray (Table 1). Rosolem et al. (2013) [16] documented that reduction in dry matter accumulation due to Mepiquat chloride sprays could be due to disturbance in source -sink ratio under Mepiquat chloride application.

Growth indices: Application of PGR (Mepiquat chloride) spray resulted in reduction in AGR of plant height of cotton (Table 2). Either one or two foliar sprays of PGR (Mepiquat chloride) have reduced rate of increase in dry weight of plant as compared to control. Reduced elongation of internode distance has resulted to reduction in dry matter in two sprays of PGR. The AGR for dry matter accumulation values were greater in control. The AGR for plant height and dry weight were drastically reduced from 30-60 DAS stage as the foliar spray was done 45 DAS. As the second application of PGR was done at 60 DAS, the AGR value was lower in two sprays of PGR since 60-90 DAS stage.

The dry matter production was reduced in PGR sprays resulting to decrease in CGR values compared to control (Table 3). Control recorded higher crop growth rate from. It was followed by one spray of Mepiquat chloride and two sprays of Mepiquat chloride. Two sprays of PGR resulted to decreased CGR as compared to other treatments. Patel et al.

(2021) [11] also observed decrease in CGR in two sprays f Mepiquat chloride as compared to control. Leaf area index (LAI) was greater in control (Table 3). Thus, highest LAI was recorded in control. Increase in light interception and leaf area might be reason for increased leaf area index under control.

Bolls m -2 and seed cotton yield: Number of bolls m -2 was increased with two sprays of Mepiquat chloride which was significantly superior over control spray (Table 4). But it was found at par with one spray of Mepiquat chloride. Prasad and Prakash (2011) suggested that energy saved in reduced dry matter production plant -1 has been diverted to increased reproductive parts leading to increased number of bolls m -2 . Two sprays of Mepiquat chloride (P 2 ) recorded significantly higher seed cotton yield (2116 kg ha -1 ) over water spray (1920 kg ha -1 ). However, one spray (2032 kg ha -1 ) and two sprays of Mepiquat chloride were found statistically equal. Khetre et al. (2018) [6] and Borge and Dalvi (2019) [2] opined that Mepiquat chloride also increased the source: sink ratio and sympodial branches. Spray of Mepiquat chloride favorably influence development and retention of fruiting bodies, thus increased seed cotton yield plant. Increase in seed cotton per hectare due to foliar application of Mepiquat chloride were also reported by Shekhar (2011) [18] , Gobi and Valyapuri (2013), Borge and Dalvi (2019) [2] , Thakur (2020) [21] , Srikala et al. (2023) [20] .

Harvest index: Two sprays of Mepiquat chloride had highest harvest index (27.46 per cent) over control (22.57 per cent) and was followed by one spray of Mepiquat chloride (25.34 per cent). Thus, increase in harvest index was noted with one and two sprays of Mepiquat chloride (Table 4).

GOT: Ginning out turn was not statistically altered due to sprays of Mepiquat chloride (Table 4). Two sprays of Mepiquat chloride had highest ginning out turn (38.20 per cent) over control (37.65 per cent) and one spray of Mepiquat chloride (38.01 per cent). Ginning percentage is governed by genetic makeup of the variety and hence was not differed by plant density. These finding are similar to Kumar et al. (2017) [7] .

Conclusion

On the basis of field investigation, it can concluded that, the plant height and number of branches were not influenced due to plant densities however dry matter accumulation plant -1 was reduced in higher plant densities. Plant density 1,11,111 plants ha -1 i.e. spacing 60 cm × 15 cm was found optimum to increase seed cotton yield of straight Bt variety of hirsutum cotton. Foliar application of Mepiquat chloride has reduced plant height and dry matter accumulation. Two sprays of Mepiquat chloride @ 25 g a.i. ha -1 at square formation and followed by 15 days has increased seed cotton yield and harvest index of Bt variety of hirsutum cotton.

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