Effect of Different Plant Populations and Naphthalene Acetic Acid (NAA) Concentrations on Growth and Yield of Cotton (Gossypium hirsutum L.)
Khaing Mar Tun *
Department of Agronomy, Yezin Agricultural University, Yezin, Naypyi Taw, Myanmar.
Aye Aye Khaing
Yezin Agricultural University, Magway Campus, Myanmar.
Phyu Thaw Tun
Department of Agronomy, University of Agriculture and Animal Science, Maubin, Ayeyarwaddy Region, Myanmar.
Aung Win
Advanced Center for Agricultural Research and Education, Yezin Agricultural University, Yezin, Naypyi Taw, Myanmar.
Htay Htay Oo
Department of Agronomy, Yezin Agricultural University, Yezin, Naypyi Taw, Myanmar.
*Author to whom correspondence should be addressed.
Abstract
Cotton is an important fibre crop in Myanmar. Cotton productivity is constrained by low plant populations, inappropriate crop management practices, and excessive shedding of reproductive structures, which limit seed cotton yield per unit area. This study was conducted to evaluate the effects of plant population and foliar application of naphthalene acetic acid (NAA), a synthetic auxin, on the growth and yield of cotton (Gossypium hirsutum L.). The field experiment was conducted at the Cotton Research and Development Farm (Lungyaw), Kyaukse Township, Mandalay Region, Myanmar, during the pre-monsoon season from February to July 2025 and the post-monsoon season from August 2025 to January 2026. Shwe Daung-10 was used as the test cultivar. The experiment was arranged in a split-plot design with three replications. Four plant populations (34,429, 43,037, 57,383, and 86,074 plants ha⁻¹) were assigned to the main plots, while three NAA concentrations (0, 20, and 40 ppm) were assigned to the subplots. NAA foliar sprays were applied at square formation, first bloom, and boll development stages. As plant population increased, plant height and seed cotton yield per hectare increased, whereas yield-contributing parameters decreased. Among the tested plant populations, P4 produced the highest seed cotton yields of 3,652.4 and 3,041.3 kg ha⁻¹ in the pre-monsoon and post-monsoon seasons, respectively, representing increases of 73.68% and 51.01% over P1. Foliar application of NAA significantly increased plant height, total reproductive structure setting points, harvested bolls per plant, single boll weight, and seed cotton yield per hectare, but did not significantly affect the number of sympodial branches, total reproductive structure points, or thousand-seed weight. Among the tested NAA concentrations, N3 produced the highest seed cotton yield, increasing yield by 15.71% and 20.70% over N1 in the pre-monsoon and post-monsoon seasons, respectively. The study indicated that increasing the plant population to 86,074 plants ha⁻¹ and applying 40 ppm NAA could improve cotton yield within the tested treatment ranges.
Keywords: Cotton, Gossypium hirsutum L., naphthalene acetic acid, NAA, plant population, planting density, plant spacing, boll retention, seed cotton yield, yield components, pre-monsoon season, post-monsoon season