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Response of Cotton to Mepiquat Chloride with Varying N Rates and Plant Populations¹

Excessive vegetative growth often limits any potential cotton (Gossypium hirsutum L.) yield increase to be expected from the use of higher N rates or plant populations. Mepiquat chloride (1,1-dimethylpiperidinium chloride) is a plant growth regulator which suppresses vegetative growth in cotton. It has been speculated that higher N rates and plant populations could be used in combination with mepiquat chloride without fear of excessive vegetative growth. Experiments were conducted at eight locations during a 3-year period to determine if the response of cotton to mepiquat chloride is affected by N rates or plant populations. Treatments consisted of three N rates, three or four plant populations on three soil types, and 0 or 49 g/ha of mepiquat chloride. Nitrogen rates were either 45, 78, and 112 kg/ha or 28, 62, and 95 kg/ha. Populations ranged from 37,000 to 235,000 plants/ha. Mepiquat chloride increased yields in four of the eight environments. The response of yield and most other agronomic properties to mepiquat chloride was usually unaffected by rates of applied N. This lack of response was found when rates of N were both below and above those necessary for optimum yield. Mepiquat chloride had no adverse effects when populations were below those necessary for optimum yield. However, the results indicated that mepiquat chloride may sometimes partially overcome the detrimental effects of above optimum populations when environmental conditions result in excessive vegetative growth and delayed maturity. In one environment with these conditions, mepiqnat chloride increased yields 10, 23, and 51 % at 37,000, 136,000, and 235,000 plants/ha, respectively. Highest yields were obtained with 37,000 plants/ha. Differential responses were observed for boll production, lint per boll, and lint percent, but most of the yield increase that resulted from the application of mepiquat chloride was due to a differential effect of the growth regulator on maturity with the varying populations. However, in all eight environments, mepiquat chloride did not alter the optimum N rate or plant population.

Key Words: Plant growth regulator • Yield • Fiber quality • Maturity • Interaction • Gossypium hirsutum

² Assistant professor, Crop Science Dep., North Carolina State Univ., Raleigh, NC 27650.

Received for publication August 17, 1982.

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