Cotton Response to Mepiquat Chloride

The potential for mepiquat chloride (1,l-dimethylpiperidinium chloride) to control excess vegetative growth and increase yield of cotton (Gossypium hirsutum L.) was evaluated under a wide range of environmental conditions in 35 replicated experiments conducted in the San Joaquin Valley of California from 1979 to 1983. Predominant soils were thermic Xeric Torriorthent's and typic Torriorthents. Yield response to mepiquat chloride (MC) has varied in production fields as well as in these tests. The focus of this study was to identify those variables which have contributed to the variability observed. Plot size among experiments varied from 15 m² to 1.3 ha. Mepiquat chloride was applied at 49 g ha⁻¹ when bloom counts reached 0.7 white blooms m⁻ˡ row and plant height was between 46 to 61 cm. Overall, MC treated plots yielded 1309 compared to 1299 kg ha⁻¹ lint for control plots and the yield response varied significantly among experiments. Forty-six percent of this variability could be accounted for by number of heat units for the growing season and by the final plant height of control plants. Yield increases occurred when the growing season was short or when final height of control plants was either shorter or taller than normal. Yield decreases occurred when heat units ranged between 2600 to 3000 and final control plant height ranged from 90 to 110 cm. Height reduction due to MC treatment varied according to plant height (r=0.57 for n= 17) and was maximal at 105 cm. Mepiquat chloride reduced main stem nodes by 1.0, gin turnout by 0.4%, and lint proportion by 0.3%; increased percent of final yield harvestable on 19 September by 90, fiber strength by 3 kN m kg⁻¹, micronaire by 0.06 units, and petiole NO₃-N at first open boll by 0.5 g kg⁻¹; and had no effect on fiber length, fiber elongation, petiole PO₄-P and K, or NO₃-N at first bloom. Typical seasonal heat units and control plant heights in the San Joaquin Valley fall within the range where a favorable yield response to MC is not predicted.