Developing management options for mepiquat chloride in tropical winter season cotton

Abstract

Little research had been conducted on managing cotton growth using mepiquat chloride (MC) during the tropical winter season. Three experiments were conducted over three seasons. Early season internodes (first nine nodes) were long compared to temperate climates. Consequently the effect of MC applied, as multiple or single doses, from four nodes to first flower was evaluated. MC dramatically reduced height and node number in proportion to a total amount of MC up to 45 g/ha, r 2=0.66 and r 2=0.81, respectively; this response was independent of the frequency of treatment. Lint yields were high averaging 2143, 2415 and 2312 kg/ha for the three seasons. Despite the number of fruiting branches declining linearly with the amount of MC lint yields were decreased ( p<0.05) in only 2 of the 28 treatments. There was a trend to increased yield (106.5%) compared with untreated plants where between 9 and 18 g/ha MC was intercepted. There was no economic effect of MC on lint quality. Yield compensation for reduced fruiting branches due to MC was via a linear increase in boll retention over all fruiting sites and vegetative branch bolls, which peaked with 35 g/ha of MC for each yield component. Where >35 g/ha was intercepted yields were the same or reduced compared to untreated plants. Compensation via boll size, possibly related to position, was implicated in this yield response. Thus treatment with 9–18 g/ha of MC, between 4 and 11 nodes, provided a compromise between suppressing plant height for operational efficiency, and optimising these yield components. Crop monitoring techniques based on the length of internodes could not aid MC treatment decisions because critical lengths, developed for temperate locations, were always exceeded and there was rarely a yield response to MC. An explanation came from the independence to MC of the height-to-nodes ratio prior to flowering. Monitoring techniques based on predicting the height and node number response from the MC concentration within the plant showed promise. MC did not affect time to maturity despite ≤15-day reduction in main-stem flowering period due to MC. This difference was attributed to high and increasing end-of-season temperatures synchronising maturity in bolls from later flowers, and it was possible vegetative branch bolls, increased by MC, matured after main-stem bolls. It was suggested future research consider modelling the MC response, irrigation interactions, the cultivar response, the effect on duration of flowering, and the interaction between MC and yield compensation from insect damage.