Improving subtropical cotton production by using late winter sowing to reduce climatic risk

Climate variability is a challenge to the reliable production of high yielding cotton, Gossypium hirsutum L. in the subtropics. Shifting the growing season earlier by sowing during late winter (August) to reduce the risk of exposure to unfavourable radiation, temperature and rainfall during mid-summer offered the potential to improve stagnating and variable yields. This study aimed to determine the effect of late winter sowing on establishment, growth, development and lint yield of cotton. Accepted knowledge from temperate climates suggested cold temperature stress would impede crop establishment, so the application of clear oxodegradable film to the soil surface at sowing to mitigate cold temperature stress was also explored. Crop establishment, biomass accumulation and partitioning, light interception and lint yield were measured for cotton sown with and without film at intervals from early August to mid-September in four consecutive experiments (2013–2016). The application of film provided no benefits for establishment and only marginal improvement for crop growth and yield despite minimum temperatures being well below accepted chilling injury thresholds in temperate environments. Greater daily solar radiation levels both solarised the soil and shortened the duration of daily minimum temperatures, enabling successful crop establishment. Early sowing did not hinder crop production, and in 3 of the 4 experiments, lint yield was significantly higher (P < 0.001) for August (2314−3305 kg ha−1) than September-sown crops (2053−2290 kg ha−1). The impact of sowing time was further explored through the measurement of biomass and yield of 29 high input commercial crops sown between August and December in 2016–2019. Final biomass and lint yield declined (P < 0.001) by 2.89 g m-2 and 7.23 kg ha−1 for each day that sowing occurred after August 1. Combined analyses of the experiments and commercial crop data confirmed that late-winter sown crops had the highest daily growth rate between flowering and maturity, declining by 37 % for summer-sown crops. RUEg (glucose equivalent) of 2.27g MJ−1 PAR was not changed by sowing time. Late winter sowing best matched flowering and boll development with the summer solstice and a lower incidence of cloud or heat stress. An earlier harvest also reduced the risk of pre-harvest weathering, collectively enabling over 40 % higher yields than traditional spring sowing without additional crop inputs. Spring and later sowings coincided unfavourable climate with the more susceptible growth phases of late boll filling or early flowering respectively and declining solar radiation and temperatures prevented yield recovery for the latest summer-sown crops. As the occurrence of excessive temperature-related weather events is expected to increase with climate change, particularly for the subtropics, this study demonstrates how a knowledge of crop response can be used to evolve sowing time strategies to better mitigate climatic risk