Optimal Sowing Time to Adapt Soybean Production to Global Warming with Different Cultivars in the Huanghuaihai Farming Region of China

Context: The Huanghuaihuai Farming Region (HFR) is one of China's largest soybean production zones, with climatic conditions in the HFR being near ideal. In recent decades, however, global warming has increasingly inhibited soybean production in this region. Objective: Our aim was to determine whether planting time and/or genetics (heat-resilient and heat-sensitive cultivars) enable adaptation of soybean cropping systems to the changing climate. Methods: Two-year split-plot design field experiments comprising five different sowing dates and three cultivars (namely heat-resilient, heat-sensitive and widely grown ones) were conducted in the HFR in 2020 and 2021. Results: Delayed sowing dates significantly elevated the average minimum temperature in the vegetative growth stage, fostering soybean growth and consequently truncating the vegetative growth stage. Later sowing reduced the occurrence of heat stress and minimized negative influences of heat stress on yield. Higher heat stress in the first two sowing treatments (1 June and 10 June) negatively influenced yields of Hedou14 (heat-sensitive cultivar) and Zhonghuang13 in 2020, while the highest yield was obtained for the third sowing treatment (20 June) for these two cultivars. In general, Jidou21 (heat-resilient cultivar) showed greater tolerance to increasing temperature, with higher yield at earlier sowing. Delayed sowing decreased temperature exposure and growing degree days (GDD), reducing the duration of grain filling, leaf area index and photosynthesis, in turn decreasing grain number per plant and 100-grain weight. Taken together, such factors inhibited yield of all cultivars in the year without heat stress. In contrast, high precipitation (2021) prolonged grain filling and increased yield. Conclusions: We contend that optimizing sowing time is crucial for mitigating heat stress in heat-sensitive cultivars. We suggest that high yield is more likely to be realized when crops are exposed to GDD > 500 degrees C & sdot;d and precipitation > 250 mm during grain filling. Implications: Future studies aiming to elicit optimal sowing times for soybean should consider cultivar heat sensitivity relative to long term climatic conditions of the target agro-ecological region.