Delayed Sowing Dates Affected Alfalfa Overwintering Survival Rates by Reducing Soil Temperature Stress

This study explored the effect of sowing time on alfalfa overwintering and its mechanisms. Gongnong No. 1 alfalfa was sown on July 1, July 16, August 1 and August 15, 2017, and was not cut at the end of autumn. The soil temperature changes in the 10 and 20 cm soil layers were monitored during the overwintering period, and bare land was monitored as a control. The growth characteristics of alfalfa were observed at the end of autumn, and the overwintering survival rate was measured at the end of April 2018. Research shows that as the sowing delay increased, the alfalfa hay yield, plant height, root collar diameter and overwintering survival rate decreased gradually. The minimum temperature of the 10 and 20 cm soil layers decreased as the sowing delay increased, and the diurnal temperature difference increased. Compared with that of alfalfa sown on August 15, the aboveground biomass of alfalfa sown on July 1 increased the average minimum temperature difference in the 10 and 20 cm soil layers by 3.12°C and 2.99°C, respectively, and reduce the average diurnal temperature difference by 3.33°C and 2.15°C. These results indicated that delaying alfalfa sowing reduces the cover on the field, which results in substantial differences in the minimum temperature and the diurnal temperature difference between the 10 and 20 cm soil layers. Our research has suggested that the minimum temperatures of the 10 and 20 cm soil layers and the increase in the diurnal temperature difference are important reasons for low overwintering survival rates in alfalfa. Therefore, sowing should occur in early July, and mowing should not be performed at the end of autumn in northern China.