Abstract
Direct planting—in which storage roots are planted instead of transplanting stem cuttings—has been studied as a labor-saving system in sweet potato. However, weeding is necessary until the shoots emerge. Therefore, it is important to estimate the days to emergence (DTE) for planning weeding practice. In this study, we estimated DTE based on growing degree days (GDD) calculated from soil temperature in direct planting of sweet potato. Seven genotypes were directly planted on four dates from 2018 to 2019, and the average daily soil temperature and date of emergence were investigated. Using these data, we calculated GDD for emergence of seven genotypes with lower limits of soil temperature of 8, 10, 12 and 15°C and upper limits of 20, 22, 24, 26 and 28°C. Then we decided the best combination of lower and upper limits of soil temperature that minimized the root mean squared error (RMSE) of the difference between the observed and predicted DTE. Consequently, the combination with a lower limit of 10°C and an upper limit of 24°C had the smallest RMSE, i.e., 2.48 days. This prediction model was also validated using data obtained by planting on three dates in 2020. The average RMSE in 2020 was 3.01 days, indicating that DTE in direct planting could be predicted with an error of about 3 days.
