Sowing Date Regulates the Growth and Yield of Broomcorn Millet (Panicum miliaceum L.): From Two Different Ecological Sites on the Loess Plateau of China

A two-year experiment was conducted to determine the optimal combinations of sowing date and variety maturity using four broomcorn millet (Panicum miliaceum L.) varieties. The results showed that sowing dates had significant effects on the leaf net photosynthesis (Pn) and chlorophyll fluorescence and multivariate analysis showed that the effects of variety, sowing date, measuring stage and their interactions were significant in both sites. The days from seeding to maturity were strongly decreased (6–35 d) and the ratios of reproductive growth to vegetative growth were increased in V2 and V4 and decreased in V1 and V3 in Baoji and increased in all varieties in Yulin. The highest yield was Jinshu 5 in Baoji and Shaanmei 1 in Yulin, and the total average yield of Yulin (2408.3 kg ha⁻¹) was higher than that of Baoji (1385.2 kg ha⁻¹) and the average yield was reduced by 12.4% and 27.2% compared to BJ1 in Baoji and 15.5%, 3.6% and 12.7% compared to YL1 in Yulin. Correlation analysis showed that the key meteorological factors which limit the growth and yield of broomcorn millet were different for the two sites. Moreover, linear fitting analysis indicated that the accumulated temperature and the number of growth days in the reproductive growth stage (R² = 0.5306 and 0.5139) and accumulated temperature during the whole growth period (R² = 0.4323) were the top three factors affecting the yield in Baoji and precipitation (R² = 0.386) affected the yield in Yulin. Overall, the results of this study determined that the varieties of broomcorn millet with a short growth period should have delayed sowing, while those with a longer growth period are suitable for early sowing in the semi-arid area.