Seeding and Harvesting Times and Climate Conditions Are Important for Improving Nitrogen and Fiber Contents of Green Manure Sunn Hemp

The efficient use of green manure enhances sustainable nitrogen (N) cycling in agroecosystems. Appropriately utilizing the differences in N and fiber of green manure between cultivars or harvesting times strengthens the N use efficiency for subsequent crops. Therefore, it is necessary to clarify these differences. We clarified differences in nitrogen and fibers from four sunn hemp ( Crotalaria juncea L.) commercial cultivars at two harvesting times after spring and summer seeding for three years. Only a few significant differences were found for any components between the four commercial cultivars. Although sunn hemp produced almost no flowers after the May seeding, the earliest flower opened at 50 days after seeding (DAS) and then significantly increased by 60 DAS ( p < 0.05), after the July seeding. Sunn hemp DMY at 60 DAS had significantly larger values than those at 50 DAS all three years after the May and July seedings ( p < 0.01). Significant differences between 50 DAS and 60 DAS were also observed for more than two years for the sunn hemp total carbon (TC), total nitrogen (TN), C/N ratio (CN), nitrate N (NO 3 -N), acid detergent fiber (ADF), and neutral detergent fiber (NDF) ( p < 0.05). The sunn hemp N and ADF yields were significantly greater at 50 DAS than at 60 DAS in 2019 and 2020 ( p < 0.05). These results suggest that the seeding and harvest time are important for decomposition estimations, although the differences between cultivars with equivalent flowering characteristics were not worth considering. Additionally, significant differences between years were found for all measurements except for NDF after the July seeding. This result suggests that the yearly effect of the difference in weather conditions, which causes differences in the yield, nitrogen, and ADF contents, is also an important factor for decomposition.