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 (NO3-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.

Background: Sunn hemp (Crotalaria juncea L.) is an important annual legume used as green manure. It contains pyrrolizidine alkaloids, which are apparently effective in suppressing nematodes. It is pertinent to know the growth stage during which pyrrolizidine alkaloid concentrations become higher. Methods: The period of days necessary for flowering, pod growth and the concentrations of one pyrrolizidine alkaloid in pods at different stages in five sunn hemp cultivars with May and July seeding were investigated. Result: Findings indicated that flowering of the two cultivars occurred later than the others, especially for May seeding. The pods grew to 20 mm length in 6-11 days from flowering. Then the pods grew to maximum size (40 mm length) in only few days. A main pyrrolizidine alkaloid in sunn hemp seed, trichodesmine, increases as pod size grow; then it increases considerably as pod mature. However, it in matured pod hull contains similar values of whole immature pod. These results demonstrate that both day-neutral and short-day cultivars with July seeding shorten the days to first flower and indicate that cutting after 10 days from first flowering is effective for increasing pyrrolizidine alkaloid concentration.