Sustainablesuccess of the Northern High Plains (NHP) crop and livestock industry is limited by market accessibility, infertile soils, short growing seasons, and availability of high nutritive value, low-cost feed. Adoption of the tropical legume, Crotalaria juncea L. (commonly known as sunn hemp), into NHP cropping systems could improve the sustainability of the rotation by fitting into summer growing windows and being used as a hay alternative. Rainfed and irrigated studies were conducted in Lingle, WY, to determine the target planting date for sunn hemp in the NHP. Later planting day of year was correlated with increased percent establishment in the rainfed study but had no effect in the irrigated study. In both irrigated and rainfed studies, planting between 8 June and 18 June maximized biomass accumulation. Biomass accumulation 60 d after planting ranged from 0.2 to 5.1 Mg ha -1 in the rainfed study and 1.2 to 4.2 Mg ha -1 in the irrigated study. For both irrigated and rainfed studies, the nutritive value of sunn hemp was high with crude protein greater than 220 g Kg -1 , acid detergent fiber less than 305 g Kg -1 , neutral detergent fiber less than 331 g Kg -1 , relative feed values above 183, and total digestible nutrients greater than 590 g Kg -1 . Planting date did not affect nutritive value. Sunn hemp grown in the NHP has a 60-d yield potential 5.1 Mg ha -1 , should be planted between 8-18 June to 60-d biomass production, and has nutritive value equivalent to supreme quality alfalfa (Medicago sativa L.) hay.

The integrated crop‐livestock systems of the northern High Plains are lacking in annual legumes that meet the nutrient demands of beef cattle when alfalfa (Medicago sativa) is in limited supply. We investigated the biomass accumulation, regrowth biomass, and nutritive value of sunn hemp (Crotalaria juncea) in response to initial harvest day and cutting height in irrigated and dryland studies in Wyoming. Net biomass accumulations in 105‐day growing period were 4.4 tons acre−1 under irrigation and 0.7 tons acre−1 in dryland conditions. Initial harvest day after planting (iDAP) affected initial and regrowth biomass accumulation but did not affect net biomass accumulation in irrigated or dryland studies. Regrowth and net biomass accumulations were affected by cutting height in the irrigated study only. Nutritive value concentrations were significantly affected by iDAP in both irrigated and dryland studies. Under irrigation, net nutrient accumulation was not affected by iDAP but was significantly greater with a cutting height of 4.2 inches compared to 2.5‐ and 6‐inch heights. In contrast, the dryland study, net nutrient accumulation was not affected by cutting height but was higher in 55–105 iDAP than 45 iDAP. In the irrigated study, a cutting height of 4.2 inches produced net accumulations higher in both biomass accumulation and nutritive value. In the dryland study a harvest time of 55–105 iDAP produced the highest net accumulation regardless of harvest time or cutting height. Sunn hemp can be harvested once or twice in a 105‐day growing season to produce biomass and essential nutrients for livestock feeding.