Alfalfa intercropping maintains intermediate wheatgrass grain yield relative to nitrogen fertilization across site conditions and stand ages
Intermediate wheatgrass (IWG; Thinopyrum intermedium) is a promising perennial grain crop, but yield instability remains a major challenge. We compared effects of nitrogen fertilization and intercropping with alfalfa (Medicago sativa L.) on IWG biomass, grain yield, and root traits across three stand ages at two nearby sites in eastern Missouri differing in soil properties (Planthaven Farms and Danforth Center Field Research Site). Four treatments (unfertilized IWG, fertilized IWG, IWG-alfalfa intercrop, and alfalfa monoculture) were established in a randomized complete block design with four replications per site. Biomass and grain yield were evaluated at per-row and field scales, while roots were measured to 1 m depth during the second stand age. Alfalfa intercropping maintained IWG grain yield without synthetic N fertilizer, although treatment responses varied by site and stand age. At Planthaven, differences became more pronounced as stands matured, with intercropped IWG producing the greatest per-row biomass and grain yield in stand age three. At the Danforth Center Field Research Site, intercropped IWG produced greater per-row biomass and grain yield than monocultures, whereas fertilized IWG tended to produce greater field-scale biomass; grain yield declined across treatments with stand age. Across sites, nitrogen fertilization increased vegetative biomass more consistently than grain yield. Despite having fewer IWG rows, intercrops maintained field-scale grain yield comparable to fertilized IWG in several site-years. Root traits responded weakly to management and were structured primarily by soil depth. These findings highlight the importance of site conditions, and stand age, when evaluating management strategies for perennial grain production.