Nitrogen fertilizer rate and timing effects on smooth bromegrass interseeded with sorghum‐sudangrass

Interseeding perennial cool-season grass pastures with annual warm-season grasses may increase forage accumulation and nutritive value. Our understanding of how N fertilizer rate and timing affect the forage mass, herbage composition, and N use efficiency (NUE) of interseeded pastures remains limited. In 2018-2020, we evaluated the responses of smooth bromegrass (Bromus inermis Leyss.) to two interseeding and five fertilizer rate and timing treatments in eastern Nebraska. Interseeding treatments included unseeded smooth bromegrass and smooth bromegrass interseeded with sorghum-sudangrass [Sorghum bicolor (L.) Moench x S. bicolor var. sudanense]. Fertilizer rate and timing treatments included: 0 lb N acre(-1) , 80 lb N acre(-1) in spring, 40 lb N acre(-1) in spring and summer, 80 lb N acre(-1) in summer, and 80 lb N acre i in spring and summer. Summer applications occurred after sorghum-sudangrass emergence. Across years, fertilizer rate and timing had the greatest effects on forage and weed responses. Applying N at 80 lb acre(-1) in spring increased spring and total spring + summer forage mass by 33% while increasing NUE compared with summer application. Interseeding did not increase summer and total forage mass, and summer N applications did not enhance the success of sorghum-sudangrass and were problematic, increasing weediness and reducing NUE. Practitioners should fertilize smooth bromegrass with 80 lb N acre(-1) in spring or 40 lb N acre(-1 )in split spring and summer applications in eastern Nebraska. Interseeding sorghum-sudangrass may increase forage accumulation and nutritive value, but establishment methods need improvement.