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Information on fi eld management practices for seed production of newly domesticated grasses native to the southern Great Plains of North America has not been well documented. This study was conducted to document seed yield responses of one accession of plains bristlegrass [Setaria vulpiseta (Lam.) Roem. & Schult.] and three accessions of streambed bristlegrass [Setaria leucopila (Scribn. & Merr.) K. Schum.] to 0, 50, 75, 100, and 140 kg N ha −1 yr −1 under irrigation at Stephenville and Beeville, TX, during 2005 and 2006. The soil at Stephenville was a Windthorst fi ne sandy loam, mixed, thermic Udic Paleustalfs and at Beeville a Parrita clayey, mixed, active, hyperthermic, shallow Petrocalcic Paleustolls. Seed yields across N levels at Stephenville in year of establishment ranged (p < 0.05) from 57 to 753 kg ha −1 yr −1 . October seed yield of accession 648 increased (p < 0.05) 305% with 75 kg N ha −1 compared to the no-N treatment at Beeville. Seed production peaked at 328 kg ha −1 for accession 648 and 352 kg ha −1 for accession 715 in spring 2006 at Beeville. Infl orescence density was positively correlated to seed yield (from 81 to 93%). Optimum bristlegrass N fertilizer management varied with accession, location, and year.