| Summary: | Interseeding annual warm-season grasses into perennial cool-season grasses has the potential to increase summer forage mass and nutritive value. Knowledge of how seeding rate affects annual warm-season grass establishment, forage mass, and vegetation dynamics remains limited. From 2016–2017, we conducted a field experiment evaluating the effects of seeding rates on sorghum-sudangrass (<i>Sorghum bicolor</i> × <i>S. bicolor</i> var. <i>sudanense</i>) density and forage mass and on the frequency of occurrence of plant species in cool-season grass sod in Lincoln, NE. The experiment had a completely randomized design consisting of six replicates of four seeding rates [0, 14, 28, and 35 kg pure live seed (PLS) ha<sup>−1</sup>] in sod mowed at a 2.5-cm height and one unseeded, non-mowed control treatment. Sorghum-sudangrass establishment increased with seeding rate from an average of 20 to 45 plants m<sup>−2</sup> as the seeding rate increased from 14 to 35 kg PLS ha<sup>−1</sup>. Forage mass depended on a seeding rate × harvest interaction, showing positive linear and cubic responses to seeding rate in consecutive harvests at 45 and 90 d after interseeding. To increase forage mass in perennial cool-season grass sod, producers should interseed sorghum-sudangrass with at least 28 kg PLS ha<sup>−1</sup>. One-time seedings into cool-season, perennial grass sod have no residual effects on subsequent forage mass and vegetation dynamics.
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