Vegetation dynamics in nitrogen-fertilized versus mixed legume-smooth bromegrass pastures interseeded with sorghum-sudangrass

Interseeding annual warm-season grasses may increase forage and pasture productivity. From 2017 to 2019, we conducted a grazing experiment that examined smooth bromegrass (Bromus inermis Leyss.) pasture responses to interseeding sorghum-sudangrass [Sorghum bicolor (L.) Moench x S. bicolor (L.) Moench ssp. drummondii (Nees ex Steud.) de Wet & Harlan]. The experiment had a split-plot design with six main plots each containing two, 0.60-ha pasture subplots (i.e., 12 total subplots). The six main plots included three replications each of two pasture types: (A) nitrogen (N)-fertilized smooth bromegrass (FERT) and (B) mixed legume-smooth bromegrass [LEG; alfalfa (Medicago sativa L.), red clover (Trifolium repens L.), and birdsfoot trefoil (Lotus corniculatus L.)]. The subplots included sorghum-sudangrass interseeded and unseeded control treatments. Each pasture subplot, which supported two steers (Bos taurus) from spring through summer, contained six, 0.10-ha sub-subplots for rotational stocking. Sorghum-sudangrass was interseeded after stocking cycle 2 in two of the sub-subplots in the interseeded subplot each year. Vegetation responses were evaluated in these sub-subplots and in two adjacent sub-subplots in the unseeded control subplot. Across years, sampling dates, and subplot treatments, the LEG main plots supported 7% greater forage biomass (2743 +/- 102 kg ha(-1)) and six more grazing days than the FERT main plots (2552 +/- 111 kg ha(-1)). Sorghum-sudangrass interseeded sub-subplots, however, had 36% greater forage biomass than unseeded sub-subplots pre-grazing in stocking cycle 5. To optimize forage biomass, producers should fertilize in early spring or maintain legumes in smooth bromegrass pastures. Interseeding sorghum-sudangrass provides an option to increase late-summer forage biomass.