Selection and Dietary Quality of Beef Cattle Grazing Smooth Bromegrass, Switchgrass, and Big Bluestem

The ability of grazing animals to enhance quality of diet by selection is important in production. The study determined the effects of selection on dietary quality of cattle grazing monocultures of smooth bromegrass (Bromus inermis Leyss; SB), switchgrass (Panicum virgatum L.; SG), and big bluestem (Andropogon gerardii Vitman; BB) as influenced by plant maturity. Three ruminally-fistulated steers (295 kg) strip-grazed SB, SG, and BB at vegetative, elongation, early reproductive, and a regrowth stage of development. Selection was maximized by providing cattle access to 40 kg of DM/d per head. Clipped samples were compared with dietary samples accumulated during 45 min grazing following total rumen evacuation. Dietary CP was enhanced 3 to 4% for SG and BB, and 8% for SB (P < 0.05). Dietary IVDMD was enhanced 1A contribution of the University of Nebraska Agricultural Research Division supported in part by funds provided through the Hatch Act. 2Corresponding author: brett_kirch@ars. usda.gov at elongation and reproductive stages for SG and BB and vegetative and reproductive stages for SB. Dietary NDF was 7 to 13% less (P < 0.05).in SG compared with forage-on-offer, whereas there was no effect with SB and BB diets. Diets of cattle grazing SG and BB had less ADF than clipped forage at elongation and reproductive stages, whereas ADF for the SB diet was less at the elongation phase (P < 0.05). Dietary lignin did not exceed 4% whereas the grass-on-offer was much greater. Regrowth produced forage and diets comparable to the elongation stage. If adequate forage is available, the selection ability of cattle can provide a superior diet compared with forage-on-offer. When the quality of warm-season grasses has declined, animal selection allows for potentially greater animal gain when grass quality is not optimum.