Ruminal degradation of switchgrass, big bluestem, and smooth bromegrass leaf proteins.

Two in situ protein disappearance experiments were conducted to determine disappearance rates of leaf protein fractions and characterize individual leaf protein fractions that escaped ruminal degradation. Fresh leaf blades of two warm-season grasses, switchgrass (Panicum virgatum L.) and big bluestem (Andropogon gerardii Vitman), and one cool-season grass, smooth bromegrass (Bromus inermis Leyss.), were included in Exp. 1. Only warm-season grasses were used in Exp. 2. Leaves were harvested from greenhouse-grown plants, placed in polyester bags, and incubated up to 48 h in situ in three ruminally fistulated steers fed diets of warm-season grass hay. The rate of protein disappearance for switchgrass (.037 h-1) was slower (P < .10) than that for big bluestem (.110 h-1). Big bluestem and smooth bromegrass (.169 h-1) disappeared at similar rates, whereas switchgrass disappeared more slowly (P < .05) than smooth bromegrass in Exp. 1. Rates of protein disappearance in Exp. 2 were similar for switchgrass (.112 h-1) and big bluestem (.116 h-1). Major protein fractions that resisted ruminal degradation in both experiments, detected using SDS-PAGE, were at approximate molecular weights of 56, 26, and 24 kDa. For switchgrass and big bluestem, total protein and individual protein fractions were generally at higher concentrations and present for longer periods of time than for smooth bromegrass. This suggests that a mechanism may exist in C4 species that allows certain protein fractions to remain undegraded for longer periods compared with smooth bromegrass.