RUMINAL DEGRADATION OF SWITCHGRASS, BIG BLUESTEM, AND SMOOTH BROMEGRASS LEAF PROTEINS

Two in situ protein disappearance experiments were conducted to determ ine disappearance rates of leaf protein fractions and characterize ind ividual 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-seaso n grass, smooth bromegrass (Bromus inermis Leyss.), were included in E xp. 1. Only warm-season grasses were used in Exp. 2. Leaves were harve sted from greenhouse-grown plants, placed in polyester bags, and incub ated up to 48 h in situ in three luminally fistulated steers fed diets of warm-season grass hay. The rate of protein disappearance for switc hgrass (.037 h(-1)) was slower (P < .10) than that for big bluestem (. 110 h(-1)). Big bluestem and smooth bromegrass (.169 h(-1)) disappeare d at similar rates, whereas switchgrass disappeared more slowly (P < . 05) than smooth bromegrass in Exp. 1. Rates of protein disappearance i n 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 mole cular weights of 56, 26, and 24 kDa. For switchgrass and big bluestem, total protein and individual protein fractions were generally at high er concentrations and present for longer periods of time than for smoo th bromegrass. This suggests that a mechanism may exist in C-4 species that allows certain protein fractions to remain undegraded for longer periods compared with smooth bromegrass.