RELATIONSHIPS BETWEEN IN-SITU PROTEIN DEGRADABILITY AND GRASS DEVELOPMENTAL MORPHOLOGY

The objective of this research was to determine the relationships betw een the morphological development and in situ ruminally degradable pro tein (RDP), ruminally undegradable protein (RUP), and microbial protei n of two cool season grasses (intermediate wheatgrass and smooth brome grass) and two warm season grasses (switchgrass and big bluestem). The initial growth of grass tillers grown near Mead, Nebraska was clipped at ground level six times during the 1992 growing season and morpholo gically classified. Mean stage was calculated. Forage was ground to pa ss a 2-mm screen and was incubated in ruminally fistulated steers for 16 h. The RUP was adjusted for microbial protein and acid detergent in soluble N. The mean stage of cool season grasses was higher than that of warm season grasses throughout the growing season. The RDP decrease d as plant maturity increased for all species. The RUP expressed as a percentage of crude protein for the cool season grasses was lower than that for warm season grasses. The RUP for intermediate wheatgrass, sm ooth bromegrass, and switchgrass remained constant across maturities, but RUP for big bluestem decreased as maturity increased. Microbial au gmentation of RUP decreased as crude protein decreased in all species. The RUP corrected for acid detergent insoluble N and microbial protei n was relatively constant across plant maturities. The quantification of RUP across a range of plant maturities provided information for inc orporating RUP content of forage grasses into the diets of animals.