In situ dry matter and crude protein degradation of fresh forages during the spring growth

We compared ruminal dry matter (DM) and crude protein (CP) degradation kine tics of fresh forages of alfalfa (vegetative, early bud, early flowering, a nd late flowering stages) and bromegrass with endophyte-free and endophyte- infected tall fescue (tillering, stem elongation, heading,and flowering sta ges) by using nonlinear models. Duplicate Dacron bags were incubated far 0, 3, 6, 9, 12, 16, 24, 36, 48, and 60 h in 2 Simmental xAngus steers fitted with ruminal cannulas. The effects of animal, forage species, and maturity within forage species were evaluated. A first-order kinetics model was dete rmined to be suitable for estimation of degradation profiles relative to mo dels with variable rate of degradations. Alfalfa had higher soluble DM (36. 6%), lower insoluble potentially digestible DM (43.0%), higher rate (13.8%/ h), and higher extent of DM degradation (66.3%) than grasses (27.2, 53.5, 6 .7%/h, and 54.6% for soluble DM, insoluble potentially digestible DM, rate, and extent of DM degradation, respectively). The extent of CP degradation was similar among forages (74.7%), but alfalfa had a higher CP degradation rate than grasses (16.1 vs. 12.5%/h). Extents of degradation of DM and CP d ecreased with maturity in alfalfa and in grasses. Ruminally undegradable CP (RUP)was higher in alfalfa (40.0 g/kg of DM) than in grasses (34.9 g/kg of DM), and decreased with maturity in grasses (40.4 to 28.3 g/kg of DM) but not in alfalfa. The amount of RUP that was potentially degradable in the ru men was not different among forage species (22.1 g/kg). As forage CP concen tration decreased, the RUP (as a percentage of CP) increased but, as a perc entage of forage DM, decreased. Species of forage had important effects on ruminal DM and CP degradation when incubated in fresh form.