Comparison of the in situ degradation parameters of fresh and frozen-thawed 15N-labelled alfalfa and ryegrass

Abstract Degradation of dry matter (DM) and nitrogen (N) in fresh or frozen-thawed alfalfa and ryegrass was studied by using in situ technique. The forages were labeled with 15N during growth in a glasshouse, harvested at similar growth phase, and fresh (F) or frozen-thawed (FT) samples were incubated in the rumen of 3 sheep. There was no difference (P>0.05) between forage type (S) for the immediately soluble fraction (a) and the insoluble but slowly degradable fraction (b) estimates for DM and N disappearance over time. However, 15N values for a and b were significantly (P<0.05) higher and lower, respectively, for ryegrass. There was a significant effect of forage type on the degradation rate of b (c) and potential degradability of DM, N and 15N; degradation rate of b for DM, N and 15N were higher for alfalfa. There was no effect due to sample preparation (M) in degradation rate (c) for DM and 15N but FT had a significantly (P<0.05) higher estimate than F for N degradation rate. Potential degradability was higher for FT than F for DM and N but not for 15N. Mean estimates of effective rumen degradable protein due to forage species were significantly (P<0.05) different, while there was no difference due to the preparation method. The undegraded protein estimates for alfalfa and ryegrass were 66.8 and 52.5 g/kg DM, respectively. The corresponding values for F and FT were 66.2 and 53.1 g/kg DM, respectively. There were significant (P<0.05) interactions betwee