In situ degradation of fresh and frozen-thawed 15N-labelled alfalfa and ryegrass

Alfalfa and perennial ryegrass were labeled with 15N during growth in a glasshouse, harested at s milar growth phases and samples (either fresh (F) or frozen and thawed (FT)) were incubated in situ in the rumen. The results were fitted to a model describing the degradation of DM and total N with time. There c as no difference between forage species for the instantly soluble fraction (a) and the insoluble but potentially degradable fraction (b) estimates for DM and N disappearance over time but alfalfa 15N value. for a and b were significantly (P<0.05) higher and lower, respectively. than for ryegrass. Forage species had significant (P<0.05) effect on the degradation rate of b (c) and potential degradability on DM, N and 15N. however, alfalfa had higher degradation rates of b for DM, N and 15N than ryegrass. Samples prepared as FT had higher a estimates than for F preparations, while estimates for b were higher for F than FT in both DM and N. There was no significant effect due to sample preparation in est mates of 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 significantly (P<0.05) higher for FT than F for DM and N but not for 15N. There were significant (P<0.05) interactions between forage species and sample preparation method for DM degradation rase and potential de gradability because FT increased the rate of degradation and potential degradability in altalla but decreased them in ryegrass. A significant (P<0.05) interaction was also found between lOtage species and sample preparation method for quickly degradable protein. Because of the differences in prediction of DM and N degradation parameters among sample preparation methods, it s recommended that for in situ degradation determination utilize the same sample preparation method throughout.