Comparison of the in Situ Degradation Parameters of Fresh and Frozen-Thawed 15n-Labelled Alfalfa and Ryegrass

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 between s and m for dm degradation rate and potential of degradability, where ft increased the rate of degradation and potential degradability in alfalfa but decreased these in ryegrass. a significant (p < 0.05) interaction was also found between s and m for quickly degradable protein where the estimate for fresh alfalfa was ranked lowest while the ft alfalfa was ranked highest. because of differences in the prediction of degradation parameters for dm and n between preparation methods it is recommended that for in situ degradation determination the same preparation method be used throughout.