Rumen digesta kinetics in cold exposed prolific sheep: impact on protein evaluation

In vivo measurements of the changes over time in rumen degraded and undegraded protein fractions were used to study effects of midwinter shearing of prolific ewes on effective rumen degradability of dietary protein (EPD). Values were compared to EPD estimated with the standard method of in situ feed characteristics and an assumed rumen passage rate. The effect of the experimental treatments (unshorn and shorn) was investigated in ewes during three years (n=11). Rumens of the sheep were emptied and sampled 1, 2, 4, 8 and 12 h after feeding to monitor rumen contents of dry matter (DM), crude protein-free organic matter (OM-CP) and nitrogen (N). The degraded and undegraded fractions of DM, OM-CP and N in rumen contents were measured by in situ rumen incubation of rumen content samples. Protein evaluation based on EPD estimated from changes in rumen digesta volume with time had more variation and 10% more digestible protein in the small intestine (DVE) than the commonly used EPD. Midwinter shearing of crossbred ewes increased the extent of degradation of DM and OM-CP in the rumen. The extent of degradation of protein tended to be higher in shorn ewes. These differences did not result in different DVE yields with an assumption of an unchanged efficiency of microbial protein growth during cold exposure. While the commonly used EPD is more suitable for routine techniques than EPD based on changes in rumen digesta volume over time, the commonly used EPD may result in an underestimation of the DVE value of feeds for prolific ewes. Protein evaluation should take account of differences in efficiency of microbial production between unshorn and shorn ewes to avoid underestimation of the DVE value of feeds.