Effects of the correction of particle microbial contamination and particle transit model in the rumen on in situ protein evaluation of grass hays

Effects of considering the particle comminution rate (k(c)) in addition to particle rumen outflow (k(p)) and the ruminal microbial contamination on estimates of by-pass and intestinal digestibility of DM, organic matter and crude protein were examined in perennial ryegrass and oat hays. By-pass k(c)-k(p)-based values of amino acids were also determined. This study was performed using particle transit, in situ and N-15 techniques on three rumen and duodenum-cannulated wethers. The above estimates were determined using composite samples from rumen-incubated residues representative of feed by-pass. Considering the comminution rate, k(c), modified the contribution of the incubated residues to these samples in both hays and revealed a higher microbial contamination, consistently in oat hay and only as a tendency for crude protein in ryegrass hay. Not considering k(c) or rumen microbial contamination overvalued by-pass and intestinal digestibility in both hays. Therefore, non-microbial-corrected k(p)-based values of intestinal digested crude protein were overestimated as compared with corrected and k(c)-k(p)-based values in ryegrass hay (17.4 vs 4.40%) and in oat hay (5.73 vs 0.19%). Both factors should be considered to obtain accurate in situ estimates in grasses, as the protein value of grasses is very conditioned by the microbial synthesis derived from their ruminal fermentation. Consistent overvaluations of amino acid by-pass due to not correcting microbial contamination were detected in both hays, with large variable errors among amino acids. A similar degradation pattern of amino acids was recorded in both hays. Cysteine, methionine, leucine and valine were the most degradation-resistant amino acids.