Influences of Dietary Nitrogen Sources on Amino Acid Composition and its Availability of Rumen Microbial Fraction

Abstract

Studies were carried out to elucidate the effect of dietary nitrogen (N) source on the amino acid composition and the availability of rumen microbial fractions. Amino acid availabilities of microbial fractions were determined by the in vitro digestion method with pepsin and pancreatin. In Expt. 1, two mature fistulated Holstein cows were allocated in a reversal method to two purified diets containing urea (diet P-U) or isolated soy-protein (diet P-S) as a sole N source. An additional amount of rice straw was fed daily to cows throughout the experiment. No appreciable influence of dietary N source was observed on the bulk amino acid composition and amino acid availability for bacterial fraction, however, some differences were detected for protozoal fraction. When cows were fed diet P-U, the numbers of protozoa within the rumen were lower than when they consumed diet P-S, and it owed principally to the decrease in the numbers of entodiniomorphs. The protozoal fraction on diet P-U contained less lysine (P<0.05), had higher digestibility with pepsin and pancreatin (P<0.05), and released less lysine and isoleucine after the in vitro digestion (P<0.05). In Expt. 2, three mature Holstein cow with rumen fistula were used in a 3×3 Latin square experiment in which they were assigned to diet P-U and the other two conventional diets containing urea (diet C-U) or soybeen meal (diet C-S). Rice straw was given to all cows throughout the experiment. No significant difference was observed among diets in the numbers of protozoa, and also in the bulk amino acid composition and the availability of isolated protozoal fraction. In conclusion, the dietary N source would have little influence not only on the composition but also on the availability of amino acid in the rumen microbial protein.