Rumen fermentation and retention time of the digests in growing cattle of the breeds Black-White Dairy Cattle, Galloway, and Highland

The objectives of this study were to describe ruminal fermentation, mean re tention time (MRT) of the particulate digesta in the whole gastrointestinal tract and the apparent digestibility of nutrients in growing cattle of the genotypes Black-White Dairy Cattle (B), Galloway (G), and Highland (H). Tw o experiments were carried out in bulls aged 11-12 months (mean weight 260 kg) or 9-10 months (mean weight 210 kg) at the beginning and fed diets vary ing in the nutrient composition and nutrition level. B bulls had a higher r umenoreticular volume, a higher MRT when fed a low nutrition level, a highe r ruminal pH and a lower acetate/propionate ratio in the rumen than G bulls (P<0.05). In comparison with H bulls, rumen pH and MRT were higher (P<0.05 ) and the acetate/propionate proportion, NH3 level, and the protozoa number in the rumen were smaller in B bulls. The differences between genotypes in apparent digestibility of organic matter (OM) and crude cellulose were sma ll. In some cases digestibility was significantly (P<0.05) higher in B bull s as compared to G and H bulls. In B bulls, the digestibility of OM at 30 < degrees>C environmental temperature was 2 units lower than at 3 degreesC or 18 degreesC (P<0.05). The results show that B bulls implement a more effic ient ruminal digestive process than do G or H bulls.