Rumen fermentation and retention time ofthe digesta in growing cattle ofthe breeds Black-White Dairy Cattle, Galloway, and Highland

Abstract. The objectives of this study were to describe ruminal fermentation, mean retention time (MRT) ofthe 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). Two 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 varying in the nutrient composition and nutrition level. B bulls had a higher rumenoreticular volume, a higher MRT when fed a low nutrition level, a higher 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, NH,sub>3 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 small. In some cases digestibility was significantly (P<0.05) higher in B bulls as compared to G and H bulls. In B bulls, the digestibility of OM at 30 °C environmental temperature was 2 units lower than at 3 °C or 18 °C (P<0.05). The results show that B bulls implement a more efficient ruminal digestive process than do G or H bulls.