Effect of the substitution of urea for soybean meal on digestion in steers

Four growing Holstein steers (305 +/- 33 kg body weight) with ruminal and d uodenal cannulae were used in a 4 x 4 Latin square experiment to assess the effect of level of dietary urea (0, 7.3, 14.7 and 22.0 g kg(-1) of dry mat ter) on the site and extent of digestion of some feed components. The diets were isonitrogenous and consisted of 60% ryegrass hay and 40% concentrate and were offered to the animals three times daily in amounts limited to 24 g of dry matter kg(-1) liveweight in order to avoid feed refusals. The addi tion of urea caused a linear increase in ruminal pH and in ruminal ammonia concentration during the first hours after feed ingestion (P < 0.01), and l inearly increased ruminal digestibility of cellulose (P < 0.01) and apparen t nitrogen (P < 0.05). However, it had no significant effect on apparent to tal digestibility of feed components, nor did it affect ruminal digestibili ty of dry matter, organic matter or starch. Flow of microbial nitrogen to t he small intestine decreased linearly (P < 0.05), but the efficiency of mic robial protein synthesis or the proportion of undegraded feed nitrogen in t he rumen that flowed to the small intestine was not significantly affected (P > 0.05). The increase in ruminal ammonia concentration above that necess ary for optimal microbial growth increased ruminal nitrogen losses into the blood. However, excess ruminal ammonia concentration also increased rumina l fibre digestion.