The digestion of barley-based diets by cattle: the effect of feeding of three diets containing different amounts of formaldehyde-treated soybean meal

Abstract

Six growing cattle, with intestinal fistulae, were fed on barley-based diets to which were added increments of formaldehyde-treated soybean meal; the latter was substituted for whole-grain barley. The digestion of organic matter (OM), �-linked glucose polymer (starch), nitrogen (N) and rumen volatile fatty acids (VFA) were measured at a restricted level of intake. The cattle were offered air-dried feed at 3.4 kg per head daily, or approximately ad libitum (i.e. 4.2 kg air-dried feed per head per day). The three diets contained 19.7, 27.8 or 42.9 g nitrogen (N) per kg and were fed in a Latin-square design. When the feed was offered ad libitum there was a significant increase in OM digestibility, from 78% on the lowest N diet to 84% on the highest N diet. Relatively large quantities of starch (140-477 g/day) entered and were apparently digested in the small intestine, especially on the 19.7 g N/kg dry matter (the highest starch diet) on ad libitum feeding. The molar proportions of VFA in rumen liquor largely reflected the amount of starch which was fermented in the rumen. The digestion of nitrogen was characterized by a net gain in flow to the duodenum compared with intake. There was a significant increase in the amount of N apparently digested in the small intestines on the 42.9 g N/kg dry matter diet. The proportion of microbial N at the duodenum declined as more formaldehyde-treated soybean meal was added to the diet. Compared with that of the lowest N diet, the amount of amino acids entering and being apparently digested in the small intestines doubled in the highest N diet, and the concentrations of plasma-free amino acids reflected this. The feed intake comparison indicated that differences in interpretations of digestion studies were likely if animals were fed solely at restricted intakes. The main discrepancies were in OM digestibility, starch digestion in the small intestine, the proportion of microbial N flow at the duodenum and in the estimation of amino acid absorption and requirement.