Urea and meat meal in the diets of ruminant calves - the sites of digestion and the nitrogen requirements for microbial protein synthesis

Abstract

Nine Friesian calves between 11 and 30 weeks of age were fed on diets based on barley supplemented with 0, 1.75 or 3.5 % urea or with meat meal. In a further experiment a purified diet was supplemented with 3-5.25 % urea and 20-50% wheat straw. The replacement of meat meal by urea reduced the flow of nitrogen to the duodenum and the apparent absorption of amino acid in the intestines. It was calculated that diets containing no meat meal would be limiting the growth of calves by a deficiency of sulfur amino acids and possibly threonine. With purified diets it was also calculated that the sulfur amino acids would be the first limiting amino acids for growth. The apparent digestion of amino acids in the small intestine varied from 66 to 76% for diets containing at least 1.8% nitrogen. The apparent digestion of essential amino acids was greater than that of non-essential amino acids. A nitrogen content of 1.8 % as urea in a purified diet was sufficient to maximize the efficiency of microbial protein synthesis in the rumen to 23-24 g nitrogen per kg of dry matter fermented. This is equivalent to 1.7 g nitrogen per MJ of metabolizable energy in the diet and corresponds to a concentration of ammonia in the rumen of 120 mg/l. The dietary nitrogen content required in barley and urea diets for maximum microbial protein synthesis was greater than with purified diets owing to the incomplete breakdown of the nitrogen in barley in the rumen. The efficiency of microbial protein synthesis in the rumen was similar for diets containing 8.3-11.0 MJ of metabolizable energy per kg of diet. The apparent digestion of a-linked glucose polymers in the rumen was lower for diets containing 1.3 % nitrogen than for those containing at least 1.8 %. As the metabolizable energy content of diets increased, there was a decrease in the apparent digestion of a-glucose polymers in the rumen.