Protein nutrition of growing lambs

1. The effects of supplementation of a cellulose-based diet with either urea, casein or formaldehydetreated (HCHO)-casein were studied in growing lambs. Responses were measured in terms of growth rate, food intake and food conversion ratio.2. In Expt 1 lambs were given free access to a basal diet containing (g/kg) oat-hulls (700) and solka floc (300) (containing 5 g nitrogen/kg dry matter (DM)) supplemented (g/kg basal diet) with either urea (25), untreated casein (75), HCHO-casein (75) or combinations of these. Food intake was increased on average by 27% above that on the basal diet by the addition of either urea, casein, HCHO-casein plus urea. Urea plus HCHO-casein when given as a combined supplement further increased food intake on average by 60% above that on the basal diet. Supplements of either urea, casein, HCHO-casein or casein or casein plus urea changed a mean live-weight loss of 40 g/d on the basal diet to a mean live-weight gain of 56 g/d. Urea plus HCHO-casein further increased lamb growth to 112 g/d.3. In Expt 2 lambs were given free access to the basal diet (plus 25 g urea/kg diet) used in Expt I. In this experiment the content of insoluble and soluble casein in the diets was varied by the addition of HCHO-casein and untreated casein of 0, 150; 50, 100; 100, 50 and 150, o g/kg basal diet respectively. Maximum lamb growth (141 g/d) was obtained with a supplement of 25 g urea plus 100 g HCHO-casein and 50 g casein/kg.4. The growth responses to these supplements suggest a requirement for soluble N by the rumen microorganisms to maximize rumen fermentation, and for maximum growth rate on this diet a further requirement for amino acids produced by protein which has escaped degradation in the rumen.5. Fermentation and the absorption of nutrients were examined in Expt 3 in lambs fitted with simple ‘T’-shaped cannula in the duodenum and ileum, and fed ad lib. one of the diets: a basal diet of oat hulls and solka floc, or the basal diet supplemented (g/kg) with either urea (25), urea plus casein (150), or urea (25) plus HCHO-casein (150). The rates of production of volatile fatty acids (VFA), methane and microbial cells were measured using isotope-dilution techniques. The apparent absorption of nutrients was determined by differences in the quantity of those nutrients in digesta at the duodenum and ileum.6. Supplements of urea, urea plus casein and urea plus HCHO-casein increased organic matter (OM) intake in lambs by 65% above that on the basal diet. OM digestibility was unchanged by the from of nitrogen supplementation. The rates of production of all fermentation end-products varied directly with voluntary food intake.7. Rumen methane production remained constant at 0.09 mol methane/MJ metabolizable energy (ME) intake on all diets, which represented an 11% loss of digestible energy (DE). Hindgut methane production was highest on the urea-supplemented diet.8. The rate of VFA production (mol/MJ ME intake) in the rumen was highest on the diet supplemented with urea in comparison with the basal, urea plus casein and urea plus HCHO-casein diets (which were not significantly different). The molar proportions of the individual VFA in rumen fluid were not significantly different between diets except for the branched chain and higher fatty acids which were highest in proportion with the urea plus casein diet.9. The loss of energy in the faeces, urine or as methane in expired air was not influenced by the form of N supplementation. DE and ME were greater on the supplemented diets, as a result of the increased OM intake of these diets.10. There was no effect of the form of N supplementation on OM digested in the rumen, small intestine or large intestine. Of an increase in OM intake, apparently 55% was digested in the rumen (of which 19% was incorporated into rumen micro-organisms) and 26% disappeared in the small intestines. The apparent digestibility of OM for all diets was 0.67.