Nitrogen metabolism in sheep fed protein sources of various solubilities with low quality roughages.

Four rumen-cannulated, mature crossbred wethers were used in a 4 x 4 latin square experiment designed to examine the effects on protein metabolism of protein sources with various solubilities fed with low quality roughages. Four diets were used: (1) grass hay plus oat straw (control), (2) control plus urea, (3) control plus soybean oil meal (SBOM) and (4) control plus sunflower oil mean (SFOM). The diets were isocaloric, isofibrous and isonitrogenous (1.60% except for the control, which contained 1.09% N). The protein solubility means determined by three solvents (Burroughs' mineral mixture, McDougal's artificial saliva and sodium chloride) were 23.7, 50.5, 18.2 and 24.2% for the respective diets. The average protein solubilities of SBOM and SFOM were 20.5 and 35.4%, respectively. Apparent crude protein digestibilites were higher (P less than .05) for diets with higher protein solubility. The Urea diet (highest protein solubility) resulted in higher urinary N excretion (P less than .05) and lower N retention than the SBOM and SFOM diets. N retention was similar for the animals fed the SBOM and SFOM diets. Ruminal NH3-N and plasma urea N levels were higher (P less than .05) in animals fed the urea diet. The protein source altered VFA concentrations but not C2 to C3 ratios of VFA. The SBOM and SFOM produced more branched chain fatty acids. These data illustrate that protein sources of higher solubility result in higher crude protein digestion and higher N loss in the urine. N balance was similar with SFOM and SBOM. SFOM and SBOM can be used interchangeably on an equal protein basis for sheep fed high roughage diets.