Optimization of rate and efficiency of dietary nitrogen utilization through the use of animal by-products and(or) urea and their effects on nutrient digestion in Holstein steers

The objective of this N balance study was to determine the potential for im proving the efficiency and rate of dietary N utilization in Holstein steers by feeding an amino acid-balanced mixture of animal by-product protein sou rces in combination with urea. The Beef NRC 1996 Model Level 2 was used to formulate a corn-based (86:14 concentrate-hay) control diet with soybean me al as the primary N supplement that would provide ME and metabolizable prot ein (MP) allowable ADG of 1.4 kg in 250-kg steers with an estrogenic implan t and fed an ionophore. A combination of porcine meat and bone meal, fish m eal, hydrolyzed feather meal, and blood meal was also formulated as an unde gradable intake protein (UIP) blend to complement those amino acids (AA) de rived from microbial protein synthesis. Four steers with an average initial BW of 259 kg were assigned in a 4 x 4 Latin square design to treatments co nsisting of control, two levels of UIP inclusion (2.6 and 5.2%; DM basis) i n combination with urea, and a negative control "urea diet" containing no U IP and no SBM. The steers were fed at hourly intervals 95% of ad libitum in take and were injected with 500 mug of estradiol-17 beta twice daily. Nitro gen intakes were 155, 160, 162, and 145 g/d, and N balances were 47, 51, 42 , and 47 g/d when the 0, 2.6, 5.2% UIP and the urea diets were fed, respect ively. Nitrogen balance was reduced with the 5.2% UIP diet (P < 0.05), and was less than the capacity estimate derived from abosmasal casein infusion studies. Apparent N digestibilities averaged 69%, but DM, OM, and nonstruct ural carbohydrate digestibilities were significantly reduced for the urea d iet. Feeding 5.2% UIP in the diet reduced (P < 0.05) the biological value f rom 46 to 38%, which was accompanied by a significant elevation of plasma u rea N. Results indicate that genetic capacity for N retention was approxima tely 51 g/d. Results demonstrate that use of an AA-balanced blend of animal by-product protein sources did not improve the efficiency of dietary N usa ge when added to corn-based diets formulated with the Beef NRC 1996 Model L evel 2 to meet nutrient requirements of rapidly growing steers. Using urea as the only N supplement achieved equal rate and efficiency of N use.