Energy and protected protein supplements to lambs on endophyte-infected tall fescue pasture

The effect of supplements on intake, digestibility, N retention, ADG and blood and body composition of growing lambs fed cut herbage or grazing KY 31 tall fescue (Festuca arundinacea Schreb.) pastures at two levels of N fertilization (92 and 318 kg/ha) was determined. Supplements were corn (C), corn with soybean meal (U-SBM) and corn with heat-treated SBM (H-SBM). Metabolism trials were run at three growth stages in the 1st yr with 24 lambs. Although all supplements increased total DMI and DM digestibility, they decreased NDF digestibility relative to grass (G), with no difference between supplements; C depressed apparent CP digestibility. Nitrogen retention increased from -2.5 g/d on G to -.4 g/d on C and 3.2 and 4.1 g/d on U-SBM and H-SBM, respectively, for combined periods and N rates. Blood urea N (BUN) concentrations differed (P less than .01) in the following order: G greater than U-SBM greater than H-SBM greater than C. In the 2nd yr, lambs (n = 64) grazed fescue pastures at the same N fertilizer rates and were given the same supplements. Gains were not different (P less than .05) on low-N (LN) and high-N (HN) pastures. Seasonal ADG were 80, 115, 122 and 130 g/d for G, C, U-SBM and H-SBM treatments, respectively, with no difference (P less than .05) between U-SBM and H-SBM. At slaughter, lambs from G had lower dressing percentages (P less than .05) and fat content (P less than .01) than lambs on C, U-SBM and H-SBM treatments, with no differences between supplements. Results indicated a better performance of growing lambs on fescue with both energy and protein supplements. Response to protected vs unprotected protein was small.