Effects of supplementation with fish meal or fish protein hydrolysate on growth, nutrient digestibility and rumen fermentation of growing cattle fed grass silage

Two experiments were conducted to determine the effects of supplementing grass silage (S) with isonitrogenous amounts of fish meal (FM; 380 g head−1 d−1) or fish protein hydrolysate (FPH; 300 g head−1 d−1) on growth, diet digestibility and rumen fermentation in beef cattle. In the first experiment, 36 crossbred steers were used in a 112 d growth trial to determine treatment effects on dry matter intake (DMI), average daily gains (ADG) and feed to gain ratio. Protein supplementation did not affect silage DMI, but increased ADG by 18% (P < 0.01) and feed to gain ratio by 6% (P < 0.08). The increase in ADG with FM was 12% higher than that obtained with FPH (P < 0.01). In a second experiment, six rumen fistulated heifers were used in a double 3 × 3 Latin square to study treatment effects on apparent digestibility and rumen fermentation. Protein supplementation increased apparent DM and N digestibility when compared to the S treatment (P < 0.04). Apparent ADF digestibility was 3.0% higher in heifers fed FM than in those fed FPH (P < 0.05). Rumen pH decreased immediately after feeding (P < 0.001), but was not affected by the treatments. Total volatile fatty acids (VFA) concentrations were not significantly affected by supplementation but by time; they were higher for the FM than for the FPH diet at all times (P < 0.02). Acetate propionate ratio was lower in heifers fed silage alone (P < 0.02). Rumen NH3N levels during the first 4 h after feeding were higher in animals receiving protein supplements (P < 0.001). From 0 to 2 h after feeding, rumen free α-amino-N increased almost twice as high in heifers fed FM than in those fed FPH (P < 0.05); however, at 4 and 6 h post feeding, the situation reversed. Plasma urea N was lower in animals fed only silage (P < 0.01). Total essential amino acids were higher in animals receiving protein supplementation than in those fed silage alone (P < 0.03) and tended to be higher in heifers fed FM than in those fed FPH (P < 0.08).