The effect of level of feeding on the response of lactating ewes to dietary supplements of fish meal

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Abstract
Thirty-six individually-penned ewes (mean live weight 69 kg), each suckling two lambs, were given one of three diets containing either 128 (low), 155 (medium) or 186 (high) g crude protein (CP) per kg dry matter. All diets contained (g/kg), milled hay, 570; molasses, 95; and a barley/fish meal concentrate, 330. The three protein concentrations were achieved by adjusting the proportions of barley and fish meal in the concentrate. Each diet was given at daily metabolizable energy (ME) intakes of 19, 23 and 27 MJ. Mean daily yields of milk in weeks 3 to 8 of lactation for ewes given the diet with the low concentration of crude protein increased from 2·32 kg at 19 MJ ME to 2·53 kg at 27 MJ. Corresponding values for the medium concentration of CP were 2·49 and 2·67 kg and for the high concentration 2·52 and 3·09 kg (P < 0·05 for differences between ME intakes and differences between dietary protein concentrations). For milk composition, interactions between the concentration of dietary protein and level of ME intake were not statistically significant but the main treatment effects were significant, with the protein concentration in milk increasing from 49·6 g/kg for ewes given the low concentration of dietary protein to 54·1 g/kg for those given the high (P < 0·001). Corresponding values for protein concentration in milk for the lowest and highest energy intake were 51·2 and 53·4 g/kg (P < 0·05). Losses of tissue protein were variable but decreased from 26 g/day for ewes given the low-protein diet to 8 g/day for those given the high. In discussing the responses in milk yield to dietary protein and ME intake attention is drawn to the modifying influence of the energy contributed from body tissue.