Effect of forage conservation (hay or silage) on chemical composition of milk

Abstract

Forty-two lactating multiparous cows were used in a changeover design with two periods of 4 weeks. During each period cows were offered an isocaloric diet composed either of grass silage (12 kg dry matter/day) or hay (12.5 kg dry matter/day). Hay and grass silage were harvested the same day (7 June) in the same field of native mountain grassland. Each diet was supplemented with the same quantity of a mixture of barley and soya bean meal in proportions calculated to keep the overall rations isonitrogenous. The cows fed grass silage yielded 0.7 kg/day (P < 0.05) more milk than those fed hay, but their milk was significantly poorer in fat (-1.2 g/kg), proteins (-1.0 g/kg) and caseins. As energy balance and live weight changes were similar with the two diets, these results are linked only to a dilution effect of milk solids secreted. The casein/protein ratio and the soluble protein concentration of the milk were not significantly different between the two types of forage. The calcium concentration was lower, and the phosphorus concentration higher when cows were fed hay.