EFFECT OF FORAGE CONSERVATION (HAY OR SILAGE) ON CHEMICAL-COMPOSITIONOF MILK

Forty-two lactating multiparous cows were used in a changeover design with two periods of 4 weeks. During each period cows were offered an i socaloric 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 t he same day (7 June) in the same field of native mountain grassland. E ach diet was supplemented with the same quantity of a mixture of barle y and soya bean meal in proportions calculated to keep the overall rat ions 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, thes e results are linked only to a dilution effect of milk solids secreted . The casein/protein ratio and the soluble protein concentration of th e milk were not significantly different between the two types of forag e. The calcium concentration was lower, and the phosphorus concentrati on higher when cows were fed hay.