THE EFFECTS OF CONCENTRATE ENERGY-SOURCE AND PROTEIN-CONTENT ON MILK-PRODUCTION IN COWS GIVEN GRASS-SILAGE AD-LIBITUM

Sixteen Friesian cows were given four dietary treatments in a 4 x 4 La tin square experiment with a 2 x 2 factorial arrangement of treatments . The diets consisted of grass silage ad libitum plus 2 kg of hay per day and two types of concentrates of either barley or a mixture of bar ley, oats and fibrous by-products [200, 200 and 600 g per kg dry matte r (DM) respectively], with two protein contents. For the low-protein d iets, barley- (B) and fibre-based (F) concentrates were given without protein supplements, while for high-protein diets 1 kg of both concent rates was replaced with fish-meal (FM). The concentrates were given at the rate of 9 kg d-1 for the cows (n = 12) and 8 kg d-1 for the heife rs (n = 4). The cows given the F diets tended (P < 0.10) to have a gre ater silage dry matter intake and produced 1.5 kg d-1 more (P < 0.05) milk with a lower (P < 0.05) protein content than those given the B di ets. Increasing dietary crude protein concentration with FM had no eff ect on feed intake but resulted in significant increases in milk yield (P < 0.01), milk protein content (P < 0.05) and yields of milk consti tuents. The response in milk yield to FM tended to be greater with bar ley than with fibrous supplement (+ 2.5 vs + 1.5 kg d-1). Compared wit h B diets. the greatest relative increase occurred in lactose yield (0 .07) when the corresponding F diets were fed, while FM produced the gr eatest response in protein yield (0.12). The calculation of the utiliz ation of metabolizable energy (ME) for milk production showed that bot h the feeding of a concentrate consisting of different carbohydrate so urces and inclusion of fish meal improved the utilization of ME, the e ffects being partially additive. It is concluded that the nutrient sup ply to the cow's tissues can be modified by the source of carbohydrate and protein supplementation as indicated by different responses in th e yield of milk constituents. The production response to protein suppl ementation may depend on the source of carbohydrate in the concentrate .