The nutritive characteristics of herbage consumed by grazing dairy cows affect milk yield responses obtained from concentrate supplementation

Abstract

The results of 7 experiments, conducted during 1994–95 (year 1) and 1995–96 (year 2) at the Kyabram Dairy Centre in northern Victoria, were collated to test the hypothesis that nutritive characteristics of the herbage consumed by dairy cows will affect responses to concentrates. In 6 experiments, lactating Friesian cows grazing irrigated perennial pastures were either unsupplemented or were offered 5 kg DM/cow.day of a high energy concentrate (pellets comprised of 75% barley and 25% wheat); in the other experiment, cows received 0 and 3 kg DM/cow.day. Four experiments, each of 5 weeks duration, were completed during year 1, and 3 experiments of 4 weeks duration were conducted during year 2.

The mean metabolisable energy concentration of the herbage consumed in each experiment varied with season, being highest in spring (10.1–11.3 MJ/kg DM), and lowest from summer to early autumn (8.3–9.0 MJ/kg DM). It is suggested that milk responses attributed to feeding high energy supplements were influenced by characteristics of the herbage eaten in conjunction with the supplement. The highest marginal responses to concentrate supplementation occurred in summer and early autumn when pastures, which were dominated by paspalum and other poor quality species, were low in energy. Responses at that time (≥1.0 kg milk per kg DM of concentrates) were greater than in spring (≤0.6 kg milk/kg DM). A significant negative relationship existed between marginal returns of fat-corrected milk and metabolisable energy concentration of the herbage consumed (100R² = 80.6) and several published data sets provided support for this result. Possible reasons for this outcome, including variations in substitution between experiments, confounding with stage of lactation, variable partitioning of nutrients and imbalances in dietary crude protein and neutral detergent fibre, were examined. Of these explanations, fibre insufficiency in spring appeared the most probable. The possibility that the lower marginal returns in spring are associated with fibre insufficiency requires further investigation.