The effect of level and type of supplement offered to grazing dairy cows on herbage intake, animal performance and rumen fermentation characteristics

A study was undertaken to examine the effect of level and type of supplement and changes in the chemical composition of herbage through the grazing season on herbage intake, animal performance and rumen characteristics with high-yielding dairy cows. Thirty-two high genetic merit dairy cows were allocated to one of four treatments in a two-phase change-over design experiment. The four treatments involved offering either high-fibre (HF) or high-starch (HS) supplements at either 5 or 10 kg dry matter (DM) per day. Animals rotationally grazed perennial ryegrass swards offered a daily herbage allowance of 23 kg DM. High levels of animal performance were maintained throughout the study with mean milk yields over the 21-week experimental period of 35.5 and 37.2 kg per cow per day on the 5- and 10-kg treatments respectively. Supplement type had no significant effect on herbage intake. Increasing the level of supplementation reduced herbage intake by 0.49 and 0.46 kg herbage DM per kg supplement DM and increased milk yield, with a milk yield response of 0.55 and 0.65 kg/kg supplement DM offered in phases 1 and 2. Supplement type had no significant effect on milk yield and milk butterfat content. However, in both phases 1 and 2, milk protein content was significantly higher with cows offered the HS supplement compared with the HF supplement, this being particularly evident at the higher level of supplementation. Supplement type also had marked effects on milk protein composition. These results indicate that high levels of performance can be achieved with dairy cows on grazed pasture with moderate levels of supplementation.