Lactation performance of spring-calving dairy cows grazing mixed perennial ryegrass/white clover swards of differing composition and height

An experiment was designed to examine the changes in clover content of three mixed perennial ryegrass/white cover swards of differing initial clover contents subjected to different grazing height management regimes and their effect on lactation performance of 48 Friesian dairy cows and heifers. Two paddocks were established for each treatment and grazed on alternate days. Treatments T17 and T13 consisted of swards with initial clover contents of 0.17 and 0.13 of the dry matter (DM) mass, respectively, grazed to maintain compressed sward heights of 6 cm throughout the season. A third treatment, S15, consisted of a sward with an initial clover content of 0.15 grazed to maintain a compressed sward height of 4.5 cm for the first 78 days of the grazing season (period 1). Throughout period 1, half the animals on each treatment each received 4 kg of a concentrate supplement daily, while the others remained unsupplemented. From days 79 to 90, the cattle on treatment S15 grazed a similar sward, while the compressed sward height of the S15 paddocks was allowed to increase to 6 cm before reintroduction of the animals. The three swards were then grazed for a further 47 days (period 2) before the animals were housed and milk yield recorded for a further 63 days (period 3). While sward T17 showed little change in clover content over the first 29 days of grazing, remaining at just below 0.18 of DM mass, swards T13 and S15 showed a marked decline in clover content to 0.05 and 0.07 of DM mass respectively. However, by the end of period 1 the clover content of all three swards had increased markedly (0.25, 0.15 and 0.15 of DM mass respectively). By the end of Period 2, dover proportions were slightly higher than initial values (0.19, 0.15 and 0.15 of DM mass for treatments T17, T13 and S15, respectively). Owing to the relatively small differences in clover content of swards T17 and T13, there were no significant, effects of these two treatments on milk yield or composition in any period. Supplementation had no effect on milk composition and had little effect on milk yield, except when sward height was maintained at 4.5 cm. There was no carryover effect of supplementation on milk yield or composition in periods 2 or 3.