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 initia l clover contents subjected to different grazing height management reg imes and their effect on lactation performance of 48 Friesian dairy co ws and heifers. Two paddocks were established for each treatment and g razed on alternate days. Treatments T17 and T13 consisted of swards wi th initial clover contents of 0.17 and 0.13 of the dry matter (DM) mas s, respectively, grazed to maintain compressed sward heights of 6 cm t hroughout the season. A third treatment, S15, consisted of a sward wit h an initial clover content of 0.15 grazed to maintain a compressed sw ard height of 4.5 cm for the first 78 days of the grazing season (peri od 1). Throughout period 1, half the animals on each treatment each re ceived 4 kg of a concentrate supplement daily, while the others remain ed unsupplemented. From days 79 to 90, the cattle on treatment S15 gra zed a similar sward, while the compressed sward height of the S15 padd ocks was allowed to increase to 6 cm before reintroduction of the anim als. 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 co ntent 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 conte nt to 0.05 and 0.07 of DM mass respectively. However, by the end of pe riod 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). O wing to the relatively small differences in clover content of swards T 17 and T13, there were no significant, effects of these two treatments on milk yield or composition in any period. Supplementation had no ef fect on milk composition and had little effect on milk yield, except w hen sward height was maintained at 4.5 cm. There was no carryover effe ct of supplementation on milk yield or composition in periods 2 or 3.