EFFECT OF ACCESS TO IRRIGATED RYEGRASS ON THE MILK-YIELD AND COMPOSITION OF AUTUMN CALVING COWS IN A TROPICAL UPLAND ENVIRONMENT

The effect of the area of irrigated temperate grass pasture available to cows during autumn and spring on milk composition was measured usin g autumn-calving Holstein-Friesian cows. The irrigated pasture (Lolium multiflorum) was used in conjunction with a raingrown tropical pastur e (Panicum maximum, Setaria sphacelata cv. Narok and Neonotonia wighti i cv. Tinaroo) pasture at the levels of 0, 0.1 and 0.2 ha/cow. Mean ov erall stocking rate in each treatment was 1 ha/cow, and cows were conf ined to treatments from May 25-December 22. Differences in animal prod uction between treatments developed progressively during the experimen t. During winter (May 25-July 27), there were no differences in milk y ield, milk composition or liveweight gain. During late winter-early sp ring (July 28-September 29), milk yield and milk protein concentration for cows with 0, 0.1 and 0.2 ha/cow of irrigated ryegrass were 21.6L at 2.99%, 22.8L at 3.11% and 22.6L at 3.16%, respectively. Liveweight gain was also increased at 0.2 ha/cow of irrigated ryegrass pasture. D uring late spring (September 30-December 1), milk yield, and fat and p rotein concentrations were all affected by amount of irrigated pasture . Following storm rains in late November, responses in milk yield were inversely related to amount of irrigation, and differences in milk pr otein concentration persisted for at least 5 weeks following rainfall. The soluble carbohydrate concentration of the tropical grasses was si milar to levels in ryegrass (14% DM) for only 3 weeks following rainfa ll, after which it declined to 7% DM. Results show that irrigated temp erate grasses were effective in maintaining both protein and fat perce ntages during winter and spring.