DRY-MATTER INTAKE AND NUTRIENT SELECTION BY LACTATING COWS GRAZING IRRIGATED PASTURES AT DIFFERENT PASTURE ALLOWANCES IN SUMMER AND AUTUMN

Three experiments investigating the effects of herbage allowance on th e consumption of nutrients by lactating cows were conducted on irrigat ed perennial pastures in northern Victoria during summer and autumn. E xperiment 1 was conducted in mid lactation (autumn-early winter) with perennial ryegrass [54% of dry matter (DM)]-white clover (22% of DM) p asture offered at allowances of 15, 20, 30 and 40 kg DM/cow.day. Herba ge intake increased (P<0.001) from 8.0 to 14.6 kg DM/cow.day as allowa nce increased and this was associated with a decrease (P<0.001) in uti lisation from 54 to 37%. The cows consistently selected a diet 11% hig her in in vitro dry matter digestibility than that in the pasture on o ffer, but selection for crude protein increased (P<0.001) from 21 to 4 1% above that in herbage on offer as herbage allowance increased. Neut ral detergent fibre content of the diet selected was lower (P<0.001) t han that in herbage on offer. Along with these changes, milk productio n increased (P<0.001) from 9.0 to 15.5 kg/day as herbage allowance inc reased at a marginal response of 0.99 kg milk/kg extra DM consumed. Ex periments 2 and 3 were conducted in mid lactation (summer) on pasture containing 28% paspalum, 26% weeds, 17% perennial ryegrass or 36% pasp alum, 19% weeds and 24% ryegrass respectively. Pasture allowances were between 20 and 70 kg DM/cow.day. Herbage intake increased (P<0.001) f rom about 8 to 17 kg DM/cow.day as allowance increased in both experim ents and was accompanied by a decrease (P<0.001) in utilisation from a bout 40 to less than 25%. In experiment 2, the cows consistently selec ted a diet with a similar in vitro dry matter digestibility to that of the herbage pregrazing, regardless of allowance, but selection for cr ude protein increased (P<0.05) from 25 to 45% above that in herbage on offer, as allowance increased. In experiment 3, the diet selected was 13% greater (P<0.001) in in vitro dry matter digestibility and 42% gr eater (P<0.001) in crude protein than the herbage on offer. Neutral de tergent fibre content of the diet selected was lower (P<0.001) than th at in herbage on offer in experiment 3, while the difference was small in experiment 2. Along with these changes, milk production increased (P<0.001) (in experiment 2, 12.3-15.0 kg/cow.day; experiment 3, 10.0-1 5.8 kg/cow.day) as herbage allowance increased, but the marginal respo nses were lower (0.28 kg milk/kg extra DM consumed in experiment 2, 0. 64 kg milk/kg extra DM consumed in experiment 3) than observed in expe riment 1 reflecting the differences in pasture quality.