Summary. Three experiments investigating the effects of herbage allowance on the consumption of nutrients by lactating cows were conducted on irrigated perennial pastures in northern Victoria during summer and autumn. Experiment 1 was conducted in mid lactation (autumn–early winter) with perennial ryegrass [54% of dry matter (DM)]–white clover (22% of DM) pasture offered at allowances of 15, 20, 30 and 40 kg DM/cow.day. Herbage intake increased ( P <0.001) from 8.0 to 14.6 kg DM/cow.day as allowance increased and this was associated with a decrease ( P <0.001) in utilisation from 54 to 37%. The cows consistently selected a diet 11% higher in in vitro dry matter digestibility than that in the pasture on offer, but selection for crude protein increased ( P <0.001) from 21 to 41% above that in herbage on offer as herbage allowance increased. Neutral detergent fibre content of the diet selected was lower ( P <0.001) than that in herbage on offer. Along with these changes, milk production increased ( P <0.001) from 9.0 to 15.5 kg/day as herbage allowance increased at a marginal response of 0.99 kg milk/kg extra DM consumed. Experiments 2 and 3 were conducted in mid lactation (summer) on pasture containing 28% paspalum, 26% weeds, 17% perennial ryegrass or 36% paspalum, 19% weeds and 24% ryegrass respectively. Pasture allowances were between 20 and 70 kg DM/cow.day. Herbage intake increased ( P <0.001) from about 8 to 17 kg DM/cow.day as allowance increased in both experiments and was accompanied by a decrease ( P <0.001) in utilisation from about 40 to less than 25%. In experiment 2, the cows consistently selected a diet with a similar in vitro dry matter digestibility to that of the herbage pregrazing, regardless of allowance, but selection for crude 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% greater ( P <0.001) in crude protein than the herbage on offer. Neutral detergent fibre content of the diet selected was lower ( P <0.001) than that 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, 10.0–15.8 kg/cow.day) as herbage allowance increased, but the marginal responses 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 experiment 1 reflecting the differences in pasture quality.
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Wales et al. (1998) studied this question.