Dry matter intake and nutrient selection by lactating cows grazing irrigated pastures at different pasture allowances in summer and autumn

Abstract

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.