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

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.