Summary. The effect of herbage allowance (20, 30, 40,
50, 60 and 70 kg DM/cow. day) on the consumption of nutrients from herbage
and milk production by cows in early lactation, was examined. The experiment
was conducted on rainfed perennial ryegrass pastures in September and October
1997 in south-eastern Victoria, Australia. The herbage on offer comprised
64% perennial ryegrass, 21% other grasses, 1% white
clover, 5% weeds and 9% dead material on a dry matter (DM)
basis. The average pregrazing herbage height was 13 cm, at an estimated
pregrazing herbage mass of 3.6 t DM/ha. The herbage on offer was of high
quality containing 11.6 MJ metabolisable energy/kg DM, 202 g crude
protein/kg DM and 525 g neutral detergent fibre/kg DM. Concentrations
of calcium, magnesium, sodium, potassium, phosphorus, sulfur and chloride were
4.4, 2.2, 4.4, 31.2, 3.5, 2.7 and 11.4 g/kg DM, respectively.
As daily herbage allowance per cow increased, dry matter intake increased
curvilinearly (P<0.01) from 11.2 to 18.7 kg
DM/cow. day. This was associated with a decrease in utilisation of herbage
from 54 to 26% and an increase in milk production from 25.9 to 29.1
kg/cow. day. The cows on all treatments grazed for less than 8.7
h/day. The increase in intake was achieved by an increase in the rate of
herbage intake from 1.5 to 2.2 kg DM/h for herbage allowances of 20 and 70
kg/cow.day, respectively.
Irrespective of herbage allowance, cows selected a diet that was approximately
10% higher in in vitro dry matter digestibility
and 30% higher in crude protein than that in the herbage on offer. The
neutral detergent fibre content of the herbage selected was lower
(P<0.05) than that on offer. The herbage consumed
contained more (P<0.05) magnesium, potassium and
sulfur, the same amount of calcium and phosphorus and less
(P<0.05) sodium and chloride than the herbage on
offer.
For rainfed perennial pastures in spring, herbage allowance is an important
factor in determining voluntary feed intake and production of dairy cows. To
achieve 30 L from herbage, without supplementation, high herbage allowances
are required. The increase in herbage intake, with increasing herbage
allowance, resulted from an increase in rate of dry matter intake and not an
increase in grazing time. No relationship was evident between herbage
allowance and the selection differentials for in vitro
dry matter digestibility, crude protein and neutral detergent fibre. Selection
differentials for rainfed perennial pastures in spring are similar to those
reported for irrigated perennial pastures in northern Victoria in spring and
autumn. When determining nutrient requirements it is important to consider the
interaction between herbage intake and nutrient concentration in the herbage.
Effects of Daily Herbage Allowance on Sward Structure, Herbage Intake and Milk Production by Dairy Cows Grazing a Pure Perennial Ryegrass Sward