Effects of variations in herbage mass, allowance, and level of supplement on nutrient intake and milk production of dairy cows in spring and summer
Authors: Wales W. J. , Doyle P. T. , Stockdale C. R. , Dellow D. W. 
Published: September 2002

Two experiments examining the effects of herbage mass and herbage allowance on
the consumption of nutrients by lactating dairy cows were conducted on
irrigated perennial pasture swards in northern Victoria. Experiment 1 was
conducted in early lactation (spring) with a perennial ryegrass
(Lolium perenne L.)–white clover
(Trifolium repens L.) sward at herbage masses of 3.1
(low) or 4.9 (medium) t dry matter (DM)/ha and herbage allowances of about
20, 35, 50 and 70 kg DM/cow.day.
Within each herbage mass treatment, there were no significant differences
between herbage allowance treatments in nutritive characteristics of
pregrazing herbage. Daily DM intake increased linearly from 7.1 to 16.2 (low
mass) or 9.9 to 19.3 (medium mass) kg DM/cow, as herbage allowance
increased which was equivalent to 2.29 kg DM/t DM increase in herbage mass
and 0.18 kg DM/kg DM increase in herbage allowance. This was associated
with a decrease in utilisation of herbage from 35 to 23% and from 52 to
29%. Also, milk production increased linearly from 21.8 to 27.1 (low
mass) or 24.7 to 32.0 (medium mass) kg/cow.day as herbage allowance
increased.
Experiment 2 was conducted in mid lactation (summer) with a paspalum
(Paspalum dilatatum L.)-dominant sward at herbage masses
of 3.0 (low) or 4.7 (medium) t DM/ha and herbage allowances of about 25
and 45 kg DM/cow.day and either 0 or 5 kg DM concentrate/cow.day.
Within each herbage mass treatment, there were no significant differences
between herbage allowance treatments in nutritive characteristics of pregrazed
herbage.
Daily DM intake increased by 0.13 kg DM for every 1 kg DM increase in herbage
allowance. Over the 2 herbage masses, 2 herbage allowance treatments and 2
concentrate treatments, cows consistently selected a diet about 1.03 higher in
in vitro DM digestibility and 1.24 higher in crude
protein than that in the herbage on offer. Substitution rate increased from
0.20 to 0.42 (low mass) and from 0.34 to 0.44 (medium mass) kg DM reduction in
herbage intake/kg DM of concentrates consumed, with increasing herbage
allowance. Along with these changes, marginal returns to supplements decreased
from 1.38 to 0.95 (low mass) and 1.07 to 0.97 (medium mass) kg milk/kg DM
of concentrates with increasing herbage allowance.
Dairy farmers should consider the effects of herbage allowance and herbage
mass on intake, nutrient selection and milk production when allocating pasture
to dairy cows. Herbage mass and allowance had a greater effect on intake in
spring than in summer, principally due to the decline in nutritive
characteristics that occurs in summer. Cows selected nutrients to varying
degrees depending on the composition of the pasture sward and this selection
may result in seasonal nutrient imbalances. Finally, it appears that the best
use of supplements will occur when pastures are short in height because this
will minimise substitution of supplement for pasture and maximise marginal
returns in milk production.