Effect of length of the period of supplementation with concentrates on pasture intake and performance of grazing dairy cows
Authors: Stockdale C. R. 
Published: September 2002

Summary. The hypothesis tested in the experiment
reported here was that levels of substitution would be lower and marginal
responses to supplementation higher in grazing dairy cows that were subjected
to long, compared with short, treatment periods. Forty cows were fed irrigated
perennial pasture which contained ryegrass
(Lolium perenne), white clover
(Trifolium repens), paspalum
(Paspalum dilatatum) and weeds at a herbage allowance of
about 40 kg dry matter (DM)/cow.day. Of the 40 cows, 20 were under
continuous treatment for 6 months (2 October 1995 to 24 March 1996, inclusive)
(long term), while treatments were applied to the remaining 20 cows every
second month (short term). The second group of cows, therefore, commenced 4
weeks of treatment on 3 occasions, 31 October, 1 January and 26 February. The
cows in the short-term treatments were re-randomised for each of their 3
periods in the experiment. There were 2 long-term treatments, one where
pasture was fed alone and the other where the same amount of pasture was
supplemented with 5 kg DM/cow.day of a 75% barley –25%
wheat grain pellet. There were 2 replicates of each treatment and 5 cows per
group. The second set of 20 cows was allocated to the same treatments on a
short-term basis. When not under treatment, all cows in this second group were
offered about 40 kg DM of pasture/cow.day plus 5 kg DM of
barley–wheat.
Cows ate less pasture when concentrates were fed although total DM intake
increased (P<0.05). The level of substitution
averaged 0.4 kg DM reduction in pasture intake for each kg DM of concentrates
consumed, and this was not affected (P>0.05) by the
length of time for which the cows were supplemented. Milk yield declined from
November to March and concentrates increased production, but the interaction
between period of the year and use of supplements was not significant
(P>0.05). However, the milk responses associated with
length of time under supplementation appeared to be different in March
relative to the other periods. Although the marginal returns to concentrates
in March were 0.9 and 1.3 kg milk/kg DM for short- and long-term
supplementation, the marginal returns to feeding concentrates for short or
long periods during November and January were the same (0.9 kg/kg DM).
However, in terms of total DM intake, the average marginal response was lower
(1.4 v. 1.9 kg milk/kg of additional total DM) for
the short-term treatments. The trends in body condition for the long-term
treatments through the experiment indicated that divergence between
unsupplemented and supplemented cows occurred consistently throughout,
resulting in 0.8 units difference in body condition score after the 6 months
of treatment. It is suggested that it was the differences in body condition
that were responsible for the variation in milk response towards the end of
the experiment.
It was concluded that, while the length of time under supplementation had no
effect on intake variables or substitution, marginal responses to the long
term use of concentrates eventually diverged from those obtained from their
use for short periods, principally due to changes in the body condition of
animals in the long-term treatments.