Effect of length of the period of supplementation with concentrates on pasture intake and performance of grazing dairy cows

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.