Threshold and linear models for genetic evaluation of visual scores in Hereford and Braford cattle

Milk production responses were measured in grazing cows offered mixed rations with or without canola meal. Experiments were conducted in spring (Experiment 1; early lactation) and autumn (Experiment 2; late lactation). The experiments used 140 (Experiment 1) or 96 (Experiment 2) Holstein-Friesian multiparous dairy cows that had calved in late winter/ early spring. Each experiment lasted 28 days including a 14-day adjustment period and a 14-day measurement period during which intake and milk production was measured. In each experiment, there were two dietary treatments: PMR: cows grazed a restricted pasture allowance (10-15 kg DM/ cow per day, measured to ground level) supplemented with a PMR comprising wheat grain (60%, DM basis), lucerne hay (21%) and maize grain (19%); and PMR + C: cows were fed the same as the PMR cows, but some wheat grain in the PMR was replaced with solvent-extracted canola meal so that it comprised wheat grain (39%, DM basis), lucerne hay (21%), maize grain (19%) and canola meal (21%). The two treatments were randomly allocated to half the cows in each experiment. Cows were then further allocated into 10 groups of 7 cows (Experiment 1) or 8 groups of 6 cows (Experiment 2). Two groups of cows were assigned to receive a different amount of each ration: 8, 10, 12, 14 or 16 kgDMtotal supplement/ cow per day for Experiment 1 and 6, 8, 10 or 12 kg DM total supplement/ cow per day for Experiment 2. In Experiment 1, yields of energy corrected milk (ECM), milk fat, and milk protein were greater for PMR+ C cows than PMR cows when 12 kg/ cow per day of supplement or more was offered. Milk protein concentration was greater in PMR+ C cows than PMR cows but only when 16 kg DM supplement/ cow per day was offered. These changes in milk yield and composition were associated with increases in the intakes of pasture, supplement and total DM intake in the PMR+ C cows compared with the PMR cows. In Experiment 2 there were no differences in milk yield or composition between PMR and PMR+ C cows at any amount of supplement offered, nor were there any differences in intakes of pasture, supplement or total DM intake. It is concluded that replacing some of the wheat in a well-formulated PMR with canola meal can stimulate DMI and increase per-cow production of ECM when high amounts of supplement are fed in early lactation, but not late lactation.