Effect of grazing severity on grass utilization and milk production of rotationally grazed dairy cows

Abstract

AbstractTwo experiments were carried out to examine the effects of grazing severity on the performance of January/February calved British Friesian dairy cows. In Experiment 1, three groups of cows were rotationally grazed across twenty‐four one‐day paddocks with high (H), medium (M) or low (L) herbage allowances. Cows on treatment M were offered a daily herbage allowance designed to achieve a residual sward height of 50 mm, assessed by a rising‐plate sward stick. The daily herbage allowance (g organic matter (kg live‐weight)‐1 on treatments L and H were 0.30 below and above that for M, respectively, to give residual sward heights of 42 and 59 mm. In Experiment 2, three groups of cows were grazed across twenty‐four one‐day paddocks to obtain residual sward heights of 50 mm (severe), 60 mm (moderate) and 80 mm (lax).Average milk yields on the L, M and H treatments in Experiment 1 were 11.8, 14.6 and 14.5 kg d,‐1 and in Experiment 2 they were 13.7, 16.0 and 17.0 kg d‐1 on the severe, moderate and lax treatments, respectively. The results indicate that the critical herbage height below which milk production per cow declines may vary with the production potential of the animal. There were no significant treatment effects on milk composition.Milk output ha‐1 and utilized metabolizable energy ha‐1 were greatest with the low herbage allowance in Experiment I and the moderate treatment in Experiment 2. Net herbage accumulation on the severe treatment in Experiment 2 was 30% lower than that on the lax treatment, as a result of treading damage in early season.It is concluded that, in a rotational grazing system, a reasonable compromise between sward utilization and animal performance can be achieved by grazing January/February calved cows to a residual sward height of 60 mm as assessed by a rising‐plate sward stick. This is equivalent to a sward surface height of about 80 mm.