Effect of grazing intensity in spring on pasture growth, composition and digestibility, and on milk production by dairy cows

Two 20-block farmlets, each of 6.5 ha, were rotationally grazed by milking cows in spring at stocking rates of 1.9 and 3.4 cows/ha to give a low (LGI) or high (HGI) grazing intensity. These grazing intensities were designed to simulate dairy farms with low or high levels of pasture conservation, and resulted in post-grazing residues of2.6 (LGI) and 1.7 (HGI) t/ha DM by late spring. Over summer, the 2 groups of cows continued to graze their respective areas, but spare cows were re-allocated between the groups so that stocking rates were the same at 1.7 cows/ha. The HGI cows produced 1 kg less milk fat (0.79 v. 0.81 kg/day.cow, P> 0.05) in spring (15 October-30 November), but produced significantly (P < 0.05) more milk fat (0.63 v. 0.59 kg/day.cow) and 1.3 kg milk protein (0.44 v. 0.42 kg/day.cow) in summer (1 December-1 February). Despite the higher grazing intensity in spring. there was no difference in intake of pasture or in digestibility of herbage selected by the cows in the two herds. In summer the HGI swards contained less dead herbage (P<0.05) but a similar quantity of green herbage compared with the LGI swards, and, compared with LGI cows, the HGI cows removed a higher proportion of this green herbage during grazing. Digestibility of the LGI swards was lower because of the higher content of dead herbage but cows in both groups rejected this dead material and selected a diet of similar digestibility. The presence of dead herbage in the LGI swards appeared to reduce animal production by reducing availability and intake of green herbage rather than by reducing the digestibility of the diet selected by the cows. The high grazing intensity increased daily net accumulation of grass leaf in spring (1 7 v. 6 kg/ha of DM, P< 0.05) but reduced that of true stem (-4 v. 28 kg/ha, P < 0.10) and of total herbage (43 v. 54 kg/ha, P<0.10). The HGI swards had higher ryegrass tiller densities in summer and higher (P<0.05) net accumulation rates of herbage in summer through to winter). Increased grazing intensity in spring above normally accepted levels increased pasture utilisation in spring and summer, and milk production in summer, and increased the quantity of spring herbage available for conservation.