Effect of grazing rotation length on milk production and composition of dairy cows strip-grazing at the same herbage allowance during a dry summer

Grazing rotation length is one of the most important tools that dairy farmers possess to control pre-grazing herbage mass (HM), pasture characteristics and utilization. An experiment was carried out in the INIA Remehue dairy farm during summer, from 29 December, 2014, to 28 March, 2015, in which two grazing rotation lengths (GRL): 30 vs. 45 days were compared. The experiment was carried out with 48 Holstein Friesian cows in mid lactation (130 days in milk), which were producing an average of 25.9 kg/d of milk with concentrations of 33.6 and 33.2 g/kg of milkfat and protein, respectively. Body condition score was 2.9 and body weight was 521 kg. At the beginning of the experiment, herbage allowance was fixed at 22 kg DM/d > 3 cm. Due to dry conditions and reduced pasture growth during summer, herbage allowance was then reduced to 18 kg DM/d, and supplementation was included: 4.4 kg DM/d of concentrate (from 28 January) and 4.6 kg DM/d of herbage silage (from 1 March). Pre-grazing HM above 3 cm was lower with 30 than 45 days GRL (2077 vs. 2656 kg DM/ha). On average, pre-grazing platemeter sward height was 8.4 cm and was not affected by treatments. Due to dry conditions, herbage presented low quality with high NDF content and low digestibility (565 g/kg and 0.69 (in vitro), respectively). Post-grazing herbage characteristics were similar between treatments. Post-grazing platemeter sward height and herbage mass > 3 cm averaged 4.7 cm and 660 kg DM/ha, respectively. On average, milk production was 14.9 kg/day and body condition score was 2.83. Productions per ha of 4% fat corrected milk (1430 vs. 1985 kg/ha) and milk solids (106 vs. 142 kg/ha) were lower with 30 than 45 d GRL. Overall, the effect of GRL during summer was null for animal performance. This may be related to similar herbage characteristics between treatments, even if significant differences in pre-grazing HM were generated. The fact of including relatively high supplementation may also have reduced the effect of GRL. On the other hand, under these grazing conditions, a longer GRL results in greater production per ha.