The effect of calving date and stocking rate on the performance of spring-calving dairy cows

A 3-year experiment on milk production systems was set up in 1989 to investigate the effect of calving date and stocking rate on the performance of spring-calving dairy cows. An early-calving herd (System A) with a mean calving date of 23 January and stocked at 2.9 cows per hectare was compared with two later calving herds (Systems B and C) with a mean calvin,g date of 15 March. System B had a similar stocking rate to System A (2.9 cows ha(-1)), while System C had a stocking rate of 2.6 cows ha(-1). The average lactation yields (kg) over the three years were as follows: 5872, 5444 and 5584 (milk) 210, 204 and 215 (fat), 187, 184 and 189 (protein) and 261, 245 and 250 (lactose) for Systems A, B and C respectively. The average milk composition (g kg(-1)) was: 36.0, 37.6 and 38.7 (fat), 31.9, 33.7 and 33.8 (protein) and 44.5, 45.1 and 44.8 (lactose) for Systems A, B and C respectively. Delaying calving date to coincide with the beginning of the grass-growing season (System B compared with System A) reduced milk yield per cow significantly (P<0.05) in all three years of the experiment. However, fat and protein concentration were increased, resulting in no significant difference in the yield of fat or protein per cow. Reducing the stocking rate from 2.9 cows per hectare to 2.6 cows per hectare for cows calving in mid-March (System C compared with System B) increased milk yield per cow significantly (P<0.05) in only one of the three years (1990). Milk fat content was increased significantly in 1990. Stocking rate had no other significant effect on milk composition. In all three years of the experiment delaying calving date (System B) resulted in lower live weights at the end of lactation (P<0.05). The system of milk production had no effect on reproductive performance, which was satisfactory.