Sward characteristics, grass dry matter intake and milk production performance is affected by timing of spring grazing and subsequent stocking rate

The objective of this study is to investigate the effect of contrasting spring grazing dates (GD) and stocking rate (SR) on sward characteristics, grass dry matter intake and milk production performance of autumn calving dairy cows during the spring period. Two swards were created by grazing in March (early grazing; E) or by delaying first grazing until mid-April (late grazing; L). Two stocking rates, high (H; 5.5) and medium (M; 4.5) were applied across each sward. Forty eight autumn calving Holstein cows (160±35 days in milk) were assigned to one of four (n =12) different grazing treatments. The experiment began on April 17th and finished after 2 grazing rotations on June 20th. Later spring grazing significantly increased herbage mass (kg DM/ha) above ground level (+933, P <0.05) and >50 mm (+738, P <0.05). Compressed sward height (+22.1 mm, P <0.05), extended tiller height (+73 mm, P <0.001) and pseudostem height (+35 mm, P <0.001) were also significantly higher for later grazed swards. In the grazing horizon (>80 mm-- extended tiller height), later grazed swards had significantly lower leaf proportion (-0.09, P <0.05) and higher dead material (+0.05, P <0.001). Daily herbage allowance (>50 mm) was on average 12.7, 15.9, 18.2 and 21.9 kg DM/cow for EH, EM, LH and LM, respectively. Daily leaf allowance (>80 mm) was 10.1, 12.3, 13.3 and 14.5 kg DM/cow for EH, EM, LH and LM, respectively. The EM (16.2 kg DM/cow), LH (+0.1 kg) and LM (0.8 kg) treatments all had similar grass DM intake, however there was evidence of an interaction (P <0.10) between GD and SR, this was due to the low grass DM intake of the EH (13.9 kg DM/cow) treatment. When expressed as UFL (Fill unit) intake the EM treatment recorded the highest value. There was a significant interaction between GD and SR (P <0.01) for milk, protein yield, 4% fat corrected milk yield (P <0.05) and protein concentration (P <0.001). Cows grazing the EM treatment produced 23.9 kg of milk, 876 and 685 g of fat and protein yield. The difference in milk production (cow/day) between EM and EH treatments was +3.6 kg milk, +98 g fat and +107 g protein. The production yield difference between LM and LH treatments was +1.1 kg milk, +27 g fat and +29 g protein in favour of the LM treatment (23.9 kg of milk, 877 and 687 g fat and protein yield). Herbage quality and morphological characteristics are clearly improved with early spring grazing as herbage mass is reduced on subsequent rotations. Swards grazed in early spring allow higher grass utilisation and high milk production performance when grazed at a medium stocking rate. Improved milk production from herbage can be achieved provided herbage mass and allowance are maintained at levels where herbage quality decreases are minimised.