Short-term response in milk production, dry matter intake, and grazing behavior of dairy cows to changes in postgrazing sward height

Postgrazing sward height (PGSH) can be altered to adjust the allowance of grass in the dairy cow's diet. This study aimed to investigate the short-term dairy cow response to a change in postgrazing height in early lactation. Ninety Holstein Friesian spring-calving cows were randomly assigned across 3 postgrazing height treatments (n=30): 2.7cm (severe), 3.5cm (low), and 4.2cm (moderate) from February 14 to April 24, 2011. From April 25, animals were rerandomized within each treatment to graze across 2 postgrazing heights: 3.5cm (low) or 4.5cm (high). Animal production measurements were taken from April 4 to 24 (measurement period 1; M1) and from April 25 to May 15 (measurement period 2; M2). The 6 treatments (n=15) of M2 were severe-low, severe-high, low-low, low-high, moderate-low, and moderate-high. During M1, increasing postgrazing height from severe to low to moderate linearly increased daily milk yield (21.5, 24.6 and 25.8kg/cow per day) and grass dry matter intake (GDMI; 13.2, 14.9, and 15.8kg of DM/cow per day). Milk solids yield was reduced in the severe (−1,518g/cow per day) treatment when compared with the low and moderate cows (1,866g/cow per day, on average). The milk yield (MY) response to change in PGSH between M1 and M2 (V M1−M2 ) was established using V M1−M2 MY=−1.27−1.89 × PGSH M1 + 1.51 × PGSH M2 (R 2 =0.64). The MY response associated with each treatment between M1 and M2 (3 wk) were −1.03kg/cow for severe-low, 0.68kg/cow for severe-high, −2.56kg/cow for low-low, −1.11kg/cow for low-high, −4.17kg/cow for moderate-low, and −2.39kg/cow for moderate-high. The large increase in energy intake in severe-high between M1 and M2 was achieved through higher GDMI per minute and GDMI per bite, which supported the positive change in MY. Treatments low-high, moderate-low, and moderate-high recorded the highest overall cumulative milk yield (74kg of milk solids/cow) over the 6-wk period, whereas severe-low and severe-high had the lowest (65kg of MS/cow). From the animal responses observed in the present study, imposing a postgrazing height of 3.5cm in early spring provides the opportunity to increase postgrazing height thereafter; the cow increases GDMI accordingly and converts the additional energy intake into milk output. The equations established in this paper provide a decision tool for dairy farmers to anticipate the animal response when postgrazing height is altered or maintained around the tenth week of lactation.