Herbage biomass and intake of beef cows with calves grazing a grass-legume pasture in southern Ontario

Intake and performance of 40 spring-calving cows with calves was investigated under a rotational grazing management system across three consecutive summers. A 19.2 ha pasture was divided into sixteen 1.2 ha fields with each field being further subdivided into eight 0.15 ha paddocks. Five cows with calves belonging to one of two breed groups (large vs. medium framed) and two body conditions (high vs. low) were randomly assigned to graze each of eight field pairs from May to October of each year. Paddocks not grazed by cow-calf pairs were grazed by yearling heifers or mechanically harvested as hay or silage. Grazing was managed by setting target sward heights for exit of each paddock at 8 to 10 cm and allowing a minimum of 25 to 30 d for regrowth. Paddocks were topclipped and fertilized with 34 kg ha(-1) of nitrogen (N) immediately following the second grazing of each paddock. Sward surface height data were collected from 40 measurements taken immediately pre- and post-grazing in each paddock as well as during the regrowth period. Herbage biomass and quality was determined in year 1 by handclipping 10 quadrats (1.22 m x 0.15 m) from each paddock pre- and post-grazing and by clipping 7 similar quadrats from alternate paddocks in each of the other 2 yr. Average pre-grazing sward heights were 24.8, 30.3 and 28.1 cm, in years 1, 2 and 3, respectively. Herbage biomass available for grazing averaged 7.59, 10.62 and 11.04 tonnes DM ha(-1) for years 1, 2 and 3, respectively. A positive linear (r = 0.93 P < 0.0001) relationship was established between sward height (cm) and herbage biomass (kg DM ha(-1)) across all 3 yr. Intake of dry matter (DM), crude protein (CP) and metabolizable energy (ME) was higher for both large- and medium-framed cow-calf pairs in year 3 than in the other 2 yr. Both large- and medium-framed cows lost (P < 0.05) body condition in years 1 and 2, but gained in year 3. Cows from the low body condition group gained (P < 0.05) more than those from the high group in years 2 and 3. Medium-framed calves gained more (P < 0.05) in year 3 than in years 1 and 2. Body condition of cows grazing a multi-species grass-legume pasture can lie maintained if not improved provided seasonal variations in quality and quantity are minimized.