Seasonal changes in quality and botanical composition of a rotationally grazed grass-legume pasture in southern Ontario

Seasonal changes in quality and botanical composition of a grass-legume pasture were investigated under a controlled rotationally grazed system. A 19.2-ha area divided into sixteen 1.2-ha fields, each subdivided into eight paddocks, was grazed by 40 cows with calves over three consecutive summers. Grazing was managed by setting target sward heights for exit of each paddock between 8 and 10 cm and allowing at least 25 to 30 d for regrowth. Herbage growth in those paddocks not grazed by cow-calf pairs was consumed by yearling heifers on a "put and take" basis. Paddocks were topclipped at 10 cm and fertilized with 34 kg ha(-1) of N immediately following the second grazing cycle. Botanical composition changed both within and among the grazing seasons. Legume content of the pasture increased (P < 0.05) throughout the grazing season, while grass content declined (P < 0.05) across all 3 yr. The amount of weeds and dead material averaged 8.9 and 3.8%, respectively, over the 3 yr. Crude protein (CP), in vitro organic matter digestibility (IVOMD), soluble protein (% total CP), rumen degradable protein (RDP) (% total CP) and metabolizable energy (ME) decreased (P < 0.05) from May to June in each season and then increased (P < 0.05) to or surpassed levels seen at the beginning of the grazing seasons (May). Neutral detergent fibre (NDF) and acid detergent fibre (ADF) content increased (P < 0.05) during June and July and were lowest in the spring and the fall across all 3 yr. Mean entry sward heights were 24.8, 30.3 and 28.1 cm for years 1, 2 and 3, respectively. Pre-grazing sward height was negatively correlated to CP (n = 786, r = -0.38, P < 0.0001) and IVOMD (n = 786, r = -0.45, P < 0.0001), but positively related to NDF (n = 786, r = 0.43, P < 0.0001) and ADF (n = 786, r = 0.68, P < 0.0001) across all 3 yr. The highest CP and IVOMD of the pasture were measured at a sward height of between 12 and 15 cm. Pasture quality varied both within and across all three grazing seasons but remained relatively high and was influenced by botanical composition and sward surface height.