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-legum e pasture were investigated under a controlled rotationally grazed sys tem. A 19.2-ha area divided into sixteen 1.2-ha fields, each subdivide d into eight paddocks, was grazed by 40 cows with calves over three co nsecutive 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 com position changed both within and among the grazing seasons. Legume con tent 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 t he 3 yr. Crude protein (CP), in vitro organic matter digestibility (IV OMD), 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 l evels seen at the beginning of the grazing seasons (May), Neutral dete rgent fibre (NDF) and acid detergent fibre (ADF) content increased (P < 0.05) during June and July and were lowest in the spring and the fal l across all 3 yr. Mean entry sward heights were 24.8,:10.3 and 28.1 c m for years 1, 2 and 3, respectively. Pre-grazing sward height was neg atively correlated to CP (n = 786, r = -0.38, P < 0.0001) and IVOMD (n = 786, r = -045, 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 an d across all three grazing seasons but remained relatively high and wa s influenced by botanical composition and sward surface height.