Herbage Allowance Effects on Leaf Photosynthesis and Canopy Light Interception in Palisadegrass Pastures Under Rotational Stocking.

Although pastures under intermittent grazing often have limited time for regrowth, sward structure changes continually during that period. Under these circumstances, leaf photosynthetic rates can decrease, due to increased mutual shading, especially towards the end of the regrowth period. The objective of this study was to evaluate grazing intensity effects, applied as herbage allowance (HAL) levels (5, 10, 15, and 20 kg/100 kg liveweight per day), on leaf photosynthesis (P_leaf) of Marandu palisadegrass (Brachiaria brizantha) and its relationship with leaf area index (LAI) and canopy light interception (LI), during two grazing seasons (warm-rainy season of 2003 and 2004). In both years, P_leaf was similar across HAL levels at the onset of the grazing season, but towards the end, when leaves under low grazing intensity grew in a low irradiance environment, P_leaf decreased as HAL increased. Late in the 2003 grazing season (last day of regrowth of the last cycle), P_leaf  was 32, 30, 25 and 26 µmol CO₂ m^-2s^-1 for HAL levels of 5, 10, 15 and 20%, respectively. In a sward with 97% LI, LAI 4 and 55 cm height (20%-HAL) the newly expanded leaves in response to light (0 – 2000 µmol photon m^-2s^-1) were the most photosynthetically active leaves of the tiller (P_max = 33 µmol CO₂  m^-2s^-1). The P_max of developing leaves was relatively high (24 µmol CO₂ m^-2s^-1), while P_max of the other three mature leaves was 20.7, 17.5 and 4.9 µmol CO₂ m^-2s^-1. Mean P_leaf was highest (26 µmol CO₂ m^-2s^-1), on the 16th day of regrowth. Increasing mutual shading within the sward was thought to be the main reason for the decline in P_leaf. This variable light environment in the sward, inherent to the intermittent grazing, led to a decline in post-grazing leaf photosynthesis, regardless of HAL level.