Wednesday, November 7, 2007
270-9
Modeling Canopy Assimilation of Xaraes palisadegrass [Brachiaria brizantha cv. Xaraés] Pastures under Rotational Stocking Strategies.
Bruno C. Pedreira and Carlos G. Pedreira. Universidade de Sao Paulo, Dept. Zootecnia ESALQ-USP, Av. Padua Dias, 11, Piracicaba, 13418-900, Brazil
Grazing strategies affect plant morphology and sward structure in pastures. These affect the physiology of canopy carbon assimilation. These responses can be assessed with the use of models that estimate potential carbon uptake from crop structural characteristics, and which allow for the simulation of scenarios and the prediction of responses in order to optimize management and resource use efficiency in animal-forage systems. The objective of this research was to measure leaf photosynthetic rates during the regrowth period and to model canopy photosynthesis of vegetative swards of Xaraes palisadegrass [Brachiaria brizantha] under contrasting management strategies of rotational stocking, with frequency of defoliation dictated either by the level of light interception (LI) by the canopy or by calendar days. Time at which carbon exchange was measured within the regrowth cycle strongly affected the leaf photosynthetic response. Early regrowth was characterized by low photosynthetic potential and for paddocks under the 100% LI treatment leaf photosynthesis was 3.3 µmol CO2 m-2 s-1, probably requiring plants to draw on reserves during that phase. Halfway through regrowth canopy photosynthesis increased to 16.5 µmol CO2 m-2 s-1 in all treatments. In paddocks under the 95% LI treatment (shortest mean regrowth periods; 22 days), canopy photosynthesis decreased to 13 µmol CO2 m-2 s-1 whereas longer regrowth intervals (28 and 32 days on average, for the 28-d and for the 100%-LI paddocks, respectively), generated less favorable light environments, probably with more competition for light, partially explaining the lower rates of 10.9 and 10.2 µmol CO2 m-2 s-1 for 28-d and 100%-LI, respectively. Under the highest grazing frequency (95% LI), the short rest period and the low mean LAI (2.08) allowed for more light penetration into the canopy. This suggests that adequate management practices coincide with highest photosynthesis over grazing cycles, represented in this study by the 95%-LI treatment.