Radiation Use Efficiency on Campos Grasslands with Contrasting Grazing Methods

In Uruguay, natural grasslands cover about 67% of the lands of the country. The Basaltic region occupies 4 millions of hectares and it has the highest proportion of natural grasslands of the country. In these pastures, livestock management is applied traditionally associated with intense grazing, with high loads, high sheep / cattle relations, with long periods of occupation and continuous grazing or with short rest periods. This livestock management is the main reason of degradation of natural grasslands. In Basaltic deep soils, it results in the loss of high value species with a decrease in productivity (Millot et al., 1987). However, with a controlled method that adjust grazing time and intensity of defoliation it is possible to avoid the degradation of the natural grasslands and reverse it in overgrazed sites. Traditionally, aboveground net primary production (ANPP) was estimated from biomass cuts made at the fields. Today, it is possible to estimate ANPP using remote sensing techniques by synthetic images of enhanced vegetation index (EVI). The EVI is correlated with the fraction of photosyntethically active radiation absorbed by plants (fPAR), providing the link of ANPP estimation covering larger areas and taking repeated measurements over time in the same place (Piñeiro et al., 2006). The radiation use efficiency (RUE) is the effectiveness with which fPAR is transformed in PPNA and is known to vary according to temperature, precipitation and species composition (Monteith, 1972; Piñeiro et al., 2006). Taking all these into consideration, we set as aims of this work: a) to calibrate RUE and b) study the temporal variability of RUE for two contrasting grazing methods