Effects of grazing regimes on the temporal dynamics of grassland communities

Questions Grazing management influences grassland community dynamics, but few studies exist on the effects of specific management regimes in subtropical grasslands. Here, we ask how rotational and continuous grazing influences the composition, diversity, and biomass production of natural grasslands in southern Brazil. Location The study was conducted at four cattle farms within the south Brazilian grassland region (Campos Sulinos), in the upper area of the Camaqua river basin of Rio Grande do Sul, Brazil. Methods On each farm, grassland that had previously been subjected to continuous grazing (freely grazed by cattle) was divided into two experimental units; one managed by rotational grazing and the other maintained under continuous grazing. In each experimental unit, 14 permanent plots were monitored over six years for species composition and species biomass. Data were analyzed using linear mixed models to evaluate the effects of grazing management strategies on composition dissimilarity, diversity indices, and plant biomass. Results Grasslands under a rotational grazing regime changed steadily over the period of evaluation. Total biomass and species distribution patterns greatly differed from those in plots subjected to continuous grazing, leading to changes in species richness, dominance, and composition, as well as to an increase in overall plant biomass. Conclusions Community changes during the six-year study period as a consequence of the distinct grazing treatments were evident. Grasslands under rotational grazing management benefitted from less selective grazing, enabling a more uniform growth of species within the communities, which in turn were richer at the end of the six-year experiment. In contrast, the proportion of grazing-resistant species or less-consumed species was higher under continuous grazing, where foraging by cattle appears to be more selective. Rotational grazing thus not only leads to higher species richness and diversity, but also to higher primary and, in consequence, secondary productivity.