Grazing Intensity Controls the Seasonal Utilization of Functionally Diverse Patches by Mediating Herbivore Selectivity

Abstract

Understanding the mechanistic link between plant functional traits and foraging patterns across seasons and grazing intensities is crucial for implementing sustainable grazing systems and predicting ecological successions. We assessed the interaction effects of grazing intensity, season, and leaf functional traits on herbivore grazing patterns in a native grassland composed of functionally diverse patches in four 10–14 ha paddocks managed under lenient or moderate grazing intensities. Seasonal botanical composition was estimated in permanent 20 × 20 cm patches. Defoliation events of species in patches were recorded every 7–10 d, four times during summer-autumn, and five times during winter and spring. Leaf dry matter content, specific area, tensile strength, and width of dominant grass species were measured seasonally, and species were allocated to functional groups. The proportion of total defoliation events occurring in each functional group and their selectivity index were estimated for each period and season. We tested the relationship between trait community weighted mean in the pasture and that grazed by cattle for each quadrant and period within seasons. Cows focused grazing on resource-acquisitive strategy species with wider and more tender leaves during summer-autumn under both grazing intensities. As herbage mass and accumulation decreased during winter, cows shifted their foraging strategy toward shorter patches and previously rejected, tougher-thinner leaves and resource-conservative strategy species stockpiled from summer-autumn, not overgrazing the resource-acquisitive species. However, this general pattern was modulated by grazing intensity; cows grazed taller patches and more tender and wider-leaved species under lenient than moderate grazing intensity. Thus, the spatiotemporal heterogeneity of desired and rejected stockpiled patches of different functional groups was managed by integrating seasonality, grazing intensity, and leaf functional traits. Therefore, the general principle of foraging selectivity toward desired species can be altered by management practices and inherent species attributes, maintaining communities of species of different ecosystem functions in equilibrium.