Moderate grazing intensities modulate canopy structure that influences short-term intake rate of heifers grazing a natural grassland

Abstract

Grazing management frequently relies on forage availability as an indicator of performance. However, canopy structure and composition can play an important role in foraging behavior and intake in native grasslands. Here, we aim to (1) assess the effect of grazing management on herbage biomass, tussock cover, average canopy and inter-tussock canopy height, and to (2) study the effect of these canopy structure components on short-term intake rate (STIR) and patch selectivity of beef cattle, in a native grassland in the Campos region in southern Brazil. Ten 1000 m² experimental units were built in five paddocks (two per paddock) managed with different herbage allowance (HA) treatments: four fixed year-round HA (1 to 4 kg dry matter [DM]/kg live weight [LW]) and one variable HA (2 and 3 kg DM/kg LW during spring and the rest of the year, respectively). The experimental units were grazed individually with four heifers during 45 min, in two grazing cycles. Before each grazing session, we assessed the total herbage mass, average and inter-tussock canopy height, and tussock cover. During the 45-min grazing sessions, we evaluated the STIR (g DM/kg LW^0.75/min) and grazing time in tussock and inter-tussock strata. Increasing the HA increased the total herbage mass, canopy height and tussock cover. The STIR was maximized with herbage mass of 2200 kg DM/ha and a tussock cover of ~7%, canopy structures related with moderate grazing intensities (i.e., 2 and 3 kg DM/kg LW), which increased the selection opportunity by herbivores. Canopy structures associated with heavy (1 kg DM/kg LW) and lenient grazing intensity (4 kg DM/kg LW) determined linear decreases of 50% in STIR.