Effects of Canopy Structure on Patch Depression by Grazers

Abstract

Patch depression is an essential premise of optimal patch use models. We studied the relationship between patch residence time and intake by Bos taurus. The hypotheses were: (1) grazing changes sward structure and thus, causes patch depression; (2) patches are grazed systematically, by horizon; (3) at equal herbage mass per unit area, tall and sparse swards yield larger bites and thus, exhibit faster and greater depletion than short dense ones. In one experiment, we constructed 0.46—m2 swards of wheat (Triticum aestivum L.) with two contrasting structures but equal herbage mass. In a second experiment we mowed a perennial ryegrass (Lolium perenne L.) field to produce 0.36—m2 patches of three heights (7, 12.5, and 18 cm). The animals were allowed to remove different numbers of bites from each patch type, and we measured bite mass, intake, and residence time. Bite dimensions and location (fresh or gazed area) were also recorded in the first experiment. In the first experiment, bite mass was initially larger and decreased more steeply with residence time in the tall sparse swards than in short dense ones. Because bite dimensions and mass declined but time per bite did not change, instantaneous intake rate declined as patches were grazed. Animals grazed systematically and removed most of the top half of the canopy before biting into the lower horizon. A rectangular hyperbola described intake curves (cumulative intake vs. time) well but ignored the abrupt change in instantaneous intake rate when the top horizon was depleted. A model based on the horizon grazing style accurately depicted this change in slope but disregarded the reduction in bite mass within horizons. Intake curves for the field patches were more variable than for hand—constructed patches, but were well described by a rectangular hyperbola. Taller patches had more herbage mass and allowed greater bite mass than shorter ones. Therefore, intake curves had initially steeper slopes and approached higher asymptotes than in shorter patches. Grazers experienced patch depression at the scale of feeding stations. Plant height and density are the causal factors of this process.