EFFECTS OF CANOPY STRUCTURE ON PATCH DEPRESSION BY GRAZERS

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 systematical ly, by horizon; (3) at equal herbage mass per unit area, tall and spar se swards yield larger bites and thus, exhibit faster and greater depl etion than short dense ones. In one experiment, we constructed 0.46-m2 swards of wheat (Triticum aestivum L.) with two contrasting structure s 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 diff erent numbers of bites from each patch type, and we measured bite mass , intake, and residence time. Bite dimensions and location (fresh or g razed area) were also recorded in the first experiment. In the first e xperiment, bite mass was initially larger and decreased more steeply w ith residence time in the tall sparse swards than in the short dense o nes. Because bite dimensions and mass declined but time per bite did n ot 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 d escribed intake curves (cumulative intake vs. time) well but ignored t he abrupt change in instantaneous intake rate when the top horizon was depleted. A model based on the horizon grazing style accurately depic ted this change in slope but disregarded the reduction in bite mass wi thin horizons. Intake curves for the field patches were more variable than for hand-constructed patches, but were well described by a rectan gular hyperbola. Taller patches had more herbage mass and allowed grea ter bite mass than shorter ones. Therefore, intake curves had initiall y steeper slopes and approached higher asymptotes than in shorter patc hes. Grazers experienced patch depression at the scale of feeding stat ions. Plant height and density are the causal factors of this process.