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.