Aw. Illius et al., COSTS AND BENEFITS OF FORAGING ON GRASSES VARYING IN CANOPY STRUCTUREAND RESISTANCE TO DEFOLIATION, Functional ecology, 9(6), 1995, pp. 894-903
1. The vertical distribution of plant tissues and the grazing response
s of goats to five grass species differing in their physical and struc
tural properties were analysed, with the aim of establishing how these
affected the loss of tissue from the plant and the gains to herbivore
s. 2. The grass species varied more widely in canopy structure than th
eir tissues vary in mechanical characteristics. Fine-leaved grasses ha
d higher tiller numbers than broadleaved species and this largely dete
rmines the higher forces and energy required to graze them. 3. A lower
proportion of plant tissue was removed in each bite taken from those
grasses which demanded higher bite forces during defoliation. 4. It is
calculated that the great majority of energy expenditure during grazi
ng is in chewing the ingested vegetation, rather than in removing plan
t tissue from the sward. Energy gain exceeded predicted energy use dur
ing grazing to any depth in the sward and could not therefore explain
why animals remove plant tissue only from the surface layer. 5. Large
animals severed herbage closer to the ground surface than small animal
s and thus were apparently less constrained by the physical properties
of the vegetation. 6. The balance between marginal energy intake rate
and the additional force required for defoliation at greater depth of
fers an explanation for the extent of defoliation. The existence of a
limit to the force an animal can apply is not a likely explanation for
the determination of bite depth.