Costs and Benefits of Foraging on Grasses Varying in Canopy Structure and Resistance to Defoliation

Abstract

1. The vertical distribution of plant tissues and the grazing responses of goats to five grass species differing in their physical and structural properties were analysed, with the aim of establishing how these affected the loss of tissue from the plant and the gains to herbivores. 2. The grass species varied more widely in canopy structure than their tissues vary in mechanical characteristics. Fine-leaved grasses had higher tiller numbers than broadleaved species and this largely determines 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 grazing is in chewing the ingested vegetation, rather than in removing plant tissue from the sward. Energy gain exceeded predicted energy use during 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 animals 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 offers 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.