Effects of sward structure on herbivore foraging behaviour in a South African savanna:: An investigation of the forage maturation hypothesis

The grass layer of savannas is characterized by strong temporal and spatial heterogeneity in the quantity and quality of forage. Besides this, there is strong variation in other aspects of sward structure, here defined as sward chemistry, morphology, architecture and species composition. The forage maturation hypothesis (FMH) is based on the temporal dynamics of forage quantity and quality of grasslands. Recently this hypothesis has been used to explain the foraging behaviour of large herbivores as an optimal solution for balancing forage ingestion and forage digestion, leading to a maximization of daily rates of forage intake in patches of intermediate forage quantity. However, so far studies using this hypothesis have been constrained to mono-specific forage resources or have ignored inter-patch variation in sward structure. We studied the foraging behaviour of cattle in a South African savanna. We explicitly addressed forage quantity and quality, and the structure of seven sward types in order to investigate (i) their effects on foraging behaviour; and (ii) the assumptions and predictions of the FMH for foraging behaviour in grasslands with pronounced variation in sward structure. The results indicated that the assumptions of the FMH were met and that forage quantity affected forage intake the most, with sward structure having little or no effect. The predicted maximum rate of daily forage intake agreed with the results of previous studies. However, the forage quantity at which this maximum intake rate was found, was larger than expected based on the results of some studies. It is likely that this discrepancy resulted from the use of artificial leaf-only grass swards in these studies. The results suggest that the FMH can be used to explain the foraging behaviour of herbivores over a wide range of sward structures.