Cattle use protein as a currency in patch choice on tropical grass swards

摘要

Patch choice by large herbivores is a major impact on grasslands and has consequences to both animals and vegetation in grazing systems. Previous studies have performed controlled experiments using artificially created patches to approach the mechanisms by which herbivores choose and use vegetation patches. However, the number of patch types offered to animals and the method of patch creation are limited, compared with diverse patch structure of actual grasslands. Furthermore, patch choice on tropical grasslands and the effect of hunger on patch preference remain relatively unexplored. We conducted six experiments to examine the effects of external (vegetation quantity and quality) and internal (hunger) environments on patch choice by cattle (Bos taurus) on tropical grass (Paspalum notatum) swards. In each experiment, three cows were allowed to graze three plots in sequence after a fast. The plots comprised 16, 36 or 9 patches of 4, 6 or 9 types created by defoliation treatments alone or in combination with fertilizer treatments. Fertilized patches were richer in protein but similar in digestibility compared with non-fertilized ones. Animals always showed highest preference for a fertilizer-treated, protein-rich patch when fertilizer treatments were incorporated into patch preparation. Among dry matter, digestible dry matter (as a surrogate to digestible energy) and protein, the allocation of grazing time to patch types best matched the predicted potential intake on a protein basis. Results showed usefulness of fertilizer for better simulating patchy vegetation of grasslands and overcoming the difficulty in discriminating between digestibility and protein as a currency for patch choice. Facing quantitative and qualitative patchiness in vegetation, animals were sensitive to protein concentration in the diet and perceived protein intake as a currency for patch selection. As a response to the internal environment, animals traded forage quality for quantity with increasing levels of hunger.