Forage ingestion: effects of sward characteristics and body size.

A model of a grazing ruminant in a range environment integrates ingestive behaviors (bite size, chewing rate, movement rate) and processing behaviors (mastication and rumination requirements) with rumen function to predict the combination of behaviors that produces the highest rate of energy digestion per unit of time (DE/T). Ingestive and processing behaviors compensate to maintain DE/T as the environment changes. Compensation for a change in forage density is more complete than for a change in forage quality. Ingestive behaviors alone have limited ability to offset environment changes, and control of passage rate through processing behaviors is an important additional compensatory mechanism. Optimal solutions of the model maintain a high intake by sacrificing diet quality and maintain a high rate of passage by increasing mastication and rumination effort when eating low-quality diets. When the model is scaled to body size, decreases in body size lead to higher selectivity and more pronounced processing behaviors. In no case are optimal solutions characterized by less than maximum rumen fill. The model predicts that the small ruminant derives an increasing proportion of its energy from the cell contents. The model suggests that selection for production has produced plastic ingestive and processing behaviors and has increased body size.