Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 22, 1996
Publication Date: N/A
Interpretive Summary: It is still very difficult to accurately predict how much feed that cattle, sheep, and goats will consume when given unlimited access to feed. Individual chemical measurements are only useful for predicting intake in certain limited circumstances. I combined estimates of digestibility, fibrosity, and protein content to model animal digestion. The model simulated filling of the stomach and reduced feeding as the stomach became overfilled and stimulated feeding as the stomach became too empty. The carbohydrate digested in each hour was simulated and the model attempted to consume more or less feed if energy supplied by carbohydrate was too high or too low. Energy can't be processed without protein so the ratio of digested energy to digested protein was also monitored. When energy and protein were out of balance then the appetite, digestion rate, and rate of flow of feed from the stomach were reduced. The model behaved very much as observations of cattle indicated it should. This model demonstrated that many of the theories describing limits to intake are generally correct but that effects of stomach fill, energy, and protein must be considered simultaneously. More complex calculations are required but predictions of ruminant intake may be greatly improved.
Technical Abstract: Little progress has been made in modeling intake regulation in spite of the many mechanisms identified as important. A simple model was developed with compartments for protein, soluble carbohydrate, digestible fiber, and slowly digesting fiber. Distention and chemostatic feedbacks were combined using a previously published equation. The ratio of current rumen fill to a modulating parameter for rumen fill represented distention feedback. The ratio of the current flow of available energy to a modulating parameter for chemostatic demand represented chemostatic feedback. The modulating parameter for rumen fill was based on literature reports of rumen contents. The current fill varied from greater than to less than the modulating parameter and the actual flow of energy varied from greater than to less than the chemostatic modulating parameter. The effect of the DOM to CP ratio on DOM intake was modeled to adjust rates of digestion, rates of passage, and the chemostatic feedback. The model tested an integrated concept of intake regulation with only CP, IVDMD, and NDF as inputs. In spite of relatively simple inputs, the model behavior in response to variation in protein, digestibility, and fiber was similar to observations of animal responses.