Effect of Forage Quality on Intake and Forage-Concentrate Interactions

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Abstract
Regulation of intake in ruminants is primarily a function of physical fill for diets that are energetically dilute and less digestible, such as high-forage diets; but intake becomes primarily a function of metabolic control for diets that are energetically dense and highly digestible, such as high-concentrate diets. Physical limitation of all-forage diets fed to growing ruminants is generally considered to be unpassed and undigested residue in the rumen. However, the physical limitation of forage-concentrate diets of low digestibility fed to lactating cows may be undigested residue passing the intestines, as implied by constant fecal production. Cell wall concentration of forage diets is the best single chemical predictor of intake. Gastrointestinal tract fill is important because it can cause a physical, or volume, limitation to intake; a difference in energy concentration of weight gain; and a potential difference in energetic efficiency. Prediction of intake is less precise than the prediction of digestibility or energetic efficiency. Intake of forages fed with concentrates is best considered as a curved response surface that is functionally related to the amount, or ratio, of concentrate fed and is continuous with the intake of forages fed alone. Depression of dry matter digestibility associated with increased intake is less for legumes than for grasses.

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