PREDICTIONS OF FOOD-INTAKE IN RUMINANTS FROM ANALYSES OF FOOD COMPOSITION

Equations used to predict intake by cattle from some chemical or physi cal characteristic of food were examined. The equations are empirical or mechanistic in nature. Mechanistic equations are not used widely, u sually only in a research context. The input to mechanistic models req uires too much time to quantify to be used routinely. Empirical relati onships form the basis of most feeding standards and are based on a wi de variety of prescribed characteristics (digestibility, chemical comp osition, etc.), but the underlying principle is a relationship between intake and digestibility. Equations are modified to take account of f eed types, animal weight and physiological state, rumen modifiers, hor mone implants, environmental conditions, and whether grazing or hand f ed. Quite significant differences exist between the equations in the i ntakes they predict in response to variation in weight, breed type, an d feed digestibility. Equations can be very precise in their predictio n when used with feed types and breed types on which they are based. N ear infrared reflectance (NIR) offers the most potential for long-term development of equations. At present, NIR is used largely to determin e chemical composition because of speed of operation, but long-term st orage of data is simple, allowing further associative relationships to be developed readily. More sophisticated statistical procedures being employed to improve the precision of the relationships between intake and prescribed characteristics of food and NIR will be vitally import ant as they enable extra parameters to be incorporated at no extra cos t or time for analysis.