Equilibrium weight in relation to food intake and genotype in twin cattle

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Abstract
Twenty-two Ayrshire twin female cattle were kept for up to seven years of age on six equally spaced levels of constant food intake, ranging from 35 to 140 lb per week of a complete pelleted diet. Analysis was confined to the period between 5 and 7 years when growth had practically ceased and equilibrium maintenance requirements could be determined.Equilibrium weight maintained was directly proportional to the level of constant food intake.This relationship obtained by controlling food intake was significantly different from the 0·73 power relationship that applies across species. In establishing the present relationship, systematic genetic differences were excluded, and this is probably the main condition required for weight eventually maintained to be proportional to food intake. In contrast, the well-established proportionality of food intake to metabolic weight is essentially based on a species comparison and is thus due almost entirely to genetic differences.A mathematical model is given which combines (1) the genetic relationship of proportionality of voluntary food intake to mature metabolic weight and (2) the nutritional relationship of proportionality of equilibrium weight maintained to level of constant food intake. The model is then used to partition variation into systematic and random components.The experiment provides some indication of the extent of individual variation in efficiency of maintenance within breeds, the amount which is genetic, and the repeatability of this efficiency from age to age. Some information is also provided on variation in voluntary food intake, mature weight maintained and efficiency of maintenance on ad libitum feeding.Methods of estimating efficiency of maintenance using ‘feed and weigh’ techniques, selection for efficiency of maintenance, the correlated response in mature weight, and selection for relative food capacity are briefly discussed.