A GENERAL-METHOD FOR PREDICTING THE WEIGHT OF WATER IN THE EMPTY BODIES OF PIGS

As water is the major component of the pig body it accurate prediction is of importance in pig growth models. It has become conventional to predict the weight of water, WA kg, from the weight of protein, P kg. The purpose of this paper is to find how this can be done across pig genotypes of different mature size. The widely used equation to relate WA to P is of the form: WA = a.P-b. This equation is examined theoretically. It is concluded that the form of the equation is reasonable and, that while the value of the exponent b is likely to be constant across genotypes, the value of the scalar a is not. It is proposed that the value of the scalar a is best estimated as a = WAPR(m) P-m(1-b) where WAPR(m) is the water: protein ratio in the body at maturity and P-m is the weight of protein in the body at maturity. The value of the parameter WAPR(m) is assumed to be constant across genotypes with a value in the range of 3.04 to 3.20, depending on the methods used for measuring body composition. The general value of b = 0.855, taken form published work, is confirmed. A consequence of the argument quantified in the paper is that the value of a is predicted to vary from a = 4.69 for a pig with P-m = 20 kg to a = 5.36 for a pig with P-m = 50 kg. The general equation is expected to give more accurate predictions of the weight of water and, hence, of body weight, in models intended to predict pig growth, food intake, body composition and efficiency.