Conversion of energy and the chemical composition of gain in bacon pigs

Abstract

Relationships between the intake of energy and the rate of gain on the one hand, and the chemical composition of gain on the other were studied in bacon pigs, assuming that: (1) the only important determinants of the energy balance are the expenditures of energy in maintenance, in protein deposition and in fat deposition; (2) the net energy cost of fat or of protein deposition remains constant in pigs of all ages and live-weights, whatever the level of nutrition; (3) the average maintenance requirements of pigs growing over the same live-weight range and kept under similar conditions are constant, irrespective of their daily gains; (4) the ratio of non-fat components to protein in the gains of pigs over a similar live-weight range is virtually constant. Deductions based on these assumptions enabled the expression of the feed conversion ratio as a function of the average daily protein deposition, the average daily maintenance requirements and the average daily intake of energy to be made. The dependence of the feed conversion ratio on each of these three factors was demonstrated. After the substitution of plausible values for the energy costs of protein and of fat deposition, as well as for the average maintenance requirements, equations were derived for the estimation of the content of protein and of fat in the live-weight gain, when the total feed consumption and the number of days on test (20 to 90 kg.) were known. Using these equations, the average gain of fat and of protein in each year were estimated for the Danish Progeny Testing Stations during the period from 1926–7 to 1962–3. It was found that the decrease of the feed conversion ratio and consequent reduction in fat deposition which were achieved during this period, were accounted for mainly by an increase in daily protein deposition.