Effects of restrictions of energy, protein or both on growth and body composition of pigs: Dietary restrictions and body composition of pigs

An experiment was carried out, using Duroc Jersey barrows, to test the hypothesis that relationships within the pig fat-free carcass remain unaltered when varying the principal dietary factors: energy, protein or both. Twelve pigs were assigned to each of the following diets: high energy-high protein (HEHP), used as control group; low energy-high protein (LEHP); high energy-low protein (HELP) and low energy-low protein (LELP). From 30 kg live weight onwards the animals were individually penned and fed once a day the experimental rations, on a restricted scale of feeding related to live weight. Compared with the control group, daily intake of metabolizable energy was approximately 37% lower in LEHP and LELP groups while daily protein intake was approximately 25% lower in HELP and LELP groups.

In each group the 12 pigs were slaughtered between 60 and 110 kg. The right half carcasses were completely dissected using strictly anatomical criteria. The main growth characteristics of the four treatments were described by second degree polynomials fitted to the live weight-age data. Each treatment produced a characteristic growth curve. In every case significant differences were found in the polynomial coefficients. Data on body composition were subjected to analysis of covariance, using the linear model y = a + bx. When growth of different parts of a whole were studied the weight of that whole was used as the independent variable. When muscle:bone ratio was examined the weight of total side muscle was used as covariate in the covariance analysis for total side bone.

The major treatment effects were on the relationship between fat and non-fat tissues in the carcass. Thus, in relation to dissected side weight, HELP animals had more fat and less muscle and bone than controls, while LEHP and LELP pigs had more muscle and bone and less adipose tissue. Within the fat-free carcass several differences were found on muscle weight distribution. LEHP, HELP and LELP pigs had a higher proportion of ‘expensive muscles’. However, the magnitude of these changes was small and likely to be of little commercial importance. On the other hand, only minor differences were found in bone weight distribution. These were confined to the scapula and femur. Muscle:bone ratio was not affected by the treatments. Within the adipose tissue differences were observed in growth rates from the different depots relative to total side fat. Thus, it would appear that the growth rate of the subcutaneous depot was higher and that of the intermuscular one lower in LEHP and LELP animals. The body cavity depot was less in these animals.