Time trends of Gompertz growth parameters in 'meat-type' pigs

Previously published data from serial slaughter trials on growing pigs of five genotypes were reanalysed. Gompertz curves were fitted to body protein and lipid mass in order to estimate mature protein and lipid mass (P-infinity, L-infinity) and the rate parameter (B-Gomp) that was presumed to be equal for the protein and lipid curves. L-infinity was expressed as its ratio to P-infinity, R-L infinity/P infinity. The maximum rate of protein deposition was derived as P-dep,P- max = P-infinity X B-Gomp/e The analysed data encompass body weights of 10 to 133 kg, 13 to 217 kg, 18 to 106 kg, 20 to 110 kg and 11 to 145 kg. The Gompertz Junction fitted all data sets well, as judged by the standard deviations and distribution patterns of the residual terms. Autocorrelations among the residuals were non-significant. Averaged over sexes (females and entire and castrated males), the P-infinity estimates were all close to 31 kg; the R-L infinity/infinity estmates ranged from 1.4 to 4.7 kg/kg; the B-Gomp estimates ranged from 0.009 to 0.017 kg/day per kg. The resulting P-dep,P-max estimates ranged from 110 to 193 g/day. The genotypes were placed in 1969, 1976, 1984, 1990 and 1993. Plotting the estimates against time (year) showed distinct time trends for all parameters except P-infinity, R-L infinity/P infinity seems to gradually reach a plateau around unity, whereas B-Gomp and P-dep,P- max increase linearly. These trends were confirmed by an analysis of body weight based on the same data plus data on three other genotypes that spanned the same time period. Analyses of the same protein and lipid data to fit a sigmoid growth function with a flexible point of inflexion did not change the apparently absent time trend of P-infinity. The estimates of the inflexion points of the fitted protein accretion curves, expressed as proportions of P-infinity, were indistinguishable from the fixed 0.368 value of the Gompertz function for the earliest three genotypes and then showed a tendency to increase, up to 0.46 for the 1993 population. These time trends must be the consequence of a combination of changes in nutritional and other environmental factors and genetic changes. They cannot be the sole result of within-line selection for growth and body composition traits, since this should increase P-infinity. It seems as if pig breeders have repeatedly initiated their sire lines from genetic resources with small mature size, to subsequently increase this trait as an indirect result of within-line selection.