PREDICTING AND MODELING FAT AND LEAN GROWTH IN SWINE OF DIFFERENT GENOTYPES AND SEXES USING COMPUTED TOMOGRAPHY

Abstract Text: Body composition of growing swine depends upon genotype and sex, however the effects of immunocastration need further study.  Computed tomography (CT) enables quantification of body components in live pigs, by converting X-ray attenuation values into quantitative pixel values. This study aimed to (1) estimate the amounts of fat and lean in pigs from 30 to 120 kg body weight (BW) using CT; (2) model the growth of fat and lean in pigs of different genotypes and sexes, including immunocastrated males, based on two data sets. The first included 20 each of Duroc x (Landrace x Large White), Pietrain x (Landrace x Large White) and Landrace x Large White gilts (G). The second included 12 each of G, boars (M), barrows (B) and immunocastrated males (I), Duroc x (Landrace x Pietrain); Improvac® was injected at 12 and 18 weeks of age. Pigs were fed ad libitum, weighed weekly and CT scanned at 30, 70, 100 and 120 kg. Subsets of pigs of each genotype, sex and target weight were harvested and dissections were performed to determine total fat and lean in primal cuts. Amounts (kg) of fat and lean were estimated for all animals, based on the volume of pixels within the fat density range (VF) and the fat free mass (FFM), respectively: FAT = 0.2209*VF^1.0184 (r² = 0.995, RMSE = 0.0571); LEAN = 0.6484*FFM^1.0625*VF^‑0.0175 (r² = 0.999, RMSE = 0.0095). Genotype and sex specific allometric regressions (Y = a*X^b) up to 120 kg BW showed that Pietrain-sired pigs were the leanest, Large White x Landrace the fattest and Duroc-sired, intermediate (a= 0.00990, 0.000887 and 0.00749; b = 1.417, 1.494 and 1.470, respectively; P < 0.001); M were the leanest and B the fattest, with G and I being intermediate (a= 0.00459, 0.000361, 0.00752 and 0.00212; b = 1.500, 1.626, 1.431 and 1.691, respectively; P < 0.001). Allometric coefficients for I reflect the transition from the male to the castrated condition at around 70 kg BW, and if grown to heavier weights immunocastrates would likely resemble barrows. In summary, the amount of fat and lean in pigs of different genotypes and sexes between 30 and 120 kg body weight may be estimated accurately and precisely from CT scan data. CT scans are a good predictor of body composition in growing pigs and should be useful for development of improved growth models.

Keywords: computed tomography, immunocastration, swine, body composition