Estimation of Genetic Parameters, Genetic Trends, and Growth Curve Parameters of Pigs Selected for Residual Feed Intake Using Quantile Regression

The objective of this study was to estimate genetic parameters and genetic trends in pigs selected for low residual feed intake (RFI) and classified into different growth curve groups based on Quantile regression (QR) methodology. We used data on 750 Yorkshire pigs selected for low RFI for 5 generations, including data on average daily gain (ADG), average daily feed intake (ADFI), and Gompertz growth curve parameters (asymptotic weight [a], inflection point [b], and decay parameter [c]). We estimated QR growth curves for the whole population for three quantiles (0.1, 0.5, and 0.9) of the body weight (BW) data. Each animal was classified into one of the quantile regression groups (QRG) based on their Euclidian distance between each observed and estimated BW from the quantile growth curves. Genetic parameters were estimated for these traits and QRG. In addition, genetic trends for each QRG were observed. Three distinct growth curves were observed for animals classified into QRG_0.1, QRG_0.5, or QRG_0.9. Animals in QRG_0.1 had a greater (P<0.05) estimate for parameter a (266.5±11.5 kg) than animals in QRG_0.5 (250.8±9.9 kg) and QRG_0.9 (243.6±11.0 kg). In addition, QRG_0.1 animals had greater (P<0.05) estimates for b (187.9±4.5 days) and c (144.7±4.3 days) than animals in QRG_0.5, which had greater (P<0.05) estimates for b (163.2±3.6 days) and c (111.0±3.3 days) than animals in QRG_0.9 (144.7±4.3 days and 98.3±3.8 days, respectively). For all other traits, animals classified into QRG_0.1 had the lowest (P<0.05) ADFI (1.86±0.03 kg) and ADG (0.66±0.01 kg), whereas those classified into QRG_0.9 had the highest ADFI (2.23±0.03 kg) and ADG (0.85±0.01 kg). Estimates of heritability for growth curve parameters were low, with 0.13±0.07, 0.07±0.06, and 0.09±0.06, for parameters a, b, and c, respectively. For all other traits, estimates were moderate to high, with 0.50±0.09, 0.41±0.08, and 0.33±0.09, for ADFI, ADG, and RFI, respectively. QRG analyzed as a trait had moderate-high heritability (0.41) and it was genetically similar to ADG, with a genetic correlation of 0.8±0.08. The genetic correlation between QRG and RFI was moderate (0.46±0.11). Downward genetic trends of each QRG were observed for all traits as a function of selection for reduced RFI, with the exception of ADG. For ADG, QRG_0.1 was the only group that had a positive genetic trend. Altogether, these results indicate that quantile regression methodology was able to identify animals with different genetic potential for feed efficiency, bringing a new opportunity to improve selection for reduced RFI.