Review: divergent selection for residual feed intake in the growing pig

Abstract

This review summarizes the results from the INRA (Institut National de la Recherche Agronomique) divergent selection experiment on residual feed intake (RFI) in growing Large White pigs during nine generations of selection. It discusses the remaining challenges and perspectives for the improvement of feed efficiency in growing pigs. The impacts on growing pigs raised under standard conditions and in alternative situations such as heat stress, inflammatory challenges or lactation have been studied. After nine generations of selection, the divergent selection for RFI led to highly significant (P<0.001) line differences for RFI (-165 g/day in the low RFI (LRFI) line compared with high RFI line) and daily feed intake (-270 g/day). Low responses were observed on growth rate (-12.8 g/day, P<0.05) and body composition (+0.9 mm backfat thickness, P=0.57; -2.64% lean meat content, P<0.001) with a marked response on feed conversion ratio (-0.32 kg feed/kg gain, P<0.001). Reduced ultimate pH and increased lightness of the meat (P<0.001) were observed in LRFI pigs with minor impact on the sensory quality of the meat. These changes in meat quality were associated with changes of the muscular energy metabolism. Reduced maintenance energy requirements (-10% after five generations of selection) and activity (-21% of time standing after six generations of selection) of LRFI pigs greatly contributed to the gain in energy efficiency. However, the impact of selection for RFI on the protein metabolism of the pig remains unclear. Digestibility of energy and nutrients was not affected by selection, neither for pigs fed conventional diets nor for pigs fed high-fibre diets. A significant improvement of digestive efficiency could likely be achieved by selecting pigs on fibre diets. No convincing genetic or blood biomarker has been identified for explaining the differences in RFI, suggesting that pigs have various ways to achieve an efficient use of feed. No deleterious impact of the selection on the sow reproduction performance was observed. The resource allocation theory states that low RFI may reduce the ability to cope with stressors, via the reduction of a buffer compartment dedicated to responses to stress. None of the experiments focussed on the response of pigs to stress or challenges could confirm this theory. Understanding the relationships between RFI and responses to stress and energy demanding processes, as such immunity and lactation, remains a major challenge for a better understanding of the underlying biological mechanisms of the trait and to reconcile the experimental results with the resource allocation theory.

Keywords: feed efficiency; genetics; pig; residual feed intake; selection.

Figure 1

Genetic trends in the divergent selection experiment for residual feed intake (RFI) on component traits and meat quality expressed in genetic standard deviations of the traits (σ _g), obtained from a linear mixed model including an animal random effect structured by the pedigree relationship matrix. HRFI=high RFI line; LRFI=low RFI line; Index=selection index; DFI=daily feed intake; ADG=average daily gain; FCR=feed conversion ratio; carcBFT=backfat thickness measured on the carcass; MQI=meat quality index; LMCcalc=lean meat content of the carcass computed from a linear combination of cut weights.

Figure 2

Sensorial meat quality in the loin evaluated on a 0 to 10 scale (none to very high) in pigs from a low residual feed intake (RFI) line (LRFI, n=60) and a high RFI line (HRFI, n=57) after seven generations of selection. Red colour intensity and marbling intensity were appreciated on raw meat, the other traits were appreciated on cooked meat (dry heat for 10 min at 250°C and then humid heat at 100°C up to a core temperature of 80°C). †0.05<P<0.1, *P<0.05, **P<0.01, ***P<0.001. Adapted from Faure et al. (2013).

Figure 3

Curves for daily feed intake and digestible lysine requirements during the growing–finishing period of low residual feed intake (LRFI) and high RFI (HRFI) lines in generations G1 and G5 of selection as predicted using the INRAPorc^® model (n=1370). First, recorded BW were used to fit a Gompertz model to the repeated BW records. Next, the daily feed intake (DFI) records were modelled for each individual with a non-linear exponential model DFI=a×BW^b (Gilbert et al., 2009). Finally the daily digestible lysine requirements were calculated individually with InraPorc^® on the basis of modelled protein deposition and observed growth and DFI curves as described in Saintilan et al. (2015).

Figure 4

(a) Manhattan plot for detection of associations between SNP and residual feed intake (RFI) (n=149 low RFI pigs, n=121 high RFI pigs), using a genomic relationship matrix to account for the pedigree structure. The y-axis corresponds to the ratio between the variance explained by successive 0.3 Mb windows as estimated from a single-step genomic selection approach and the empirical 5% significance threshold at the genome level computed by simulation under the null hypothesis. (b) Chromosomal location of SNPs with suggestive (small italic) or significant (large bold) P for each group of traits (growth rate, feed intake and feed efficiency in black, carcass traits in dark grey and meat quality traits in light grey). Adapted from Riquet et al. (2014). FCR=feed conversion ratio; RFI=residual feed intake; DFI=daily feed intake; ADG=average daily gain; LMC=lean meat content of the carcass computed from a linear combination of cut weights; backfat=backfat thickness measured on carcass or backfat weight; Length=carcass length; Yield=carcass yield; pHu=pH determined 24h after slaughter on adductor, semimembranosus, gluteus superficialis or longissimus muscles; L=lightness, a=redness, b=yellowness, measured 24 h after slaughter on gluteus superficialis or gluteus medius muscles; WHC=water holding capacity; MQI=meat quality index.

Figure 5

Effects of line and high ambient temperature on rectal temperature and respiratory rate in growing pigs from the low residual feed intake (LRFI) and high residual feed intake (HRFI) lines. The thermoregulatory responses were modelled using the following equation: Y=y ₀+v ₁ h−r ₁(v ₁−v ₂) ln{1+exp[(d−th₁)/r ₁]}−r ₂(v ₂−v ₃) ln{1+exp[(d−th₂)/r ₂]} where Y is the response variable; d the day of exposure, y ₀ the value of Y on day 0 L; th₁ and th₂ the threshold days of the first and second phase of response, respectively; v ₁, v ₂ and v ₃ are the linear variations of Y before and after th₁ and th₂, respectively. Only the th₁ parameter for rectal temperature was significantly affected by the line (0.85 v. 1.88 day, for the LRFI and the HRFI line, respectively). Adapted from Campos et al. (2014).

Figure 6

Correlated responses to selection for residual feed intake (RFI) after seven generations of divergent selection for sow reproductive traits. LRFI=low RFI line; HRFI=high RFI line; SDFI=sow daily feed intake; loss-BW=body weight loss during lactation; loss-BFT=backfat thickness loss during lactation; D-LW21=litter weight gain from farrowing to 21 days of age; SRFI=sow residual feed intake during lactation. All line differences were significant at P<0.001. Adapted from Gilbert et al. (2012).

Figure 7

Impacts of the reduction of residual feed intake (RFI) on major physiological functions in growing pigs. GIT=gastro intestinal tract, AA=amino acid.