Direct and correlated responses to selection in two lines of rabbits selected for feed efficiency under ad libitum and restricted feeding: I. Production traits and gut microbiota characteristics.

To get insights into selection criteria for feed efficiency, 2 rabbit lines have been created: the ConsoResidual line was selected for residual feed intake (RFI) with ad libitum feeding and the ADGrestrict line was selected for ADG under restricted feeding (-20% of voluntary intake). The first objective of this study was to evaluate, after 9 generations of selection, the direct and correlated responses to selection on production traits in the 2 lines for traits recorded during growth. Second, applying the 2 feeding conditions used for selection to both selected lines plus the control unselected line (generation 0, G0) in a 2 × 3 factorial trial, the line performances were compared and the gut microbiota of the lines was characterized. The correlated responses in feed conversion ratio (FCR) were remarkably equivalent in both selected lines (-2.74 genetic σ) but correlated responses in other traits were notably different. In the ConsoResidual line, selection for decreased RFI resulted in a small negative correlated response in BW at 63 d old (BW63) and in a null response in ADG. In the ADGrestrict line, on the contrary, the correlated response in BW63 was substantial (+1.59 σ). The 2 selected lines had a FCR reduced by 0.2 point compared with the G0 line, and the same difference was found in both feeding regimens ( < 0.001). Indeed, selection on ADG would lead to heavier animals with no significant reduction of feed costs, whereas selection on RFI leads to lower feed costs and no increase of animal BW under ad libitum feeding. Altogether, our results do not suggest any genotype × environment interaction in the response to feeding regimens. The intestinal microbial communities from efficient rabbits differed from their unselected counterparts in terms of fermentation end products and microbial phylotypes, suggesting a central role of these microbes in the better feed efficiency of the rabbits.

Direct and correlated responses to selection in two lines of rabbits selected for feed efficiency under ad libitum and restricted feeding: II. Carcass and meat quality.

To get insights into selection criteria for feed efficiency, 2 rabbit lines have been created: the ConsoResidual line was selected for residual feed intake (RFI) with ad libitum feeding and the ADGrestrict line was selected for ADG under restricted feeding. The aim of the present study was to evaluate the impact on carcass and meat quality of the genetic selections. This comparison was performed using 2 different feeding strategies corresponding to the selection design. Carcass and meat quality traits were recorded for the 3 lines (ConsoResidual, ADGrestrict, and an unselected control [generation 0 {G0}]) in the 2 feeding systems (ad libitum and restricted) for 163 animals. Concerning the line effect, the BW at 63 d old was higher for the ADGrestrict line compared with the G0 and ConsoResidual lines ( < 0.0001). There was no line effect on the gastrointestinal tract. The rabbits did not exhibit a different carcass yield but showed different carcass traits. Indeed, the ConsoResidual rabbits had a higher hind leg yield ( < 0.0001) but no difference in the meat-to-bone ratio of the hind leg. On the contrary, the ADGrestrict line had a higher proportion of forelegs plus thoracic cage ( = 0.03). We also found lower perirenal ( < 0.0001) and scapular fat yields ( < 0.0001) in ConsoResidual rabbits. The ADGrestrict line had an intermediate perirenal fat yield compared with the other 2 lines. The G0 line always exhibited higher fat yields. Concerning meat quality, the ConsoResidual rabbits showed a lower ultimate pH ( < 0.0001) and higher water loss (drip and cooking loss; < 0.002) compared with the G0 and ADGrestrict rabbits. The feeding level had a strong effect on the gastrointestinal tract ( = 0.0004) and the carcass yield ( = 0.001). The latter was decreased in restricted rabbits. The effects of feeding strategy on meat quality were detrimental in the case of restricted feeding. Even if the ultimate pH was slightly higher in restricted rabbits ( = 0.0002), the carcass chilling loss ( = 0.0005) and the drip loss during 6 d ( < 0.0001) were higher. The selection on ADG with restricted access to feed seems to be less adversely correlated with carcass traits and meat quality traits but it leads to an increase in the live weight of the rabbits. Adversely, the selection on RFI, without affecting growth rate, would not modify age at slaughter because growth rate is not affected, but it might induce detrimental effects on meat quality.

Genetic parameters for two selection criteria for feed efficiency in rabbits.

Improvement of feed efficiency can be achieved by genetic selection directly on feed to BW gain ratio or for alternative traits. In the present study, 2 different traits were explored in the growing rabbit and their heritability and genetic correlations with traits recorded between weaning (30 d) and 63 d of age: i) residual feed intake (RFI), to select animals having low ad libitum feed intake independently from their production level, and ii) ADG under restricted feeding (ADGR; with a restriction level of 80% compared with ad libitum feeding of a control group), to select animals having high growth rate despite limited feed intake. To study these traits, 2 rabbit lines were established named i) ConsoResidual line and ii) ADGrestrict line. Under ad libitum or restricted feeding, it comes to select animals that waste less energy for maintenance, metabolism, or activity and retain more for tissue deposition. The selection process was similar in both lines. Data comprised records from generations 0 to 6 for about 1,800 rabbits per line measured for their BW at weaning and 63 d of age (BW63) and their individual feed consumption. Under ad libitum feeding, the heritability estimates were moderate for RFI (0.16 ± 0.05), ADG (0.19 ± 0.05), and feed conversion ratio (FCR; 0.22 ± 0.05). The high genetic correlation estimated between RFI and FCR (0.96 ± 0.03) was in accordance with the literature. The genetic correlation between RFI and ADG traits was not significant. Thus, selection for low RFI with ad libitum feeding was confirmed as a potential trait to improve FCR and reduce feed intake, with little effect on ADG. To our knowledge, there is no previous selection experiment on growing rabbits with restricted feeding. Our heritability estimates for ADGR and feed conversion ratio under restricted feeding (FCRR) were moderate (0.22 ± 0.06 and 0.23 ± 0.07, respectively) and had very high negative genetic correlation. Both selection criteria were found with high and favorable genetic correlations with feed efficiency recorded under each feeding regimen. However, their different genetic correlations with BW at weaning and at 63 d of age (BW63R; respectively, 0.85 and 0.17 for RFI and -0.25 and 0.81 for ADGR) suggested different impacts on major production traits that need further analyses to decipher the relative advantages of the 2 selection criteria, together with interactions between genotypes and feeding regimen.

A genome scan for QTL affecting resistance to Haemonchus contortus in sheep.

Gastrointestinal nematodes are one of the main health issues in sheep breeding. To identify loci affecting the resistance to Haemonchus contortus, a genome scan was carried out using 1,275 Romane × Martinik Black Belly backcross lambs. The entire population was challenged with Haemonchus contortus in 2 consecutive experimental infections, and fecal egg counts (FEC) and packed cell volumes were measured. A subgroup of 332 lambs with extreme FEC was necropsied to determine the total worm burden, length of female worms, sex ratio in the worm population, abomasal pH, and serum and mucosal G immunoglobulins (IgG) responses. Pepsinogen concentration was measured in another subset of 229 lambs. For QTL detection, 160 microsatellite markers were used as well as the Illumina OvineSNP50 BeadChip that provided 42,469 SNP markers after quality control. Linkage, association, and joint linkage and association analyses were performed with the QTLMAP software. Linkage disequilibrium (LD) was estimated within each pure breed, and association analyses were carried out either considering or not the breed origin of the haplotypes. Four QTL regions on sheep chromosomes (OAR)5, 12, 13, and 21 were identified as key players among many other QTL with small to moderate effects. A QTL on OAR21 affecting pepsinogen concentration exactly matched the pepsinogen (PGA5) locus. A 10-Mbp region affecting FEC after the 1st and 2nd infections was found on OAR12. The SNP markers outperformed microsatellites in the linkage analysis. Taking advantage of the LD helped to refine the locations of the QTL mapped on OAR5 and 13.