Selection for feed efficiency: Direct and correlated responses on production traits in two rabbit lines selected under ad libitum and restricted feeding

Abstract Text:

Two alternative traits to feed to gain ratio (FCR) were studied in growing rabbits to compare strategies for the genetic improvement of feed efficiency: Residual Feed Intake (RFI), to select low ad libitum feed intake independently from production level, and Average Daily Gain (ADG) under fixed restricted feeding (ADG_R) (80% of ad libitum). One line has been selected for each trait (generations 0 to 9) from the same population G0. Records comprised about 2 450 rabbits per line, for body weights at weaning (30 days of age) (BW30) and slaughter (63 days of age) (BW63) and feed consumption, and ADG, FCR and RFI were computed. Heritability and genetic correlations were computed, together with responses to selection in each line. Selection criteria showed similar heritabilities (0.21±0.04 for RFI, 0.29±0.05 for ADG_R) and responses to selection in the two lines (-0.34 genetic standard deviations (σ²_a) per generation for RFI, +0.29 σ²_a per generation for ADG_R). Responses to selection were -0.30 σ²_a per generation on FCR in both lines, and genetic correlations with selection criteria were close to unity. Genetic correlations between selection criteria and BW30 and BW63 differed between lines: estimates with BW30 and BW63 were 0.72±0.33 and 0.10±0.17, respectively, with RFI, and -0.44±0.19 and 0.63±0.15 with ADG_R. The genetic correlation in the RFI line between RFI and ADG was -0.09±0.14. Consequently, responses to selection were null on ADG and limited on BW63 in the RFI line, whereas they were highly significant in the ADG_R line. To further examine responses to selection, in generation 9 the two selected lines have been compared with the G0 control population (using frozen embryos) under ad libitum and restricted feeding by testing 30 individuals per line and feeding level combination. All lines had the same BW30. The ADG_R animals fed ad libitum ate the same amount as G0 animals. When fed ad libitum RFI animals grew at the same rate as G0 rabbits and ADG_R animals grew faster than these groups (P<0.001), whereas both selected lines grew faster than G0 animals when restricted (P<0.001). As a result, despite different feed intakes and BW63, independently from the feeding level both selected lines had a similar improved FCR (2.62±0.02) compared to the G0 line (2.82±0.02). Thus, the breeding objectives for body weight at slaughter and feed intake are the key to choose a selection criterion to improve feed efficiency, together with the feeding level during selection.

Keywords: Feed efficiency, residual feed intake, Average daily gain