Abstract:
The aim of this experiment was to determine whether divergent selection of parents on post-weaning residual feed intake (RFI) was accompanied by differences in feed efficiency in steer progeny on pasture. The Angus steers were the progeny of approximately one generation of divergent selection for low RFI (high efficiency; HE) or high RFI (low efficiency; LE). Pasture intake by 22 HE-line and 31 LE-line steers was measured during Spring using the alkane technique. The two groups of steers were of similar age (mean 15.5 months) at the start of the pasture-intake measurement period. Comparison of alkane profiles of pasture species and in faeces indicated that the steers were consuming predominately perennial ryegrass (Lolium perenne) and some tall fescue (Festuca arundinacea), with a trend (P<0.1) for the steer progeny of HE parents to have a higher proportion of ryegrass in their diet. Digestibility of dry matter (DM) in the diet consumed was not associated with genetic variation in RFI (P>0.1). Steers from the HE-line tended to grow faster than steers from the LE-line (0.50 v 0.42 kg/day; P<0.1), consistent with the negative regression coefficient for ADG with mid-parent Estimated Breeding Value (EBV) for RFI (-0.11; P<0.05). The difference in daily feed intake between the selection lines was not significant (3.04 v 3.23 kg DM/day; P>0.1), nor was the regression coefficient with mid-parent EBV for RFI (P>0.1). Feed conversion ratio (FCR) was 6.36 � 0.35 (se) for HE-line steers and 8.51 � 0.74 for LE-line steers but this difference was only significant at P=0.1. The positive regression coefficient for FCR with mid-parent EBV for RFI (2.9; P<0.1) provided further evidence for low RFI in the parents being genetically associated with superior efficiency of feed conversion on pasture of the steer progeny.
