Effects of physiological age on residual feed intake of growing heifers

Using a retrospective evaluation of feed efficiency, this study investigated the effects of physiological age on residual feed intake (RFI) in growing heifers. Data were collected during 1973 and 1974 at the McGregor location of the Texas Agricultural Experiment Station. Heifers (n = 77) were obtained from a large crossbreeding program utilizing a five-breed diallel mating scheme using Angus, Brahman, Hereford, Holstein and Jersey breeds. At approximately 6 months of age, pre-pubertal heifers were individually penned and received ad libitum access to a balanced diet. Individual feed intake and body weight data were collected at 28-day intervals for 84 days prior to puberty and for 90 days after puberty. The diet was changed at puberty to provide a lower energy density. Considering all females as cohorts, RFI was calculated for each heifer for each period using separate models for the preand post-pubertal periods. A moderate, positive Pearson correlation (r = 0.48; P < 0.001) was detected between preand post-pubertal RFI. Furthermore, heifer RFI rank was compared between the preand post-pubertal periods using Spearman rank order correlation and a similar correlation (r = 0.46; P < 0.001) was revealed. This suggests that RFI determined during the pre-pubertal period may only be a moderate predictor of post-pubertal RFI. As a result, physiological age should be considered when evaluating cattle for feed efficiency using RFI.