The effect of preweaning growth restriction on the feed intake and efficiency of cattle on a grain-based diet before slaughter

Records on 78 steers and heifers were used to determine relationships among different measures of growth, feed efficiency and carcass traits, and to evaluate the effects of 2 preweaning growth rates on growth and efficiency in a feedlot. The preweaning growth rates [g/day; 871 (high) or 466 (low)] were imposed on the steers and heifers for 115 days before they were weaned at a mean age of 220 days. They were 'backgrounded' for 7 months on high quality pastures before finishing on grain-based diets in a feedlot for 85 days. The cattle were then slaughtered and carcass measurements recorded. Estimates were made on 10 growth and feed efficiency indices during the feedlot, and 5 carcass traits were assessed at slaughter. Of the indices, the Kleiber ratio which does not require measurement of feed intake (r = 0.98), was highly correlated with relative growth rate (RGR), as was feed conversion ratio (FCR, r = –0.82), which includes feed intake in the calculation. Residual feed intake, calculated using the expected feed intake estimated from the dataset (RFIREG), was correlated (r = 0.62) with RFI calculated from Australian feeding standards (RFISCA), and with FCR (r = 0.45). FCR was not correlated with either carcass rib or hot P8 fat depths but was significantly correlated with carcass weight (0.39), dressing percentage (0.41) and eye muscle area (0.33). RFIREG was not correlated with any of the 5 carcass traits. Whilst the average daily gain in the feedlot was similar between the 2 preweaning groups (1.544 kg/day), the low group was significantly (P<0.05) lighter at feedlot entry (323 v. 376 kg). Daily feed intake was lower (P<0.01) in the low group (10.9 v. 11.8 kg) such that the FCR tended to be lower (P<0.10) relative to the high group. Differences between the preweaning treatment groups were not significant for the other measures of feed efficiency. The results indicate that early life nutritional restriction has some influence on FCR in the feedlot, but little effect on those feed efficiency measures, such as RFI, that account for energy required for both maintenance of liveweight and growth.