The records of 263 young Angus (AN) bulls representing 21 sire families and 270 young Hereford (HE) bulls representing 26 sire families were analyzed to evaluate the influences of dietary energy planes on growth and heritability estimates of postweaning growth traits in young beef bulls. AN bulls were heavier throughout the test period and grew faster in the first 140-d of the test than HE bulls. In the last 28-d period, growth rates in the AN and HE bulls were similar. Although bulls receiving medium-energy diet (MED) were 2.3 kg heavier than those receiving high- energy diet (HED) at the start of the test, the bulls on the HED were 78.7 kg heavier than those on the MED at the end of the test, indicating that the HED resulted in high growth performance of feedlot bulls. Although the estimate of direct heritability (h2a) for start of test weight (WTO) was somewhat higher in the HED line (0.19) than in the MED line (0.11), the estimates of h2a for end of test weight (WT1 68), average daily gain on test (ADG0_1 68) and relative growth rate on test (RGR0_1 68) were much higher in the MED line than in the HED line. Between the HED and MED lines, maternal heritability estimates (h2m) for WTO and ADG0_168 were similar (0.14 vs. 0.12 and 0.27 vs. 0.24, respectively). The estimate of h2m for WT168 was much higher in the HED line (0.28) than in the MED line (0.17). The reverse was true for RGR0_1 68, with h2m being 0.12 in the HED line and 0.33 in the MED line. All direct-maternal genetic correlations (ra™) were negative. The estimates of ram for WTO were high (-0.99) in both the HED and MED lines Estimates of ram for WT168, ADG0_168 and RGR0_168 were -0.98, -0.62 and -0.86, respectively, in the HED line compared with -0.54, -0.31 and -0.21, respectively, in the MED line! indicating that the antagonism between direct and maternal genetic effects for postweaning growth traits in the MED line was much less than that in the HED line. Total heritability estimates in the MED line (0.25 to 0.41) were substantially larger than those in the HED line (0.09 to 0.14) except for WTO, indicating that after many generations of selection, the MED line retained more genetic variation in postweaning growth traits compared with the HED line. Therefore, selection on individual performance would be more effective in the MED line than in the HED line.

D. RC Bailey, M. F Liu, S. H Shannon

Proceedings of the World Congress on Genetics Applied to Livestock Production, Volume 17. Genetics and breeding of dairy and beef cattle, swine and horses, , 233–236, 1994
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