Abstract: United States seedstock sheep producers have selected sheep for higher weaning and post-weaning weights using estimated breeding values for approximately forty years. Despite this, little work has investigated if the increased performance has inadvertently resulted in increased feed intakes, increasing total feed costs for the producer. Eighty range-breed ewe lambs (forty Targhee and forty Rambouillet) were enrolled in a residual feed intake (RFI) trial shortly after weaning at approximately four months of age. Feed intake was recorded utilizing a GrowsafeTM feeding system modified for sheep. Following a two-week adaption period, feed intakes were recorded daily for forty-two days. Body weights were collected at the beginning, end, and weekly throughout the trial period. Residual feed intake was calculated utilizing mid-trial body weights and feed intake measurements. Six ewes were removed from the trial due to failure to adapt to the feeding system. Linear regressions were utilized to determine the association between post-weaning weight estimated breeding values and RFI and body weight. As expected, there was a significant relationship (p< 0.05) between post-weaning estimated breeding value and body weight during the trial. No statistically significant relationship between post-weaning weight estimated breeding value and RFI was identified, indicating selection for increased body weight has not resulted in decreased feed efficiency during the growth phase. Additional feed efficiency measures will be collected on these same ewes at a later life stage to represent the maintenance phase of production as opposed to the growth phase. In addition, genetic and rumen microbial analysis is ongoing. Together these approaches will aid in the construction of a framework for eventual genetic and genomic selection of feed efficiency in U.S. range sheep.
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