PSI-5 Relationships of Feed Efficiency and Growth Estimated Breeding Values in Range Sheep

Abstract

Abstract United States sheep producers have selected range sheep for larger post-weaning weights through estimated breeding values (EBVs) for the past forty years. This has led to producers perceiving more feed being required than historical averages. Therefore, the objective of this study was to quantify the relationship between lamb growth estimated breeding values, feed intake, and feed efficiency. Eighty ewe lambs (40 Targhee and 40 Rambouillet) were weaned at four months of age and put onto a GrowsafeTM feeding system for a 42-day feeding trial with a 14-day adaption period. Ewe lambs were weighed weekly and at the beginning and end of the trial. Body weights, average daily gain (ADG), and daily feed intake measurements were used to calculate Residual Feed Intake (RFI). The ewe lambs’ post-weaning weight EBV was associated to feed intake, mid-trial metabolic body weight, and RFI. These experimental procedures were repeated in a second trial when the same ewes were yearlings and at maintenance. While post-weaning weight EBVs were predictive to dry matter intake (P < 0.05), there was no relationship between post-weaning weight EBVs and RFI (P > 0.05), this association was constant across both trials. A ewe’s post-weaning weight EBV was also predictive of mid-trial body weight in both trials (P < 0.05). This suggests that while selection for higher post-weaning weight animals may increase sheep producers’ feed usage due to larger body size, feed efficiency is not associated with the post-weaning weight EBV. To investigate if the RFI measurements from the growth stage are predictive of maintenance requirements, we further compared the RFI, ADG, mid-trial body weights, and DMI measurements between the two trials. Growth RFI measurements were found not to be predictive of RFI in maintenance. Therefore, caution should be used when utilizing RFI from the growth phase as an indicator for feed efficiency in maintenance. Alternatively, we found that DMI from the growth stage was predictive of DMI of the maintenance stage (P < 0.05). Ewes’ mid-trial body weight in the first trial was also predictive of the mid-trial body weight of the second trial (P < 0.05). Mid-trial body weight was positively associated with DMI in both trials (P < 0.05). Thus, the differences in ewe body weight were likely driving the differences seen in dry matter intake. Overall, our results suggest that range producer’s selection towards sheep with larger post weaning weight EBVs may be increasing ewe DMI due to increased body size but is not negatively affecting the ewe’s overall feed efficiency.