# A Genomic and Phenotypic Investigation of Feed Efficiency and Growth Traits in Targhee and Rambouillet Sheep

## TL;DR

This study investigates whether breeding sheep for higher weight has reduced feed efficiency, finding no link between weight and efficiency but identifying a genetic marker associated with feed intake.

## Contribution

The study identifies a genetic marker in DMRT2 associated with feed efficiency and confirms no indirect reduction in efficiency from selecting for higher weights.

## Key findings

- Post-weaning weight EBVs were associated with dry matter intake but not with residual feed intake.
- A single SNP within DMRT2 was associated with feed efficiency metrics in the second experimental period.
- Selecting for higher weights did not reduce feed efficiency, as EBVs for weight were not linked to RFI.

## Abstract

Over the past forty years, United States sheep producers have selectively bred range sheep for larger post-weaning weights using estimated breeding values as part of their breeding objectives. However, producers have observed increases in feed intake, prompting concern that higher weaning weights were inadvertently lowering feed efficiency of their animals. To test this hypothesis, 81 ewe lambs were enrolled in a feed efficiency trial to measure residual feed intake during growth and maintenance phases and genotyped to determine the relationships between these traits. A single genetic marker within DMRT2 was associated with feed efficiency, warranting further investigation into its possible role in feed efficiency and growth. No relationship was observed between feed efficiency and weight estimated breeding values, indicating range sheep producers have not indirectly reduced feed efficiency as they selected for higher weights.

The U.S. range sheep industry uses estimated breeding values (EBVs) as part of their breeding objectives to increase post-weaning weight. The study objective was to quantify the relationship between lamb growth EBVs, feed intake, and feed efficiency. Eighty-one range ewe lambs were enrolled in the study to measure residual feed intake (RFI) over two 42-d periods at both the weaning and yearling stages. The ewe lambs’ post-weaning weight EBVs (PWWT EBVs) were linearly associated with their phenotypic traits. Preliminary genome wide associations (GWAs) were also performed with Dry Matter Intake (DMI), RFI, mid-test body size, and average daily gain (ADG) and Ovine 50K SNP genotypes. Post-weaning weight EBVs were associated with dry matter intake (DMI) (p < 0.05) but had no association with residual feed intake (RFI) (p > 0.05) in both experimental periods. However, PWWT EBV was predictive of mid-test body weight in both periods (p < 0.05). A single SNP at Oar2:68,812,505, located within DMRT2, was associated with DMI and RFI in the second experimental period (Bonferroni corrected p <0.05). While selecting for higher post-weaning weight range ewes may increase feed consumed due to a larger body size, it was not associated with feed efficiency.