Impacts of age on residual feed intake and its effect on reproductive parameters and profitability in ewes

Final Report for GW11-007

Project Type: Graduate Student
Funds awarded in 2011: $24,990.00
Projected End Date: 12/31/2012
Grant Recipient: University of Wyoming
Region: Western
State: Wyoming
Graduate Student:
Principal Investigator:
Dr. Kristi Cammack
University of Wyoming
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Project Information


Feed intake measurements were collected on Targhee x Rambouillet (n = 61) ewes to determine residual feed intake (RFI) ranking. Effects of diet on growth, intake and RFI ranking were determined. Ewes were further selected to measure reproductive efficiency based on their forage ration RFI ranking: highly efficient (LRFI; n = 6), moderately efficient (MRFI; n = 6) and lowly efficient (HRFI; n = 6). Surveys from Wyoming sheep producers were used to evaluate producer knowledge of RFI, willingness to incorporate RFI into current breeding strategies, adoption of new technology, and to determine the economic impacts of RFI selection.


For residual feed intake (RFI) to be an appropriate measure of feed efficiency, it must not be unfavorably associated with traits of economic significance such as carcass merit, growth traits or reproductive efficiency. Research has suggested that RFI is independent of carcass and growth traits (Koch et al. 1963; Francois et al. 2002; Baker et al. 2006), but little information is available on the relationship with reproductive performance. Redden et al. (2010a) reported RFI ranking differences in Targhee ewes (9 months and 13 months age) tested on a pelleted grower diet (16% CP) compared to a chopped grass hay maintenance diet (8% CP). However, it is unknown if differences were due to size of feedstuffs used, growth stage of the ewes or type of diet. Furthermore, Redden et al., (2010b) suggested that selection for low RFI in ewes resulted in unfavorable reproductive efficiency as more efficient ewes produced less offspring than less efficient ewes, and twin born ewes were less efficient than their contemporaries. Cattle, swine and poultry industries have extensively researched the effects of RFI; however, little information is available on the economic repercussions associated with incorporating RFI into individual breeding programs.

In cattle, selection for feed efficiency could result in a 9-10% reduction in maintenance costs, 10-12% decrease in feed intake and 25-30% reduction in methane emissions (Nkrumah et al., 2006; Hegarty et al., 2007). We hypothesized that RFI ranking would differ in ewes fed a concentrate versus a forage-based diet, and that reproductive measures would differ in ewes more or less feed efficient. Furthermore, economic estimates for cattle may differ from sheep as the two industries differ greatly in structure and breeding emphasis.

Project Objectives:

Objective 1. Determine how age affects RFI ranking.

Objective 2. Estimate heritability of RFI in sheep and determine the impact of selecting for RFI on reproductive efficiency.

Objective 3. Investigate the economic impacts of RFI selection on the sheep industry.


Click linked name(s) to expand/collapse or show everyone's info
  • Dr. Kristi Cammack
  • Rebecca Cockrum


Materials and methods:
Animal procedures

All animal procedures were approved by the University of Wyoming Animal Care and Use Committee. After an adjustment period of two months for diet and the GrowSafe System, Targhee x Rambouillet (n = 61; 7 months of age) ewes were tested on a pelleted concentrate diet for 62 days. Ewes were retested at 10 months of age on a pelleted forage diet for 66 days after an adjustment period of two weeks. Individual feed intake information was measured using the GrowSafe System and weekly weights were collected throughout the performance trials to determine RFI. Weekly jugular blood samples were collected and stored at -20 C for progesterone analyses. Ewes were selected based on their forage ration RFI measure to evaluate effects of RFI selection on reproductive measures. The top 10% RFI ranked ewes (LRFI; n = 6), the middle 10% RFI ranked ewes (MRFI; n = 6) and the bottom 10% RFI ranked ewes (HRFI; n = 6) on the forage diet were selected.

Upon cessation of the feeding trials, ewes (19 months age) mated with a pre-determined top 15% ranked (8 out of 70 rams) feed efficient Hampshire ram (RFI = -0.2568 kg/d) from the University of Wyoming Meat Breed Ram Performance Test. Reproductive traits measured for ewes included: pregnancy (0 = not pregnant and 1 = pregnant), estimated day bred (presence of estrus and ram marking), puberty date (2 consecutive days of 1 ng/mL plasma progesterone), mothering ability (1-4 scale), utter score (1-4 scale), dystocia (0 = normal birth and 1 = lamb/dam died). Reproductive traits measured for lambs included: lamb sex (male = 1 and female = 2), birth weight (BW) and lamb vigor score (1-4 scale).

Survey procedures

Human research procedures were approved by the University of Wyoming Institutional Review Board. Producers (n = 14) who consistently participate in the University of Wyoming Ram Performance Test agreed to complete initial and post study surveys gauging their knowledge of RFI, acceptance of incorporating RFI into current breeding programs and the use of technology such as marker-assisted selection (Table 1). Producers (n = 4) were then contacted to gather pricing details on rams sold (n = 65) from performance tests and determine relationships between slaughter weight attainment and feed efficiency in rams (n = 378).

Statistical procedures

Residual feed intake rank differences were determined using the CORR procedures in SAS. The GLM procedure in SAS was used to evaluate differences in RFI and ADG, MMWT and average feed intake between diets and between each week. Selected ewes (LRFI, MRFI, HRFI) were analyzed for differences in RFI values, RFI ranking, ADG, MMWT, average feed intake and reproductive measures using the GLM procedure in SAS. The FREQ procedure in SAS was used to analyze survey responses before and after producers were provided information on RFI and ram RFI ranking results. Raw counts for operation size, type of operation and breeds used were collected from producers for operation demographics. Relationships between RFI and ram sale price and the age on test at which rams achieved slaughter weight were determined using the CORR procedure in SAS.

Research results and discussion:
Objective 1: Determine how age affects RFI ranking.

Average daily gain, MMWT, and average daily intake were higher (P < 0.001) in the forage diet compare to the concentrate diet; however, RFI did not differ (P = 0.999) between rations. As expected, component traits ADG and MMWT were not correlated (P = 1.000) with RFI in either concentrate or forage rations. By definition, RFI is independent of traits (ADG and MMWT) used to calculate expected feed intake (Koch et al., 1963; Arthur et al., 2001b; Basarab et al., 2003). Results confirm previous research that RFI is highly positively correlated with actual feed intake (Arthur et al., 2001b; Baker et al., 2006). Though RFI variation was consistent within each diet, it was notably higher (P < 0.001) on the forage ration than the concentrate diet. This may be attributed to feed conversion inefficiencies associated with forage-based rations or differing growth stages of individual ewes. Residual feed intake ranking did not differ due to diet, and there was a positive correlation (r = 0.69; P < 0.001) between RFI rankings across the diets (Figure 1). Correlations for cumulative weekly RFI estimates between rations were moderate to high (r ? 0.35; P ? 0.006) from week 1 to the end of each trial. Though ewes differed in actual feed intake, RFI ranking was highly correlated between rations. Redden et al. (2010a), however, detected RFI ranking differences between pelleted and chopped hay diets in ewes. Differences may be associated with ration form as previous research used both pellets and chopped hay rather than comparing rations similar in form. Both current and previous research suggests that ration form rather than diet composition may have a more significant effect on feed efficiency.

Due to increased variation and the most recent intake measurement, RFI selection was based on the forage diet. In the concentrate ration, LRFI and MRFI ewes had lower (P ? 0.002) RFI values and lower average feed intakes than HRFI ewes; RFI tended (P = 0.071) to be lower in LRFI ewes compared to MRFI ewes. In the forage ration, LRFI ewes had lower (P < 0.001) RFI estimates compared to MRFI and HRFI ewes, and HRFI ewes had higher (P < 0.001) RFI values and average daily intake compared to LRFI and MRFI ewes (Table 2). Feed efficient ewes (LRFI) consumed 33% less feed than HRFI ewes on the concentrate diet, and 39% less feed on the forage diet than HRFI ewes with a 6% difference between diets. Redden et al. (2010a) also reported that low RFI Targhee ewes fed a pelleted grower diet at 9 months of age and then a chopped grass hay diet at 13 months of age had a 19% reduction in DMI on the pelleted diet and a 29% reduction in DMI (Redden et al., 2011). Differences in performance may be attributed to form of feedstuff, as both rations were in pellet form but had similar diet composition. Results suggest that despite pellet composition, form may play a larger role in individual animal feed efficiency. There was a high positive correlation (r = 0.69; P < 0.001) between LRFI, MRFI and HRFI ewe RFI ranking on the concentrate diet and LRFI, MRFI and HRFI ewe RFI ranking on the forage diet (Figure 2). Less efficient ewes remained inefficient regardless of diet. Therefore, selection against feed efficiency rather than for feed efficiency may enable operations to more efficiently achieve genetic gain for RFI.

Objective 2: Estimate heritability of RFI in sheep and determine the impact of selecting for RFI on reproductive efficiency.

There was no effect (P ? 0.269) between RFI and first conception rate, date bred, achievement of puberty, mothering ability, utter score or difficulties during pregnancy with selected ewes (LRFI, MRFI, or HRFI). No relationship (P ? 0.221) was detected between dam’s RFI status and lamb birth type, sex or vigor score. However, lambs born to LRFI dams had lighter (P = 0.047) birth weights than lambs born to HRFI ewes. Lambs born to MRFI ranked ewes did not differ (P ? 0.123) in BW between LRFI and HRFI ewes (Table 3). Redden et al. (2010b) suggested that highly efficient ewes produced less offspring than lowly efficient ewes, and twin born ewes were less efficient than singles. Less efficient ewes had higher expected progeny differences for live count percentage and weaning rate index compared to more efficient ewes. No differences in offspring number or lamb vigor were detected in the current research. However, lambs born to more efficient dams had lighter birth weights than those lambs born to inefficient dams.

Objective 3: Investigate the economic impacts of RFI selection on the sheep industry.

Approximately 28% of surveyed sheep producers had < 100 head of sheep, 56% had 101-1,000 head of sheep and 16% had > 1,000 head of sheep in their operation. The sheep operations surveyed were composed of 68% purebred, 16% commercial and 16% a combination of operational types. The majority of ranchers surveyed produced dual-purpose breeds (56%). Surveyed producers agreed that they had a selection program for feed efficiency and were knowledgeable about RFI. Selection for traits that unfavorably affect reproduction, carcass merit and wool traits should be avoided. If available, at least 50% of sheep producers would use genetic markers for feed efficiency. Overall, there were no differences (P ? 0.133) survey answers before or after producers were provided with information on RFI and the performance of their rams. In the final survey, 100% of producers agreed or strongly agreed that they would select for RFI if such information were available. Results agree with producers from the Red Angus Association of America that at least 43.6% of commercial and seedstock producers were willing to adopt RFI as an alternative measure of feed efficiency based on RFI data.

The outcome (i.e. ram sold versus ram used for breeding) was not dependent (P = 0.917) upon RFI status. There was no relationship (r = 0.14; P = 0.402) between RFI and the price of rams sold (Table 4). Furthermore, there was no correlation (r = 0.01; P = 0.829) between RFI and the age at which rams achieved slaughter weight while on test (Table 4). Because rams in the current study were tested in a feedlot design, economic estimates were most applicable to the feedlot side of the production system. Results from this study indicated that RFI selection has no effect on ram sale price nor is there a relationship between RFI and attainment of slaughter weight. Therefore, data does not support an immediate increase in profitability from RFI selection in a feedlot situation. Long-term savings in feed costs may prove to increase profitability for sheep producers.

Participation Summary

Research Outcomes

No research outcomes

Education and Outreach

Participation Summary:

Education and outreach methods and analyses:

As cited below, an annual report for the University of Wyoming Department of Animal Science was generated. Departmental reports are then sent to area producers and department alumni to provide benefactors of the current research being conducted. Results were also disseminated at the International American Society of Animal Science Annual Meeting in Phoenix, AZ in July 2012. Results were presented to regional producers at the Colorado Nutrition Roundtable in the fall of 2011. An invited talk, “Incorporating Feed Efficiency with Selection Programs in Sheep,” was given at American Sheep Industry and National Lamb Feeders Association Annual Convention in Scottsdale, AZ in January 2012. Finally, this research was included in the dissertation, “Improving Feed Efficiency Traits in Sheep through Genetic and Reproductive Analysis,” of R. R. Cockrum that was successfully defended in July 2012.

Publication citations

Cockrum, R. R., S. L. Lake, R. H. Stobart, C. Molle, K. M. Cammack. 2011. Effect of pelleted concentrate versus forage diets on feed efficiency ranking in ewes. University of Wyoming Department of Animal Science Annual Report.

Cockrum, R. R., S. L. Lake, R. H. Stobart, K. M. Cammack. 2012. Effect of concentrate versus forage diet on feed intake and reproductive traits in crossbred ewes. American Society of Animal Science Joint National Meeting, Phoenix, AZ.

Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.