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

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

Summary

Targhee x Rambouillet (n = 61; 7 months of age) ewes were evaluated on the GrowSafe System to determine individual residual feed values and ranking. Ewes were first tested on a concentrate diet (14.2% CP) for 62 days and then retested (10 months of age) with a forage diet (15.2% CP) for 66 days. Effects of diet on growth, intake and RFI ranking have been determined. Ewes (n = 18) were further selected to measure reproductive efficiency based on their forage ration RFI ranking: highly efficient (n = 6), moderately efficient (n = 6) and lowly efficient (n = 6).

Objectives/Performance Targets

Determine how age affects RFI ranking. This will be accomplished by comparing RFI measurements of forage-fed ewe lambs before and after attainment of puberty.

Estimate heritability of RFI in sheep and determine the impact of selecting for RFI on reproductive efficiency. This will be accomplished by 1) regressing lamb RFI estimates on respective dam RFI estimates, and 2) estimating correlations between dam RFI measurements and reproductive performance measurements.

Investigate the economic impacts of RFI selection on the sheep industry. This will be accomplished by performing cost-benefit analyses in collaboration with agricultural economist Dr. John Ritten. Information needed for this analysis, such as input costs, labor costs, etc., will be solicited from Western region sheep producers.

Accomplishments/Milestones

To determine how age affects RFI ranking, progesterone was measured in plasma collected from weekly blood samples. By 8 months of age, 10 ewes were confirmed to have undergone puberty from the presence of > 1ng/mL in plasma for two consecutive weeks; however, by 9 months of age, all ewes underwent anestrus as day length increased. Due to the low numbers of puberty estimates, the shift from how age affects RFI ranking to how diet affects RFI ranking was made. The original proposal aimed to further distinguish if diet or age contributed to the ranking differences observed between a pelleted diet and a chopped hay diet in ewes (Redden et al., 2010). Targhee x Rambouillet (n = 61; 7 months of age) ewes were evaluated on the GrowSafe System to determine individual residual feed values and ranking. Ewes were first tested on a concentrate diet (14.2% CP) for 62 days and then retested (10 months of age) with a forage diet (15.2% CP) for 66 days. Overall, average intake, ADG and MMWT were higher (P < 0.001) in the forage diet compared to the concentrate diet. There was a high positive correlation (r = 0.69; P < 0.001) between RFI rankings based on the concentrate and forage rations in both overall and selected ewes. Results indicate that there is no difference in feed efficiency when tested on a pelleted ration despite differences in feed composition.

To estimate heritability of RFI in sheep and determine the impact of selecting for RFI on reproductive efficiency the most efficient (n = 6), moderately efficient (n = 6) and the least efficient (n = 6) ewes were selected based on forage ration RFI ranking. Residual feed intake and average feed intake on the concentrate ration were lower (P ? 0.002) in the highly efficient and moderately efficient ewes compared to the lowly efficient ewes but did not differ (P = 0.161) between the highly efficient and moderately efficient ewes. As expected, RFI and average feed intake differed (P < 0.001) between the highly efficient, moderately efficient and lowly efficient ewes when on the forage diet.

Selected ewes were mated with a highly efficient ram (determined from the 2011 Summer Ram Test) and are expected to undergo parturition in March 2012. Age of puberty, day bred and pregnancy status did not differ (P ? 0.337) among selected ewes. Birth type, rearing type, birthweight, date of birth and sex will be recorded and regressed against the dam’s RFI value to determine relationships. At 9 months of age, lambs will be tested on the GrowSafe System to determine heritability and impacts associated with selecting for feed efficiency. An extension is expected to be requested to fulfill objective 2, as lambs must be greater than 36 kg to obtain accurate feed intake measurements with the GrowSafe System.

To investigate the economic impacts of RFI selection on the sheep industry, the assistance of agriculture economist, Dr. John Ritten, will be solicited. Currently, input costs and profits are being collected from local sheep producers in Wyoming, Colorado and South Dakota to determine the economic repercussions associated with incorporating RFI into their selection program. Participating sheep producers have collaborated with our group in previous research projects. Results from objective 3 are expected to be available by June 2012.

Impacts and Contributions/Outcomes

Results indicated that the least efficient animals remained in the bottom 15% of RFI ranked ewes regardless of diet (forage versus concentrate). However, re-ranking of RFI is possible in more efficient animals when diet is changed. For sheep producers, this means that genetic gain can most effectively be made by the selection against feed inefficiency rather than for feed efficiency. Additionally, particle size of the ration rather than the composition of the diet may play a bigger role in determining the feed efficiency status of an animal. This may be due to intrinsic differences in rumen bacterial populations to break down feedstuffs, differences in gut fill signals in the appetite regulatory pathway, or a combination of both. Further research will be required to elucidate the mechanisms associated with feed efficiency variation.

At this time, little evidence from this research is available to support that RFI selection unfavorably impacts reproductive parameters in sheep; however, preliminary reports in sheep and cattle have implicated that reproductive efficiency is impaired by the selection for RFI. Once lamb performance information is obtained, more comprehensive results will be available to determine the effects of selecting for RFI on reproduction.

Collaborators: