Breeding Objectives for the Western Range Sheep Industry
By Rodney Kott¹, Brent Roeder¹, Lisa Surber¹, Randy Borg², and David Notter²
¹Montana State University
Bozeman, MT 59717
²Virginia Polytechnic Institute and State University
Blacksburg, VA 24061
Background
Expected progeny differences (EPD’s) tend to be used for animal comparisons with each EPD focused on a single production trait. From their inception, EPD’s have been geared toward outputs, focusing on end products or weights with the presumption that more is better. However, in many commercial applications many traits, and relationships between traits, can greatly influence profit. A selection index is a formal method of combining EPD’s for different traits into one single value on which some selection decisions can be based. A profit or value index combines these production parameters with broad or general economic estimates. Whereas EPD’s have focused on measuring output of individual traits without any consideration to their relationship to other traits, profitability indexes offer a simplified approach to multi-trait selection and, with economic inputs, weight the value of outputs against the expenses incurred to achieve them. They enable breeders and their commercial ram buyers to select sires that maximize profit under commercial production situations.
Purpose Statement
Selection indexes for profitability were derived for Targhee sheep under several different management systems: 2 lamb pricing policies (with or without discounting of prices for heavy feeder lambs) and 2 production/forage cost scenarios (high or low). Traits included in the breeding objective were those for which EPD’s are calculated in NSIP. Relationships between these different traits and their relationship to profitability in western range sheep operations were examined.
Summary of Findings
Increasing the percent lamb crop born (PLC) generally had the largest impact on profitability, although an increase in weaning weight (WW) was almost equally important. Increases in PLC were recommended at all prolificacy levels. Many producers in range environments express a clear desire to avoid triplet births, but our results suggest that such a strategy will limit opportunities to increase profit. The impact on number of lambs weaned from increasing PLC was largest at the intermediate level of flock prolificacy (average litter size in adult ewes of 1.55 lambs per lambing). Even with low triplet survival (50%), the value of increasing PLC did not substantially decline until mean litter size was 2.15 lambs/ewe lambing. At a mean lambing rate of 1.55 lambs per ewe per lambing, every ewe that moves from having twins to triplets through selection is accompanied by 5 ewes that move from having singles to twins.
There are direct relationships between weaning weight, yearling weight (YW), and mature body size. Generally a 1-lb increase in weaning weight corresponds to a 2.5-lb increase in mature ewe weight. The relative importance assigned to increasing WW vs. limiting YW to control ewe maintenance costs varies with the management system. The impact of increasing ewe size on fitness under extensive grazing conditions remains subject to debate. If the range is heterogeneous and selective grazing is favored, larger ewes may be less capable of harvesting required quantities of preferred forages.
The scenario identified as most representative of typical western range sheep operations and validated such a production system involved pricing policies that discount prices for heavy feeder lambs and relatively low forage costs (termed the "Western Range Index"). In order to validate this index lifetime production comparisons between yearling ewes with high and low indexes are being evaluated (Table 1). These results clearly indicate that there is a direct relationship between the Western Range Index and pounds of lamb produced.
Conclusions
- Selection using the western range profitability index increased pounds of lamb weaned per ewe.
- Number of lambs born is the single trait most affecting profitability for the lambing rates evaluated in this analysis (1.41, 1.55, and 1.70 lambs/ewe lambing).
- In most scenarios, selection for increased weaning weight should be balanced by selection pressure to limit the increase in mature body size.
- There appears, at larger body sizes, to be a modest antagonistic relationship between increases in body size and stayability and ewe productivity. The point at which that antagonistic relationship becomes economically important to a particular producer is probably highly dependent on the quantity and quality of available feed resources.
Applications
Profitability indexes can be useful in assisting growers in formulating a plan for using multiple traits in selection. The goal is to encourage purebred producers to utilize genetic performance records and provide those records to commercial sheep producers to utilize in their ram selection program. This year, genetic records and selection index values were available on rams offered at the Montana Wool Growers Association’s Miles City Ram Sale ( www.sheepinstitute.montana.edu/08sales/RamSaleCat_9_9_final.pdf).
| Table 1. Comparison of three years lambing data for yearling ewes selected for high or low Western Range Index | |||||
| Treatment Group | Average Initial WRI of Group | Pounds of Lamb Weaned/Ewe Exposed by Year | Total Pounds of Lamb Weaned³ | ||
|---|---|---|---|---|---|
| Year 1 | Year 2 | Year 3 | |||
| Low WRI | $1.00 | 65.8 | 71.7 | 62.7 | 3,019 lbs |
| High WRI | $5.60 | 72.8 | 73.0 | 69.1 | 3,698 lbs |
| Difference | +$4.60 | +12.0 | +1.3 | +6.4 | 679 ibs |
| ³aThis value is the total pounds of lamb weaned from that group of ewes per year. Initially there were 20 ewes in each group. More ewes were culled for failing to lamb in the low WRI. | |||||
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