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An Inbreeding Assessment of California Red Sheep

H. D. Blackburn and C. S. Welsh

National Animal Germplasm Program

Agricultural Research Service USDA

1111 S. Mason St. Ft. Collins, CO

Harvey.Blackburn@ars.usda.gov 

March 27, 2012

 

An evaluation of the California Red sheep was performed to determine the level of inbreeding that has accumulated since the association started maintaining pedigree records. In general, all breeds with closed herd/flock books experience an increase in inbreeding over time. The rate the inbreeding accumulates in a breed is a function of animal numbers, selection intensity, and how well breeders manage inbreeding levels via mating choices.

Inbred individuals are the result of mating parents that share a common ancestor (e.g., a common grand sire or grand dam). The genetic makeup of an individual tends to be more homozyogous than non-inbred individuals. While increased inbreeding tends to make individuals within a subpopulation more similar in performance and appearance it can also increase the frequency of mutant or lethal recessive genes. In fact, intentional inbreeding of mating sons to mothers or brothers to sisters is a common strategy livestock breeders’ have used to determine if a bull or ram carries an undesirable allele. The livestock sector has developed and used inbred lines at various points in time. If it is the intention to develop inbred populations this can be achieved with close management by the breeder. However, in general unplanned inbreeding levels can result in depressed performance. In general performance decreases are not particularly noticeable until inbreeding levels exceed 0.20 to 0.25.    

Data

The association’s pedigree file contained 2032 animals. Of these animals there were 215 unique sires and 847 unique dams.  The pedigree records were used as input to the program Animal Breeder’s Tool Kit to estimate the genetic relationship between animals and compute inbreeding levels for each animal in the file. The results were summarized and put in the following figures.

Results

               Figure 1 reports the number of animals registered by birth years. In 2004 and 2005 registrations peaked and then entered a period of substantial decrease. Registrations for 2011 are not complete, so it is anticipated that 2011 registration may be similar to 2010.

               Figure 2 illustrates the number of registered animals by birth year that had some level of inbreeding. The first year where 100% of all registrations were inbred was in 1997. This level of inbred individual did bounce some in subsequent years but by 2004 and continuing forward all sheep being registered were inbred.  

               Figure 3 gives the average inbreeding level for animals born in a specific year. Clearly as time has progressed the average inbreeding level has increased. During the last four years average inbreeding level per birth year ranged from .12 to .15. The regression line in Figure 3 is provided to illustrate the rate that inbreeding is increasing. It was found that inbreeding was increasing .004 per year. The equation provided can also be used to project future inbreeding levels assuming mating plans continue as in the past. Therefore by 2020 the average inbreeding level will be 0.216. At this level the potential for observing reduced performance due to inbreeding depression increases. 

               Figure 4 reports the percent of animal born in 2006-2011 with varying categories of inbreeding. Approximately 75% of the population has inbreeding levels ranging from 0.06 to 0.15. It is important to note that under current management conditions the peaks observed in Figure 4 will shift to the right. In-other-words inbreeding will continue to increase.

Conclusions and Recommendations

               This analysis would suggest that inbreeding is increasing in the California Red sheep breed. The results obtained are typical for small livestock populations. Over the long term with no changes in breed wide mating strategies the observed trend will continue eventually resulting in a substantial decrease in performance due to inbreeding depression. The following are some general recommendations the association may wish to consider.

               Due to the existing levels of inbreeding all breeders will need to monitor the genetic relationships between potential sires and dams and inbreeding levels of potential offspring computed before mating. This will allow breeders the opportunity to predetermine how much inbreeding will occur in future generations. However, this process will not stop the accumulation of higher inbreeding levels. As part of such a strategy breeders may want to decrease the ratio of rams:ewes. As this ratio nears 1.0 more genetic diversity will be maintained and inbreeding will proceed at a slow pace. However, this will greatly decrease selection intensity. It has been suggested that ram circles be formed as a tool to slow rates of inbreeding. However, this again is a stop gap measure and requires a substantial number of association members to act in a unified manner, which can be difficult to organize and maintain.

               An alternative strategy would be for the California Red Association to re-open its herd book and allow the registration of newly formed California Reds (Tunis x (Tunis X Blackbelly Barbados)). This strategy can drive inbreeding levels back to zero, depending on how widely breeders use the new composite animals. This strategy has been successfully used by a number of cattle breed associations, some with annual registrations of approximately 10,000 head per year. For example, the Brangus association allows a continual influx of newly formed animals into the herd book and they maintain the parentage of the animals as they go through the upgrading process. Not only does this allow inbreeding to be controlled it allows breeders to take advantage of the hybrid vigor generated. California Red breeders could also modify the Brangus approach and allow the formation of new California Reds for a number of years and then reclose its herd books. Either way breeders will how more options in how they may want to manage the breed’s genetics than trying to work within the confines of the existing population.  

 

 Rebuilding Plan for California Red Sheep Breed

1.  Registry will be re-opened to accept new foundation stock bloodlines approved by the registrar.

2.  A California Red Sheep Heritage Registry, separate and apart from the main registry, will be established to record the Tunis, Barbados Blackbelly and first crosses used to establish new foundation lines for the breed.

3.  New foundation lines must meet all California Red Sheep breed standards and be 62.5% to 75% Tunis and 25% to 37.5% Barbados Blackbelly (polled).  American Barbados are not acceptable bloodlines for this program.  Any Tunis or Barbados Blackbelly animals being used in this program must be registered with the California Red Sheep Heritage Registry prior to being used to establish new California Red Sheep foundation stock.

4.  Tunis sheep used in this program must be registered with the National Tunis Sheep Registry, Inc.

a.  In addition:

i.  Tunis sheep used in this program must be devoid of wool forward of the horn line. 

ii.  Tunis sheep used in this program must not have dewlaps.

5.  Barbados Blackbelly sheep used in this program must be registered with the Barbados Blackbelly Sheep Association International in the Barbados Blackbelly (polled) book.

a.  In addition:

i.  Barbados Blackbelly sheep used in this program must be devoid of wattles.

ii.  Barbados Blackbelly sheep used in this program should not have white.

6.  (Optional) Those participating in this rebuilding program should be cognizant that both Tunis and Barbados Blackbelly breeds are rare and animals used in this program should be used for 2-3 years and then returned to a breeder in their own breed.  Be respectful of the fact that these animals are needed by their respective breeds. 

7.  Participation in the Rebuilding Plan is limited to CA Red Sheep breeders in good standing for the past two years. However, a CA Red Sheep breeder may sponsor an independent non-member, provided that member joins the CA Red Sheep Association while in the rebuilding program. The sponsor would be required to verify in-person all applications for registration. Members of the California Red Sheep Registry participating in this program must submit five photos and a copy of the breed registration papers for any Tunis or Barbados Blackbelly used in this rebuilding program for the animal to be included in the Heritage registry.  Both parents must be in the Heritage Registry for a First Cross animal to qualify for application. First cross animals do not have to meet the breed standards of any of the breeds involved.

8.  Members of the California Red Sheep Registry participating in this program will submit an annual breeding report listing the ewes covered and indicating the covering ram. 

9.  Members of the California Red Sheep Registry participating in this program will submit an annual lambing report which includes flock numbers, genders, birth type (single, twin, triplet, etc), birth weight and color descriptions with 1-3 photos of each lamb. 

10.  Once a member has a candidate that meets the California Red Sheep Registry Standards and falls into the breed percentages outlined in line 3 above, they may submit an application for registry following the regular registration requirements for mail in registry.

11.   This program can be temporary or permanent.  The Board of Directors of the California Red Sheep Registry will review this program every five years beginning with 2017 to evaluate progress and assess need for continuation or changes to this program. 

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