1. Determine selection goals. Where do you want your flock to be in 5, 10, 20 years?

2. Assess current production through recordkeeping of economically relevant traits.

3. Determine tools required to meet selection goals.

4. Consider components of key equation (selection accuracy, selection intensity, genetic variation, and generation interval).

5. Select replacement stock that is adapted to your environment and desired by your market.

Appropriate use of genetic tools and selection practices can have positive effects on small ruminant productivity and enterprise economics. After four generations, more than 90% of the genetics in a flock are the result of sire decisions when replacements are retained within the flock. Since just a few sire decisions may account for the vast majority of genetic potential within your flock, well-informed ram or buck purchases and mating decisions are essential.

Selection starts with goal setting and recordkeeping. Determine where you want your flock to be in the next 5, 10, or 20 years. You must have a goal to progress toward. Traits of economic relevance based on your goals can then be measured and recorded to provide a benchmark for moving forward. These records can be used to determine which traits need further emphasis and which traits may already be satisfactory. Attention should be placed on those traits that have an impact on the productivity and economics of your flock. Selection decisions will depend on your market and production system and may vary from farm to farm, even in the same region. Your flock needs to thrive in your environment and bring premiums, where possible, when marketed.

Any trait that contributes to the revenue and expenses of a flock is considered an economically relevant trait (ERT). These ERTs should be the focus of a selection program. Some ERTs may vary from operation to operation depending on markets and production systems, but many ERTs apply to all operations in the Southeast. Improving revenue-associated traits and managing expense-associated traits can improve the profitability of a small ruminant enterprise. These traits can be categorized as maternal, terminal, and fitness traits. Maternal traits include number of lambs or kids born and weaned, and milk production. In small ruminants, number weaned is frequently the most valuable ERT, as it has the greatest impact on revenue potential (Figure 1). This trait includes survivability related to dam mothering ability. Terminal traits are those associated with slaughter lamb or kid production. These include growth traits, muscling, and fat. The most common fitness trait is fecal egg count, as it is an indicator of parasite resistance or susceptibility.

Figure 1. Comparison of individual weaning weights (orange) with litter weaning weights (blue) by birth and rear type for sheep. As litter size increases, individual weights tend to decrease, yet total weight of the litter produced per ewe increases, resulting in greater return when marketed.

Data from Southwest Virginia Agricultural Research and Extension Center Katahdin sheep flock.

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Figure 1. Comparison of individual weaning weights (orange) with litter weaning weights (blue) by birth and rear type for sheep. As litter size increases, individual weights tend to decrease, yet total weight of the litter produced per ewe increases, resulting in greater return when marketed.

Data from Southwest Virginia Agricultural Research and Extension Center Katahdin sheep flock.

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Often, there is more variation in ERTs within a single breed than between breeds (Figure 2). For example, the difference between the most parasite-resistant Katahdin sheep and the most parasite-susceptible Katahdin sheep is greater than the difference in average parasite resistance between Katahdin and Texel sheep. Selecting the right breed is important. But identifying the right breeding stock within that breed is even more important. Find the breed that has the best potential to work in your production system and market. Then select individuals within that breed that will help you reach your production goals.

Figure 2. Variation within a breed is often greater than the variation between breeds for economically relevant traits.

Adapted from Lush, 1943.

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Figure 2. Variation within a breed is often greater than the variation between breeds for economically relevant traits.

Adapted from Lush, 1943.

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Appropriate use of selection tools can improve the accuracy of selection decisions (Table 1). Observable traits are called “phenotypes” and include structural attributes as well as data records. The appearance or performance of an individual is the result of that individual’s genetics (genotype) and the environment they were given in which to perform. Genetics can be passed from generation to generation, farm to farm, and year to year. Environment will vary. Care should be given to ensure selection decisions are not biased by environmental influences. Table 1 is a summary of selection tools and pros and cons to their use.

Since the traits we observe (phenotypes) are influenced by genetics and environment, we must control the environment to be able to accurately predict the contribution of genetics to animal performance. A contemporary group is a group of animals that has been given the same opportunity to perform (same environment). When the environment is the same, differences in performance within a contemporary group are likely a result of genetic differences. To accurately assess genetic differences in your flock, maintain sound contemporary groups with similar nutrition, health, and housing management. A greater number of animals in a contemporary group increases the opportunity for genetic variation within the group, improving selection potential for superior animals. An example of a contemporary group would be lambs born in a similar period (within 34 days), raised together with their dams in the same area (barn, pasture, etc.), vaccinated at the same time with the same vaccine, and provided the same nutritional opportunities (creep feed, creep grazing, etc.). Lamb or kid age, dam age, number born, and number reared in the litter will influence animal performance. These traits can be accounted for through adjusted performance records. Animals born in separate lambing or kidding seasons, managed in different barns or pastures, or fed different rations would be in separate contemporary groups and cannot be directly compared.

Phenotype = Genotype+ Environment

EBVs predict genetic merit. They are the most accurate selection tool available because they represent a genotype independent of environmental influences, such as management practices that may differ between operations. They combine not only the individual’s performance, but also the contributions of all other individuals in the known pedigree of the individual and contributions of correlated traits, making them more robust than a single data point on one individual at one point in time. EBVs exist for many ERTs. For more information about EBVs, trait definitions, and EBV use and interpretation, see the National Sheep Improvement Program.

Crossbreeding can be a powerful tool to improve animal fitness and marketability. Crossbreeding provides two primary benefits: breed complementarity and hybrid vigor (heterosis). Breed complementarity describes the opportunity to maximize the strengths and minimize the weaknesses of breeds. Find breeds to cross that suit one another well. Hybrid vigor is the superiority of the crossbred individual relative to the average of the straightbred parents. Levels of heterosis for a variety of traits in both crossbred lambs and crossbred ewes are summarized in Table 2. A common use of crossbreeding is terminal sire mating systems. Terminal sires can be used to improve market animal quality while maintaining females in the flock that excel for maternal traits. Terminal sires selected for growth and muscularity can be crossed with ewes selected for prolificacy and milk production, resulting in offspring that have improved marketability while not sacrificing ewe maternal performance. The greatest advantage of crossbreeding is generating a crossbred lamb out of a crossbred ewe.

Note: These values represent the percent increase in performance relative to the average of the straightbred parents. Data summarized from Nitter, G. 1978. “Breed utilization for meat production in sheep.” Anim. Breed. Abstr. 46:131.

\(∆G=\frac{Accuracy\ \times\ Selection\ Intensity\ \times\ Genetic\ Variation}{Generation\ Interval}\)

The equation above summarizes the components that contribute to genetic progress. ∆G indicates change in genetics (∆ stands for change, G stands for genetics). Genetic progress can be improved by increasing those components in the numerator (accuracy, selection intensity, and genetic variation) and decreasing those traits in the denominator (generation interval). Each component is described in greater detail below.

Accuracy: Accuracy values represent the relationship between the “estimated” breeding value and “true” breeding value. Increased accuracy results from greater records in the genetic evaluation (individual and progeny records). Accuracy can also be improved by using more accurate selection tools (EBVs vs. raw performance records, for example). Genomic technology is now available for Katahdin hair sheep, and its use can improve selection accuracy. Accuracy or genomics alone do not make an individual more genetically superior. They simply allow us to more accurately identify those individuals with superior genetic merit based on EBVs.

Selection intensity: Selection intensity (selection differential) is the difference between the selected population for breeding and the average of the population. By selecting individuals further from the average, greater intensity is applied to selection and greater progress can be made. Breed percentile reports can be used to identify superior individuals within the breed for particular traits. For example, selecting individuals in the top 5% vs. top 50% will allow for greater selection intensity. The 50th percentile is the breed average.

Genetic variation: Genetic standard deviation describes the variation in genotypes for a given trait within a population. Traits with more variation give us more opportunity to identify and select superior individuals. However, this component is relatively constant within the population and difficult to change.

Generation interval: The generation interval is the average age of the parents when the offspring are born. To increase genetic improvement, generation interval needs to be decreased. Therefore, greater use of ram lambs/buck kids and breeding ewe lambs/doe kids can be very beneficial. The youngest animals on a farm should have the greatest genetic potential. Therefore, the earlier they can enter the breeding population, the sooner those genetics can be multiplied. However, some ERTs, such as fertility and longevity, cannot be measured until later in life, so generation interval should be balanced with other opportunities for genetic improvement (accuracy, selection intensity, and genetic variation).

Culling is a form of genetic selection. Removal of individuals with lower genetic merit or decreased productivity due to environmental factors can improve the overall productivity and profitability of the operation. So when is it time to cull? Culling decisions may be based on a variety of factors. If it is costing more to maintain that animal than what they are returning or have the potential to return in the future, then it is economically beneficial to sell that individual. Common criteria for culling are outlined in Table 3. Individuals may also be culled for health-related reasons to minimize disease spread.

Note: These criteria are not meant to be exclusive and should be adapted to fit specific operations’ needs and goals.

In summary, genetics provide the foundation for productive livestock. Without favorable genetics, sheep and goats will perform only so well even given the best environmental conditions and management. To make measurable genetic progress, you need to first determine the goals of your operation. Next, assess current production status using records to create a baseline for improvement. Identify areas of weakness and then select breeding stock that will help improve these areas using the most accurate selection tools you have available (that is, EBVs). Ideally, find and select animals that are adapted to your production system and environment that possess traits sought after by your market and capable of generating premiums. Consider each component required to make genetic progress and continue to keep records to see if progress is being made. Finally, cull stock that is not living up to expectations. Be patient. Change takes time.