The practice of choosing desirable sheep for breeding purposes started with domestication. Today sheep breeding or improvement has become a highly
technical management practice. Improvement is dictated by the desire to have particular products.
Factors to consider when breeding sheep
Size of the population – Population provides selection pressure or intensity
Genetotypic variation – Without variation selection is not possible.
Heritability – refers to the fraction of phenotypes or phenotypic superiority that can be transmitted. Reproductive traits have low heritability hence are a function of the environment.
Birth weights are due to other factors such as the uterine environment. Selection based on weaning weight is not very effective because weaning weight has maternal influence. Selection based on age at one year is better because maternal influence has been eliminated. The rate of gain from birth to weaning has low to medium heritability. Gestation length and fleece traits also have high heritability but milk production traits have medium.
Accuracy of selection – this is achieved by use of information that can measure traits therefore production records are required. Low heritability traits have problems with interpretation of records and estimation of errors.
Selection pressure – this is limited by reproduction rate and by individuals that are needed for breeding i.e. number of traits in the flock. Therefore intensity of selection will be dictated by the size of the flock, whether the flock is being expanded or reduced and the reproductive rate.
Selection objectives are difficult to define because each sheep breeder is faced with a unique set of circumstances that influence his selection objectives. For example
Different environments e.g. to develop a selection index in Kenya confinement in one environment is important
Access to different genotypes
Different personal goals
If information is available on the sheep flock then it is easy to develop a selection program. The choice of the program will depend on the annual genetic gain expected.
Considerable genetic progress in sheep is possible mainly because improvement depends on small segments of the population. For example in Kenya there are farmers like ADC who keep pedigree sheep and raise their own replacements. They generate cash from fat lambs production, mutton and sale of replacement. Some elite farmers produce pedigree flocks where accurate measurement of traits is important.
The actual breeding objectives will aim to improve efficiency in production of wool, mutton, fat lamb, milk and skin. For fat lamb consideration is in terms of reproductive rates, growth rates and good mothers. The final goal is to maximize profits. In order to improve efficiency records obtained under appropriate conditions are necessary.
Any selection program is punctuated by the following
The fact that sheep are managed under a wide range of conditions
The that they produce and provide a wide range of annual products
In designing a selection program, consider the following:-
Products desired influenced by market demands
The right breeds for the particular area capable of surviving
Relative economic weightings
It is difficult to determine relative economic values because of market behavior. Most markets are not stable hence there are no constant values. But it is important to develop some figures e.g. when keeping Dorpers in marginal areas consider the benefits in terms of
Water penetration of wool
Wool tolerance (rotting)
Review genotypes available before looking elsewhere
Actual program development
Choose the breed
Predict the rate of improvement
Know the heritability estimates of the priority traits and the phenotypic and genotypic correlation
Estimate the cost of selection program
NB: Traits with medium to high heritability can be improved through selection
Where heritability is low, traits expressed in one sex or traits are measured after death / slaughter the traits cannot be measured through selection but through progeny testing or family performance.
Genetic improvement returns or benefits accumulate over time but there is a limitation with sheep. A system of evaluating economic returns has not been established.
The genetic potential of a sheep can be changed through selection and cross mating. When farmers develop a selection program they have decisions to make i.e. whether they want a cross or a pure breed. In cross breeding, most farmers use it as a tool to improve the local breed. However, a cross breed is not possible without a pure breed.
For those interested in pure breeding, selection is a primary tool. For it to be effective, selection index is important and should be developed. Selection index is a definition of traits to be improved or expressed. It actually defines the objective of the farmer and aims at improving several traits simultaneously and establishing economic values of the traits. In selection, multiple trait selection is recommended.
But consider a few traits for improvement each time because if too many traits are involved the lesser would be the improvement in any one of them. Select or improve traits that have economic returns. Genetic progress through selection is slower than possible improvements through management (nutrition) but progress due to the genes is more permanent.
Traits of economic importance
Prolificacy and fertility
Heritability is low and phenotypic variations are high hence used as a basis of selection. Fertility is a function of the environment therefore different approaches are employed. For example use of twins from young mothers within the first and third lambing. Twinning can be increased by up to 5%. Fertility / prolificacy can be done by measuring twinning rate or number of lambs per year. Some farmers use lifetime productivity but there are limitations because records are required and performance is measured very late in life.
3. Growth rate
Lambs should be marked sooner and on farm risks should be reduced. measurements include birth weight of lambs. Although the birth weight is affected by maternal environment, it has a correlation with the rate of gain and survival. Weaning weight is therefore useful but correction should be done for maternal effects in the first 6 weeks of life.
75.5% of differences in lambs’ growth rate are due to milk production by the mothers.
4. Yearling traits
These are very accurate for selection hence preferred. use weight of sheep 60 days after weaning.
4. Carcas merit
Select for rate of gain
Fat deposit – those that deposit fat early do not have good growth rates. Dorpers tend to deposit subcutaneous fats. Select against fat deposition.
5. Fleece merit
Wool traits are high in heritability therefore can be improved through selection. Clean fleece weight has the greatest effect on economic returns than any other wool trait. Grease fleece weight and clean fleece weight are not positively correlated hence grease fleece weight can also be used for selection.
Staple length determines weight (quantity) and affects use of the wool. Fibre diameter, uniformity and crimps determine quality. White color is recommended.