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
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.
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
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
In designing a selection program, consider the following:-
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
Actual program development
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
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.
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