Reproduction and breeding values - selection tool that works!

 

Weaning percentage is the single most important factor contributing to higher reproduction and therefor to flock profitability. This trait can be divided into:

·          Fertility (ewes lambed/ewes joined)

·          Fecundity (lambs born/ewes lambed)

·          Survival (lambs weaned/lambs born)

Selection for multiple births is an efficient method of improving reproduction.

·          Mortality at birth and the desertion/starvation/exposure complex were responsible for the majority of lamb deaths.

·          Selection against ewes that fail to lamb is now regarded as the most important factor for higher reproduction.

·          Selection against reproductive failure by using the simple wet and dry technique to be a low input method for improvement of reproductive performance in sheep flocks.

·          The lifetime reproductive performance of ewes should be carefully recorded and breeders must be encouraged to apply direct selection methods for improvement of reproduction.

·          The reproductive performance of ewes is an important criteria for selection of rams.

The factors to be taken into account are:

·          Age at first conception

·          Regularity of conception

·          Fecundity

·          Survivability of lambs

Body weight, wool production and fibre diameter

The price paid per kilogram of wool reflects the influence of wool characters on processing performance.

·          The important parameters of a top are fibre diameter, hauteur (mean fibre length of the top), coefficient of variation of hauteur, dark fibre content, average colour and vegetable matter.

·          Hauteur in turn influenced by fibre diameter, tensile strength, staple length, position of break and vegetable matter.

·          Mean fibre diameter is overwhelmingly the determinant of price per kilogram.

·          The most important decision for a breeder to make is how much emphasis to place on fibre diameter.  The weighting applied to fibre diameter will reflect the breeder’s attitude to future market premiums for micron.

·          Other raw wool characters should be added to the breeding objective as desired. 

·          Staple strength may be considered a priority.

·          It is important to keep in mind the relationship between various raw wool traits. 

·          Selection for lower mean fibre diameter will also decrease variation in fibre diameter (CV).

·          Selection for lower fibre diameter variation will lead to considerable progress for increasing staple strength, without the considerable expense of measuring it directly.

·          Increasing bodyweight will result in a slight increase in fleece weight and fibre diameter.

 

A procedure for the selection of sheep to increase the total income per sheep by recording body weight, clean fleece weight, fibre diameter and wrinkle score, combining these in a single index according to the relative economic importance and selecting sires on their ranking was developed.

·          Body weight at 365 days, clean fleece weight and fibre diameter are combined into a selection index for the purpose of ranking young rams in order of the relative economic value for wool sheep.

·          While selecting for cash value characteristics, woolled sheep breeders should be ever mindful of the relationship between the various production traits and the effect that this will have on the physiological functions of the animal.

·          There is a need to continually evaluate the results being achieved by the selection strategy applied, and a need to modify and adapt according to changing economic and environmental circumstances.

·          Only then will the productivity and profitability of woolled sheep flocks be improved.

The main selection objectives for ram breeding flock should be:

·          Hardiness and adaptability

·          Reproduction and maternal ability

·          Lamb growth rate

·          Clean fleece weight

·          Lowering fibre diameter

        

Success of selection

The success achieved by selection for a particular trait is basically dependent on the following factors:

·          How accurately an animal is identified for superior genetic merit (accuracy of breeding value prediction).

·          What is the variation within the group of animals from which selection takes place and what proportion of the variation is due to actual genetic differences (hereditability)

·          What proportion of animals are required as parents (selection differential).

·          The inclusion of several factors, which adversely affect each other, will limit progress in each individual trait (genetic correlation).

Estimated breeding values

·          The estimated breeding value (EBV) is the predicted genetic value of an animal as a parent compared with other potential parents.

·          Since selection is about picking parents, this measurement is important to a breeding program.

·          The estimated breeding value assumes that parents are of little interest, except as producers of replacements.

·          Without doubt the most powerful tool to arrive so far, that enables breeders to make more accurate and predictable breeding decisions

Genotypes and breeding values

·          The genotype is the set of genes that affects a particular trait.

·          Each parent passes a sample half of their genes on to their progeny.

·          So, siblings can have various genotypes because they have obtained dissimilar genetic samples.

·          The value of a genotype, or the breeding value, is the sum of individual gene effects.

·          Breeding values cannot be measured exactly. They are always a estimate.

·          Each offspring obtains a sample half of its sire's genes. So, if a sire is mated to a random sample of females, the average performance of his progeny measured as a deviation from the population average is an estimate of half of his breeding value.

Two points are important:

·          Breeding values are a characteristic of a population, such as a flock. They are expressed as a deviation from some population mean, or average.

·          Breeding values are estimated from phenotypes, which are observations or measurements of a trait, and not directly measured.

Environmental effects:

·            Environmental effects cannot be calculated simply

·            They are related to the phenotype of an animal and the population mean.

·            Phenotype = population mean + breeding value + environmental effect

·            Therefore, a measurement of production contains environmental effects as well as genetic effects.

To estimate the breeding value from the phenotype, use the formula:

Breeding value (EBV) = phenotype - population mean - environmental effect

The simplest form of EBV is based on individual performance testing. In this case, the EBV is calculated as:

EBV = hereditability x (measured performance - flock mean - environmental effects)

·            To obtain an accurate EBV, correct the measured performance for environmental effects using standard adjustments. Recording schemes do this automatically as part of the processing procedures.

·            This formula emphasises the EBV's dependence on population mean.

·            Only differences in the environmental effects can change the ranking of the sheep, since all other factors are constant within a flock.

·          For a more accurate estimate, use as much information as possible to calculate EBVs.

Some sources of information for the estimation of breeding values are:

·          individual performance records;

·          an average of several records on the individual;

·          ancestors' records; and

·          half-sibs' records.

The main environmental influences that might affect a group of sheep are:

·          the dam of the individual;

·          age birth and rearing status; and

·          birth date (or age at test time).

Failure to correct performance records for environmental effects might result in:

·          less accurate EBVs;

·          genetically superior, but environmentally handicapped sheep being culled;

·          the generation interval increasing owing to the progeny of older dams being chosen; and

·          a slow deterioration in lambing percentage, if multiple­born sheep are culled.

All of these factors apply whether EBVs are being calculated or not and will decrease the rate of annual genetic improvement.

EBV is a selection tool that really works.

·          In essence it has become a tool for breeders to make more money.

·          EBVs allow us to identify those potential parents with the highest probability of producing superior offspring. This is the basis of any successful breeding system

·          EBV remove environment effects (things like whether the animal has been born and reared as twins or singles, date of birth.

·          EBVs also add the average performance of each animals brothers and sisters, and parents to the EBV value.

·          EBVs are used to link sires, in different management groups in the same year, in one year and than the next, on one stud and than at another stud or site.