- The importance of progeny testing
|Last update: March 27, 2012 08:36:26 AM|
The importance of progeny testing
The purpose of the National Wool led Sheep Performance and Progeny Testing Scheme is to supply objective data for the selection of breeding stock and specialist advice in order to promote the improvement of woolled sheep. In this article, Mr Gert Erasmus, head of the Fleece Testing Centre, Middelburg, and Dr Jan Hofmeyr, director of the Animal and Dairy Science Research Institute at Irene, discuss the development, functioning and general status of the Scheme which has become an important source of information apart from its proven value in breeding improvement.
A FLEECE ANALYSIS SERVICE for breeders of wool led sheep has been in existence in South Africa since 1934 following the studies and recommendations of the late Dr V Bosman, who realised the need and benefits of objective selection for wool traits. In order to extend the original service and to include it in a national fleece testing scheme similar to the performance testing schemes that already existed for pigs, dairy and beef cattle, the South African Fleece Testing Centre was established at the Grootfontein College of Agriculture at Middelburg, Cape, in 1965 with funds supplied by the wool industry.
The Scheme has two closely related aims: To provide breeders with objective data to serve as selection criteria, and to provide a specialist advisory service on relevant aspects of sheep breeding.
As far as the first aim is concerned, it is essential that selection criteria be constantly reviewed and revised when necessary. Although the Scheme has undergone important refinements since its inception, no real change in the basic selection criteria has yet been introduced. They are, for instance, still largely directed at increasing quantity and quality of wool production.
The reasons for the apparent reluctance to adapt the Scheme to accommodate increased mutton production from wool led sheep are twofold:
- Firstly, it proved necessary to give the highest priority to an intensive effort to persuade breeders to use objective measurements in selection for wool.
- Secondly, the services of an independent performance-testing scheme for mutton sheep were already available to all breeders of wool sheep. New or alternative options with regard to breeding policy would almost certainly have led to confusion. As it turned out, the call for changes and additions to existing criteria is now coming from a large section of breeders themselves.
The widely known reluctance of breeders to record and to involve themselves with what they, initially at least, regard as superfluous and unnecessary details was an important consideration in excluding the recording of mutton characteristics in the Scheme.
However, the fact that very few breeders of the important wool led sheep breeds have made use of the Mutton Sheep Performance Testing Scheme, in spite of the fact that they openly advocate genetic improvement in the mutton production qualities of their sheep, might suggest that breeders prefer a one-stop performance testing service with which they can totally associate themselves.
The National Wool led Sheep Performance and Progeny Testing Scheme is therefore now moving in that direction, but it will remain a flexible policy with open options, based on experience.
Performance testing programmes and back-up advisory services are inseparable. Supplying data to a breeder without advice on how to use them is entirely ineffective. On the other hand, advice can be equally ineffective if no data are available on which to base it.
The most important and arduous task to date, however, has been the effort to close the communication gap between traditional breeder and modern animal breeding theory and methods. In recent years these efforts have proved extremely effective mainly as a result of intensive four-day courses offered by the Scheme on the principles of animal breeding and performance testing and held throughout the country. More advanced courses are conducted for livestock advisors. Over 2 500 breeders and officials have attended such courses.
Since its modest beginning, when just over 2000 wool samples were analysed annually in 1966 to the present time when well over 35000 samples are dealt with per year, the Scheme has had a growing impact on the wool led sheep breeding industry. It is significant to note that the greatest increase in participation occurred over the last five years when the number of samples analysed increased by more than 130%.
The sheep industry of the RSA (including national states) consists of 27,4 million sheep of which 21,4 million are wool led sheep. Of the latter at least 16,1 are Merino sheep and the rest are other white wool led breeds (Mutton Merino, Dohne Merino, Dormers, etc). Presently there are 0,8 million Karakul sheep.
The Scheme is administered and supervised by the Fleece Testing Centre of the Animal and Dairy Science Research Institute. It is still basically designed for mass selection because of the widely reported high heritabilities of the traits measured and owing to the fact that none of the traits currently measured are sex-linked. It should also be stressed that most of the input data for which provision is made are optional. Experience has shown that a policy of rewarding the breeder with either more extensive information or more accurate information for every extra recording effort he is prepared to make, is very effective and most breeders eventually end up by voluntarily supplying all the required data.
The breeder is supplied with only one input list which provides for the following data: Shearing date, birth status, age of dam, body mass, wrinkle (fold) score, subjectively evaluated grade, sire identity and the relative weightings he wishes to use for the calculation of a selection index. The latter values are normally established by the specific breed society in conjunction with the Scheme.
At the Fleece Testing Centre the following measurements are made: Clean yield percentage, fibre diameter, staple length and number of crimps per 25 mm.
Provision is made for four different outputs, depending on the data supplied by the breeder:
- A basic output that gives the uncorrected absolute values of the traits measured (fleece traits are, however, always corrected to 12 months' wool growth) and their percentages deviations from the mean of the group are supplied in all cases. It includes all the traits measured on the farm and at the Fleece Testing Centre.
Fibre diameter (d) and number of crimps per 25 mm (n) are given separately and the relationship between the two is expressed as where d' is the estimated fibre diameter according to the number of crimps as indicated by the tables compiled by Duerden. An alternative would have been to calculate the product: nd, which is a reasonably reliable estimate of the resistance to compression.
The decision to use a d:d' ratio was based merely on the fact that many breeders were already familiar with it. Evidence does exist that the relationship between fibre diameter and crimp frequency could have an effect on processing and the quality of the end product, as well as the amount of weathering in the fleece. Because selection for increased fleece mass should lead to more undercrimped wools, it is felt that the relationship between crimps and fibre diameter should at least be monitored.
- For the second possible output all values are corrected for birth status and age of dam (maiden or mature). Sheep are divided into different classes, e g twins born from mature ewes, singles born from maidens, etc, and the mean of each class is corrected to the mean of the mode class. Fixed correction factors are considered inaccurate as the magnitude of the correction is highly dependent on the level of feeding and management.
The economically sound practice of preferential treatment for maternally handicapped animals sometimes leads to a negative correction. the correction factors applied are supplied in each case for the information of the breeder concerned. If there are fewer than six animals in a class, no correction is made from the class, but each animal is identified as belonging to that class.
No correction for age of the individual is made at present, because birth dates are not always available and, since animals are not evaluated before the age of 12 months or even more, it is not really deemed necessary.
- Whereas the previous output lists sheep in sequence according to ear tag number, they are ranked according to selection index in the third output. It serves as the actual selection guide.
- Sire Summary. The predicted breeding value in respect of each trait of sires individually mated is calculated. In order to pool the estimates over groups, the percentage deviations are used and the mean of each sire is multiplied by a regression coefficient (b), b→2 with increasing number of offspring (n), b = 2 when n = 35.
Although progeny testing is generally not recommended as part of a breeding plan for wool led sheep, the identification of sires with proven superior breeding values for more widespread use could give a marked genetic boost to the industry. The demonstrative value of the sire summary is, however, of even more immediate importance and, accordingly, it has become an invaluable part of the scheme.
The National Performance and Progeny Scheme for wool led sheep has become entrenched in the South African sheep industry by virtue of the growing acceptance of performance as a basis of selection. The establishment of ram testing stations has been considered from time to time and, although the system of control flock rams suggest far greater advantages, there remains strong pressure from the industry to employ this demonstrative tool as an educational medium and supplementary technique in performance testing.
With the vast amount of data already available and still accumulating, further refinements, changes and adaptations to the Scheme can now be made with confidence. Apart from the said effort to include other characteristic related to mutton production, a trial has been planned to measure traits having a possible relation to fleece weathering. An amount of R42 000 has been budgeted for by the Wool Board for apparatus to measure these characteristics.
This has become important in view of the fact that selection for higher fleece masses has led to a marked increase in clean yield percentage in most flocks and there is evidence suggesting that these "drier" fleeces are more prone to weathering damage.
As collector and processor of production data of benefit to the sheep breeding industry, the Scheme is becoming an important source of information which is already proving most useful as an aid to flock management, apart from its present proven value in breeding improvement.
The Scheme has set its target to test approximately 40000 stud rams annually by the end of this decade. This would indeed be a meaningful number of rams and would provide the stud industry with a wide enough choice for effective selection.
Golden Fleece Vol 15 No 5