- Performance testing as an aid to more efficient woollen sheep farming
|Last update: March 27, 2012 08:31:26 AM|
PERFORMANCE TESTING AS AN AID TO MORE EFFICIENT WOOLLED SHEEP FARMING
S.A. Fleece Testing Centre, Middelburg
In the recent past, performance testing has led to many disputes in the sheep industry. Even some of the so-called "experts" had serious doubts as to whether or not performance testing would be viable. More recent debates on the subject have made way for actual implementation of performance testing programmes. Partly on account of economic pressures, more breeders are attempting to implement performance testing as an aid to identifying superior genetic material in their flocks. Nevertheless, a certain amount of uncertainty still exists on the concept of performance testing.
Performance testing can be described as the total process of providing the breeder with accurate information on economically – important properties of his animals.
Performance testing is often brushed aside as a scientific breeding plan in which the scale and perhaps one or two measuring-instruments form the beginning and end of the selection of superior sires. Actually, it is a more embracing concept: All possible parents of the next generation must receive a similar chance to exhibit their genetic potential during a well-defined testing period. Only hereafter can objective measurements and subjective assessments play their role in the accurate evaluation of the relative potential of individual animals to produce a saleable product (the stud animal itself or some commercial product).
It is necessary to consider a few genetic principles for the correct interpretation of performance testing in the broad sense and its application in practice.
Although the term "selection" is widely used in a great many popular articles, there is still some misconception in respect of the aim, techniques and consequences of selections.
The aim of selection is twofold: Firstly, to increase production in the current flock by selecting superior animals which will maintain a large proportion of their production superiority during the rest of their lifetimes. The second (and most important) aim is to increase production in the following generations by selecting parents whose offspring will exhibit a large proportion of their superiority. An attempt is thus made by selection to shift the average of a population in respect of one or other characteristic. This is accomplished by increasing the proportion of genes, which influence a characteristic in a desired direction and at the same time decreasing the proportion of undesirable genes.
Visual appearance or performance
Selection oil visual appearance or performance is known as mass selection or phenotypic selection. This technique can be based on a combination of visual characteristics (show judging); one measured characteristic, e.g. body mass; a combination of objectively- measured characteristics, e.g. body mass, fleece mass, fibre diameter, staple length, etc; a combination of visual and measured characteristics.
Family selection involves the use of performance records of the relatives (ancestors or progeny) of individuals under selection and is usually adopted in the case of a low heritability. Clearly the principle of family selection and especially progeny testing is totally different from that of performance testing which is essentially a measuring technique which can be used in mass or family selection, depending on the situation.
Sheep breeders are in a fortunate position in that, with the exception of Iambs born, milk production and other sex-limited characters, all important traits can be measured quite easily in both sexes and have reasonably high heritabilities. Mass selection is therefore the obvious technique to be used in the selection of superior wool -producing animals. Unfortunately, mass selection requires a fairly long test period, in that accurate selection is impossible before the age of 16 to 18 months. It must, however, be remembered that selection is postponed a full generation when progeny testing is applied. An additional complication of the application of progeny testing is that a large number of sires (at least three to four times the number required) must be retained in the flock for one generation before evaluation. Pedigree selection (ancestral records) does not suffer from the drawback of an increased generation interval, but the following must be borne in mind: 1. The genetic composition of each animal represents a sample of fifty percent of each parent's genes. Thus the influence of a grandparent on the genetic composition of the individual under selection is only 25 percent 2. Although an outstanding sire may have a relatively high percentage of "good genes", he may also transfer a sample of "bad genes" to one of h is progeny because of the effect of random sampling of genes. It is therefore possible that excellent sires can produce a few inferior progeny, although the average performance of their progeny will be better than that of other sires in the flock. The conclusion can be made that the value of pedigree in selection of superior wool-producing breeding material is very limited. 3. Accurate parent-offspring relationships must be available in cases where pedigree selection is considered. 4. Accurate performance-records of the ancestors are essential.
From the foregoing it is evident that the only cases in which pedigree selection is a feasible proposition is when selection is based on accurate dam/sire records and when the characters considered have a very low heritability or are sex-limited. The various components of reproduction rate represent characteristics, which conform to this situation. Lambing ability has the additional complication that it is a threshold characteristic. In other words, the characteristic is observed in a qualitative way as a ewe has lambed or is dry. The underlying genetic variance which is responsible for the visual expression is, however, quantitative. The selection procedure to be used in such a case will be expanded on later.
ACCURACY OF SELECTION
Any animal's phenotype (visual appearance or performance) is the product of environmental and genetic effects. Therefore it is not always a reliable indication of a specific individual's breeding value. Environmental effects can be classified as external or internal. External factors include differences between regions, climates, farms and also between seasons on the same farm. These are likely to affect the whole flock's average production. Some effects like disease or paddock differences might affect the whole flock or, in some cases, only part of it. Internal effects affect individuals but not the whole flock. These include each animal's grazing habits, the animal's own age and sex, age of the dam (offspring of maiden ewes vs those of older ewes), birth status (e.g. single Iambs vs twins), rearing status (e.g. born as a twin reared as a single, etc.) and even the degree of Inbreeding of the individual or its parents.
At present it is impossible for the individual breeder to control any of these between flock environmental effects. The central testing stations in many countries (where samples of animals from different flocks are tested under the same environmental conditions), however, proved that the problem of between-flock comparisons is not insurmountable. The biggest problem of these testing stations is the differential effect of pre-testing environment and genotype x environmental interactions on test results, e.g. a Iamb's intra-uterine and early post-natal environment affects the number of wool follicles, which develop, and therefore its lifetime wool production.
A genetically expensive alternative is for the breeder to mate his own rams, as well as a purchased group of rams, to random samples of his own ewes, on his farm. The performance of each ram's progeny is then compared to its contemporaries. The drawback of such a test is that the progeny of purchased rams might exhibit heterosis. This effect is of special importance where reproduction rate of the progeny is considered. Breeders in South Africa can only hope that the present development of the system of control-test in conjunction with the rather futuristic statistical technique called "blup" will be the solution to this problem.
The individual breeder can, however, ensure a standardized environment on his farm for an accurate performance test. The following precautions are strongly recommended: 1. Shorten the mating period to minimise age differences. Interrupted or too long mating periods is a definite drawback for accurate test results. 2. Ensure a long enough test period (at least six to eight months' wool growth) before testing at the specified age (16 to 18 months). 3. Standardize grazing conditions for the whole Iamb crop or, at least, for progeny of the same sex. In this regard the artificial rearing of a few rams for show purposes cannot be recommended. The breeder himself must decide on the advantage of maximum genetic improvement and the advertisement aspect of the good condition of his stock.
Accurate measuring of characteristics at the end of the testing period naturally forms an integral part of performance testing. It is quite logical that measurable properties such as clean fleece mass, crimp frequency, fibre diameter, crimp/diameter ratio and staple length can be more accurately assessed by a measuring instrument than by any "hand and eye" method. On the other hand, all immeasurable properties such as functional faults and conformation should always be evaluated subjectively. Both objective measurements and visual appraisal forms an integral part of a breeding plan to maximise the normal slow rate of genetic improvement. The conclusion can be made that the foolish arguments on the merits of performance testing, as opposed to "hand and eye" selection, are not only detrimental to genetic progress in isolated studs, but also to the economy of the whole sheep industry.
CHARACTERISTICS UNDER SELECTION
It is generally known that the genetic progress in every characteristic declined when the number of characteristics included in a selection programme is increased. Thus selection for a large number of fancy points invariably leads to a lowered selection differential for economically important characters.
Reproduction and meat production must be considered equally important, if not more important, than quantity and quality of wool produced in the selection of (especially) dual-purpose sheep. It is also not far-fetched to say that the dual-purpose breed of the future will be the one with the highest fertility! In this regard, one of the most sophisticated performance recording schemes is the one implemented by New Zealand. This system boils down to the following: Every Iamb's weaning mass is corrected for the effect of internal environmental factors and each ewe is credited with her corrected total mass of Iambs weaned. Total weaning mass is not only am excellent parameter for total ewe productivity but it also surmounts the problem of reproduction rate as a threshold characteristic. The New Zealand scheme combines this parameter with selection for body mass at 18 months age (the accepted indirect selection criterion for growth at an early stage) and the other normal parameters for wool production.
The South African National Performance and Progeny Testing Scheme now also facilitates a parameter for total ewe productivity. Except for the normal measurement of total mass of Iambs produced at 42 days age, the parameter also includes wool production (greasy) by the ewe during the same period. These two characteristics are then combined in an index according to their relative economic values and the index is accumulated on a yearly basis to give an indication of every ewe's lifetime productivity. This parameter may therefore be utilized to improve the productivity of the current flock and for genetic gain. The normal service of the South African scheme provides a wool analysis, together with data processing by computer to supply the breeders with corrections for birth status and age dam, a selection index (combination of the values of individual characteristics into one figure), ranking according to the selection index (to have some order in the data) and a progeny test. All the above options may therefore be utilized to help identify breeding material more accurately, but none is compulsory.
Every breeder should realise that performance testing is a valuable aid to bring about genetic change in his flock, as well as to control changes in specific wool properties such as fibre diameter and crimp frequency/fibre diameter ratio. No breeder can afford not to adopt more accurate and more sophisticated breeding practices, since the sheep industry is rapidly becoming more scientifically orientated.
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