- Genetic Improvement of Fertility in Merino Sheep
|Last update: April 3, 2012 10:46:27 AM|
The Genetic Improvement of Fertility in Merino Sheep
The ability to reproduce is surely the most important characteristic in any breed of farm animal. In the case of the merino it is not only of great economic value, but can be seen as the biological key to both products of the merino, wool and mutton.
From a breeding point of view we can state that reproduction tempo and breeding influence each other very strongly. An increased reproduction rate resulting in a larger percentage of sheep which can be drafted out, i.e. an increased selection intensity and consequently faster selection progress. On the other hand reproduction rate can be increased if Ii portion of the selection pressure is applied to it. This causes a type of flow-back mechanism, if selection for fertility is effective.
There are chiefly two ways in which selection for fertility can be carried out, namely indirect selection, where selection for or against a characteristic genetically correlated to reproduction rate is carried out and direct selection for higher reproduction rate.
1. INDIRECT SELECTION
To be effective, the heritability of the characteristic which is being selected for must be high, and its genetic association with reproduction rate must also be high. In other words the same requirements of high heritability, high repeatability and easy measurableness apply to a characteristic of this nature, with the further proviso that it must have a high genetic correlation to fertility.
The effect of selection on many such characteristics has already been studied and the following appear to be the most important.
(a) Body mass
There are naturally arguments against selection for a higher body mass which are chiefly based on the supposition that bigger sheep simply eat more, and, therefore, have no economic advantage. According to experimental evidence this seems to be so, although it has not been confirmed under natural conditions. The question may also be asked, whether efficiency of feed conversion under extensive conditions is of greater importance than the ability to remain alive in times of scarcity, and then - this is the cardinal point - in times of plenty, utilize the grazing to the maximum, a property that can be associated with a big sheep. With merino sheep we are in the fortunate position that selection for mature body mass will not easily result in over fat carcasses. Although not true for all breeds, it has been proved that body mass in the merino is a very good indication of the total quantity of edible meat and, therefore, carcase value. Body mass, therefore, has very definite advantages, the most important of which is the strong association with reproduction rate. Though selection for body mass, in order to increase fertility, is not as effective as direct selection for fertility.
(b) Pleat development
This is one of the most important characteristics of the woolled sheep, because it effects wool production, fertility and constitution. The association with wool production is known to all - so much so that it is very generally regarded by breeders as synonymous with quantity of wool, and as such has been completely wrested out of context. There is an association between pleat development and fleece mass, but this association is confined rather to greasy fleece mass than the actual quantity of clean wool. The reason for this is that pleats contribute more to an increase in yolk production than to actual wool production. The genetic association between pleats and quantity of wool appears to be very small. At Trangie research station in Australia , selection for clean fleece mass resulted in plainer bodied sheep with more wool. Although a certain amount of development may be necessary to maintain so called "sheep classers density" in order that the fleece will be protected from weathering, it is not necessary for increasing wool production. This will be confirmed by many breeders who do performance testing. On the other hand pleat development has clearly been proved a constitutional fault, and selection for pleats can upset the physiology of the sheep completely. Pleats are negatively correlated to fertility of the ewe and semen production in rams, especially in warm climatic conditions. It would appear that pleat development has a greater detrimental effect in unfavourable conditions.
The tendency away from pleats, which is at present noticeable, could have a beneficial influence on the woolled sheep industry. This is particularly so if pleats are to be replaced by selection for measured clean fleece mass. It must be remembered, however, that the negative genetic correlation between pleats and fertility is not very high, and that breeding plainer bodied sheep will not increase fertility significantly, except in cases where completely unadapted, pleaty types of sheep with very low fertility are encountered. With regard to wool production, the basic principle should be not to discriminate against productive sheep merely because they do not have big neck folds. If the different components which make a real or assumed contribution to wool production are reduced to the one characteristic which brings money into the pocket, namely measured clean fleece mass, we will find that here is no reason why prominent skin development should be regarded as a prerequisite for flock productivity or show standards. This is also true for other assumed important characteristics. One of the greatest advantages of actual measurements is that it helps us to see things that we do not want to see. High producing fertile sheep should be our breeding policy, and these sheep often have little pleat development and do not always look as we thought they should.
(c) Woolly face
There is a negative association between wool in the face and fertility. Woolly faces are not a problem in South Africa and, therefore, selection against this trait has no great possibility for increased fertility.
(d) Early oestrus and regularity of ovulation
There is a relationship between the stage at which ewes first show heat and their later fertility. Ewes that show heat early are usually the more fertile ewes there is also a positive relationship between regularity of heat cycle and fertility. After two heat cycles of 17 days, the most fertile ewes will normally have been fertilised and those which have not, are usually the shy-breeders and can be eliminated from the flock to advantage. By not making the mating period too long and eliminating the ewes which did not Iamb, the reproduction rate can be increased within limits.
2. DIRECT SELECTION
Selection for one or even all the mentioned characteristics collectively, is not as efficient to increase fertility genetically as direct selection for fertility.
The difference between selection for fertility and selection against infertility must be very clearly defined. In the latter case not much genetic progress will be made due to a low heritability. Nature has been selecting against infertility for centuries, in that infertile animals do not leave progeny. It is, however, sound practice to aid nature by culling such animals early because a ewe which does not lamb is a boarder which cannot be afforded.
Selection against infertility, strictly speaking, has the object that each ewe must have one Iamb, in other words a lambing percentage of 100. We must realise that fertility is controlled by a large number of genes and that environment also makes a significant contribution. Absolute likeness is therefore impossible as is the case with other polygenic characteristics. We are, therefore, forced to "put the stream in thick" and select for multiple births if we want all the ewes to have at least one lamb.
This has been proved to be so in practice. Selection for multiple births can bring about a considerable genetic and, therefore lasting, increase in fertility. Multiple birth selection causes more egg cells to be shed per ovulation and if a ewe normally ovulates a large number of egg cells the chances will be good that, when conditions are unfavourable, she will shed at least one egg cell and consequently still have a Iamb. Favourable environmental conditions like good feeding and management have the same influence, as selection, and the results attained in this manner are faster and more spectacular. Seeing that more multiple births occur under favourable conditions it is a very good method to identify ewes and rams which have genetic potential for fertility. Because fertile ewes will normally produce single lambs under poorer conditions, the ewe and her progeny cannot be identified.
Good management and selection for fertility are therefore complementary to each other. The economic justification of better management, in order to be able to wean more lambs, is so obvious that it is not necessary to emphasise the fact.
The golden rule for genetic improvement of fertility is then, to give priority to sheep born as multiples.
As selection criterion "multiple births" appears to be the easiest and most effective as the mortality of twins is never double that of single Iambs, and therefore twins will result in a higher weaning percentage. There is no better investment on a sheep farm than extra trouble taken and money spent to give twins a better chance in life. In many cases today no preference is given to twins - in fact the discrimination against twins is often very drastic.
The whole concept of improvement of fertility boils down to less emphasis on outward appearance and greater stress on actual recorded performance, in this case the birth status or reproductive performance of the dam. For the breeder who buys rams and lays emphasis on fertility, it is important to know whether the stud breeder from whom he buys his rams selects for fertility and does not discriminate against it, because twins do not grow out well and consequently don't sell well or don't look attractive in his stud.
At Trangie research station in Australia researchers have been developing a so called "fertility flock" since 1958, in an effort to breed a dual-purpose merino. In 1969 already the percentage of lambs born relative to the number of ewes mated was already roughly 150% while an increase of 22% in clean fleece mass was attained. Dun and Eastoe maintain that this flock is a very good starting point for the development of a dual-purpose merino, in other words, the project is only now starting in earnest. An interesting fact is that "born as a single" is considered a culling fault in this flock.
The "Booroola" flock, a private flock, selected by the Sears Brothers in New South Wales for multiple births has also attained fantastic results. In 1964 the percentage of lambs born in relation to ewes mated was 120% for young ewes and 180% for five year old ewes (in this case selection for fertility was only practised on ewes).
The effect of age on reproduction performance is very clear from the above figures. A young ewe giving a single lamb can, when she is older, regularly deliver twins, but a ewe which has already given a twin at first lambing is relatively much more fertile.
The reproductive performance of young ewes must, therefore, be judged and recorded separately. The keeping of proper ewe records, in which reproductive performance is recorded, is an essential to any breeder of woolled sheep. If an increase in fertility is desired, definite preference must be given to the progeny of more fertile ewes.
If the keeping of records is impossible, due to circumstances, progress can still be made by the following procedure: Select the rams which have the highest fleece and body mass from those born as twins. Keep all twin ewes and make up the numbers with those born as singles with the highest body and fleece mass. The theoretical expectation is that, after 10 years, 40 more lambs will be born for every 100 ewes mated and an increase of approximately 1, 2 lb. clean wool per head will be attained.
The genetic improvement of fertility in sheep does not require complicated techniques. It merely requires a more objective approach and a change in disposition by breeders towards twins. Constitution ideals at present have in many cases very little to do with actual constitution. Both production and constitution can and should be measured and this can be combined in a breeding plan which will result in productive, as well as hardy sheep. The disposition towards twins is very often not favourable for the genetic improvement of fertility and it is often said that a single Iamb which has grown out well is preferable to a twin which remains stunted. Economically, twins justify additional care and if we are to be objective, twins must always receive preference due to their higher fertility which is transmitted to the offspring.
Purposeful selection for fertility in the merino sheep has become very important at the present time where the "dual-purpose characteristics" of the merino are being stressed. The most important characteristics which contribute to profitable mutton production are fertility and growth rate. These two characteristics can be improved genetically by simply selecting those sheep with the highest adult mass, born as multiples. Selection for higher adult body mass is at present the best method known to bring about a genetic improvement of growth rate and the production of lean meat. Merino breeders who feel that they have "too many eggs in the wool basket" need not forsake their breed. The merino has the necessary genetic potential, and there is no reason why it cannot be developed into one of the best mutton breeds in the country without affecting its wool characteristics detrimentally.
Merino Breeders Journal 38 (1)