G. Wiener1,2 and S.C. Bishop1
1. Roslin Institute (Edinburgh), Roslin, Midlothian EH25 9PS, UK
2. Edinburgh University, Centre for Tropical Veterinary Medicine, Roslin, Midlothian EH25 9RG, UK
Opportunities for the improvement of yak production are discussed. The non-genetic routes to improvement include nutritional inputs—limited by the availability of supplementary feeds—changes in range management, disease control and increased marketing opportunities. Genetic routes to improvement, examined in more details, include selection, crossbreeding and hybridisation. Some of the opportunities and difficulties inherent in each are referred to. Consideration is also given to the potential use of genetic markers, locations on chromosomes identified by molecular techniques. These could be an aid to selection if found to be favourably associated with performance traits or disease resistance in the yak. However, in addition to investment in the molecular procedures, such associations have to be established through recording of performance and parentage identification on a large scale. These prerequisites are therefore the same as for conventional breeding schemes for yak. In the medium term, such conventional schemes are likely to be less expensive and more certain to be effective, in achieving the desired results, than reliance on molecular techniques. We concluded that until the molecular technology has advanced much further, to see what it can realistically offer to the improvement in yak productivity, it is important to support long-established practices to safeguard a future for domestic yak and for a way of life that it underpins.
Keywords: Crossbreeding, genetic markers, genetics, improvement opportunities, selection, yak
The title of this paper reflects the fact that we want to stress the importance of improvement in yak production—in those situations where it can be applied for the benefit of the herdsmen. There are several ways of achieving such improvement, which will be referred to. However, much of the paper will deal with genetic options for change.
A substantial improvement in productivity from yak is of great importance to provide a significant increase in the income and standard of living of yak herders and their families. Without any improvements, it seems likely that over the next 20 years or so there will be a significant decline in the yak population and a major shift in human population away from these highland regions. This is only a prediction, and the trends are not yet very obvious in China. But the basis for believing that such a decline may happen, if not prevented, is already apparent in some of the smaller countries with severely declining yak populations such as Nepal and India, to name but two. Movement of people away from remote rural areas is also seen everywhere in the world as a result of people wanting to improve their standard of living and have available some of the modern facilities offered in towns and cities—in spite of the risk that the aspirations of such a move may not be realised. Improvement in income from yak production would be one important component in restoring some balance and in helping to maintain a way of life and keep the yak as a unique genetic resource.
The opportunities for improvement of yak production are constrained by the environment and by access and importantly by social, cultural and economic considerations. Some of the main components of change, where these are applicable, will now be discussed.
Nutritional inputs: Major changes are limited by distance from areas where crop by-products are available and by the limited opportunities for growing supplementary feed in situ, even with the possible advent of genetically adapted crops, although systems for such production are under investigation (Liu 1999). There is clearly a potential for providing supplementary feed in the form of feed blocks for emergency use especially during harsh winters to prevent death of animals (Zhang 1998; Long et al. 1999). However, this does little to improve output except in terms of reducing immediate losses. Strategic feeding in late winter or early spring and over the calving period can reduce losses among adult animals and assist both the survival and growth of calves through the better condition and milk output of their dams. The costs and benefits of such strategic supplementations will need to be evaluated against the costs of doing nothing.
Range management: Long and Ma (1996) discussed an increasing degradation of rangelands used by yak, and the consequent threat to yak nutrition and survival, along with measures needed to reverse this situation. There are good opportunities for improvements in animal productivity by techniques to improve range management, including a reduction in overgrazing, when that is its cause, which would also help to reduce parasite burdens. Early disposal of surplus stock and of castrate males at much younger ages than is commonly practised would reduce pressure, particularly on winter pastures. Although the castrates so disposed of, if not needed as pack animals, may be smaller and perhaps fetch a lower price (provided they can be marketed), the benefit in terms of extra feed made available for the more productive females should improve the overall output from the herd. In the longer term, progressive changes in plant varieties and the introduction of legumes may improve pasture output. However, taking account of the large areas involved, these improvements will take a long time to achieve in full.
There have also been some relatively recent changes, made or proposed, which involve fencing of individual holdings and affect the access to land by yak herders. There may be good reasons for these changes, but it has yet to be shown whether they are neutral in their effect on good range management or actually harmful. Richard (2000) discussed both positive and negative consequences of such subdivisions of grazing and rangelands.
Disease prevention: There are real opportunities for prevention and cure of a number of parasitic and infectious diseases and other causes of ill health in yak. The limiting factors appear to be access to professional help and the costs in relation to the benefits.
Marketing: The provision of additional outlets for the major yak products and the development of more niche markets for specialist yak products is a route to increase income—but this depends on finance and political will to initiate such schemes. Substantial benefits could come from a concerted marketing effort of yak products with a high add-on value. An example from Nepal is the systematic introduction of cheese factories for the use of yak milk since 1952 (Thapa 1996; Joshi et al. 1999).
Genetic improvement, once achieved, is permanent and does not involve recurrent costs except in terms of maintaining the more productive animals. Several different breeding schemes could be developed to provide significant improvements in productivity. The opportunities are largely within the direct control of the herders themselves and represent a challenge to them, but also require assistance from the scientific communities and government departments. In particular, the investigations needed to identify and develop the best breeding practices need scientific and technical inputs and support from public funds. The importance of this cannot be overstressed.
We will now review, briefly, the genetic options for change, some of which have been discussed at previous yak congresses. Included in our consideration are the possible new opportunities from advances in molecular techniques.
Variation: Without genetic variation in the yak population there can be no genetic change. Both the maintenance and exploitation of genetic diversity is therefore important. Genetic variation is found in the putative existence of different yak breeds, although the extent and nature of these differences is not yet well established, and in variability within breeds.
Breed differences can be exploited by substituting, over a period of years, one breed for a more productive breed (if a real difference in performance has been accurately established). However, direct importation of a new breed is unlikely to be practical or cost-effective if the different breeds are isolated from each other by large distances. Crossbreeding is a simpler way of introducing the characteristics of one breed into another. Initially, crossbreeding may also bring additional benefits from hybrid vigour, but this will be progressively lost as the existing breed is 'graded-up' to the new breed. In a wider context, breed substitution should not be extended to the point of losing genetic diversity in the yak population. Crossbreeding among yak breeds and the creation of new 'synthetic' breeds, however, need not lead to loss in genetic variation. (A point to be borne in mind is that the first cross of two breeds may, as already referred to, show heterosis. The extra performance due to this should not be attributed to the 'better' parent breed and then lead automatically to breed substitution. Clear differentiation between additive and non-additive genetic effects is a prerequisite of effective crossbreeding strategies).
As a start, it is important that the nature and extent of breed differences and the effects of crossbreeding should be clearly shown through proper investigation. Also, any role which genotype-environment interactions may play in affecting the performance of different breed and crossbred types needs to be established. Wiener (1996) and Wiener (1997) discussed these points. The exploitation of breed differences through crossbreeding, and the concomitant reduction in risks from inbreeding within herds, have been argued previously as a practical and immediate route to the genetic improvement of yak productivity.
The use of semen from wild yak is a special case of such crossbreeding and requires no separate consideration. To provide a valid comparison, however, the crossbred and the purebred animals have to be managed and treated in the same way and in the same place. Otherwise, it is impossible to apportion any improvement to heterosis on the one hand or additive genetic effects on the other, or to distinguish genetic from the non-genetic effects on performance. The possibility that crosses of domestic yak with wild yak may be given better conditions than the ordinary yak makes it difficult to provide a genetic interpretation of some of the results to date.
Hybridisation: The crossing of yak with Bos taurus cattle is well established, particularly at the lower elevations and where better feed may be available. It is apparent that the hybrid females produce more milk and are superior in growth and in other respects to pure yak. However, it is far from clear to what extent this is due to heterosis or to additive genetic effects and what role genotype—environment interactions play in the results. The main reason for this is that of the three types—the cattle, the yak and the hybrids—the pure cattle are generally not present as females in the same environment as the yak. This is particularly the case when these cattle are exotic breeds like the Holstein —the types from which most of the benefits of hybridisation are claimed. A secondary reason is that, in all probability, the hybrids get better treatment than the purebreds. A further problem with the use of hybrids, as a means of increasing herd output, derives from the sterility of the hybrid males. This limits the breeding systems that can be employed. Moreover, because of the relatively low reproductive rate of the yak in traditional production systems, only a limited proportion of yak can be used for hybridisation if the pure yak population is to be maintained. Thus, the overall economics of hybridisation is not simple to establish and is unlikely to be as beneficial as the better milk yield or growth rate of a hybrid animal might imply. Wu (2000) discussed some of these matters
Conventional selection: In the medium- or long-term, selection of superior genotypes is the principal way to improve the genetic potential for animal productivity. For the yak, the major constraint, at present, is the lack of recorded information on pedigrees and on the performance of the animals—performance in terms of reproduction, milk production and milk composition, growth rate and meat characteristics, specialist fibre production for niche markets, disease resistance, and so on. The colour of animals, the size and shape of horns and similar traits are often admired by herders but are of very doubtful value. Such traits are entirely counter-productive to schemes for the improvement of productivity. Cai and Wiener (1995) and Wu (2000) described a traditional scheme.
The establishment of effective selection schemes for the improvement of yak productivity is complicated and detailed consideration cannot be given here. Wiener (1994a) outlined the principles and Wiener (1994b) and Wiener (1994c) gave greater details. It seems likely, however, that the application most likely to succeed in yak would be through the establishment of some form of group breeding scheme and concentrating attention on a very restricted number of the economically most valuable traits-perhaps only one or two.
Such schemes depend on the willingness of a number of yak herders to agree on their objectives for improvement and to pool their animal resources for selection purposes. Superior females and males are identified over the whole of the yak population and it is these alone, which are allowed to breed the bulls, which are then used over the whole group. The superior females derived from all the participating herds need not be kept together in a single nucleus herd, but in the case of yak this may be the best option, as the necessary recording of pedigrees and performance can then be largely restricted to the nucleus group. The use of artificial insemination (AI), to distribute semen from the best bulls, is something that might be appropriate in some areas, but is not an essential component of such schemes. For the most part, the use of AI faces formidable difficulties in yak rearing areas. Further advances, given the technological inputs—which are not, as yet, successfully developed in yak—could come from the use of multiple ovulation and embryo transfer (MOET) using the very best females as donors of embryos and as the dams of the future bulls. Such technology can be incorporated into formal MOET schemes (Nicholas 1996), but, worldwide, the application of this methodology has not been as great as the theoretical advantages might have suggested. The control of excessive inbreeding, resulting from the use of a relatively small number of parents of future generations, has been a major constraint even where the technology itself is established. Similarly, other new techniques which may be broadly regarded as increasing female reproductive rate such as in vitro maturation of oocytes and cloning have, at best, very limited potential as solutions for practical breeding problems and are potentially associated with serious disadvantages in terms of reducing genetic diversity (Nicholas 1996; Woolliams and Wilmut 1999).
We will now consider briefly whether new molecular techniques may assist in the process of selection by better identifying superior genotypes.
Marker-assisted selection: The most important tool used in genome research is the genetic marker. Currently, the most useful genetic marker is the microsatellite marker. These markers may simply be thought of as signposts along animals' chromosomes, i.e. they mark known locations on the chromosomes. An important feature of these markers is that they are very variable, i.e. have many alleles. In other words, a known marker at a known location on a chromosome will vary between animals, and even within animals: the copy or allele received from an animal's dam may differ from the copy received from the sire. Currently, many hundreds of markers are known for each major domestic species (pig, cattle, sheep and goat) and a lesser number, to date, for yak.
1. Parentage testing: The simplest use of genetic microsatellite markers is for parentage testing. To assign the correct parent to an animal from a number of potential parents, it is necessary to characterise the animal for a number of markers (e.g. 6–10), and characterise the potential parents for the same markers. By the rules of heredity, an animal must inherit the marker from its parent. Therefore, the parent will be the only animal, which, for each marker, has an allele in common with the progeny animal.
2. Enhancing genetic progress: Markers can be used to detect chromosomal regions likely to contain one or more genes affecting performance. If important regions containing beneficial genes have been found, it is possible to demonstrate which versions of the markers are consistently associated with desirable performance characteristics. Typically, this must be done in large populations to avoid obtaining associations with markers that are due to chance, rather than a true genetic relationship. These regions are known as Quantitative Trait Loci (QTL). Selection, which uses these marker associations, is known as marker-assisted selection.
Marker-assisted selection is currently being performed in dairy cattle, where markers for milk yield have been identified in commercial populations (breed) (Arranz et al. 1998; Coppieters et al 1998) and in pigs, where markers have been identified for litter size. Marker-assisted selection is most beneficial for traits that are difficult or expensive to measure, traits that only occur in one sex (e.g. milk or reproduction) or traits that are expressed late in life (e.g. longevity). A potentially and particularly beneficial use of markers is for selection for disease resistance (Crawford et al. 2000). If a genetic marker is available that indicates resistance to a particular disease, it would be possible to reduce or even eliminate the disease without having to expose the animals to the disease.
Another potential use of genetic markers is for importing a desirable copy of a gene from one population or breed (A) to another population or breed (B). In this case, populations A and B are crossed and the crossbred offspring are bred back to population B. Markers are used to ensure that all the animals used for breeding have the desirable gene. This process is known as introgression.
3. Genetic diversity: A further use of genetic markers is for quantifying genetic diversity. The more similar different breeds are, the more similar will their markers be. Conversely, breeds, which are genetically more different, will show greater differences between their markers. Markers are especially important when considering genetic conservation programmes. Steane (1997) has discussed the importance of maintaining genetic diversity in yak and some of the strategies for this.
In general: The molecular technology is being developed in the yak. DNA analysis, mostly of microsatellite origin, is in progress with yak both in China and Bhutan, and in the UK (Han Jianlin, personal communication, 2000). It must be re-emphasised, however, that variation at the molecular level does not, on its own, provide information about differences in animal performance or disease resistance, between or within breeds, or on the likely effects of crossbreeding or inbreeding. There is a possibility that genetic markers, found to be associated with particular aspects of performance or disease resistance in other Bovidae, or in other species of animals, may also be found to be similarly associated in the yak. However, having regard to some major differences between yak and other species, there is no safe alternative to establishing the actual correlations, if any, between markers and performance traits in the yak. As referred to earlier, these investigations would have to be done with large populations to avoid the risk of chance associations—and such work would be demanding, time-consuming and costly. Moreover, conventional selection programmes could be expected to be effective on their own, without the adjunct of genetic markers, if the prerequisite of parentage and performance recording was widely practised, and if AI and embryo transfer techniques could be made widely available across the yak population, instead of being localised as at present.
We should also add that because different yak populations differ in a number of respects readily seen by eye, such as differences in colour, which are almost certainly genetic in origin, it is not surprising, that some differences in DNA profiles are evident from microsatellite studies. Thus, it is all the more important to establish whether variation in DNA profiles is also related to genetic variation in animal performance and health. Microsatellite and DNA profile information may be used to choose between breeds for crossbreeding or breed substitution. If two populations are very similar in their DNA profiles, it is probable that these would not be good first candidates for crossbreeding trials or for breed substitution. This is the converse of saying that populations, which differ substantially in these profiles, are especially useful as candidates in schemes for conservation of genetic diversity.
Inbreeding: As argued in earlier papers (Wiener 1994a; Cai and Wiener 1995; Wiener 1996; Wiener 1997) there is a distinct risk of inbreeding in yak. This arises because, within herds, a small number of males are often used for several years—possibly only one dominate male having most of the progeny. He is then likely to be followed by a son or other close relative. Recent evidence based on investigations using microsatellite markers suggests that a high level of heterozygosity (around 60%) may be present in the yak population studied (Dorji 2000) and that inbreeding may not therefore be the concern once thought. However, it is a feature of inbreeding that the process moves genetic variation from within families (e.g. herd) to between families. It is possible therefore that the inference of inbreeding from traditional breeding practices, within herds, can be reconciled with the maintenance of genetic diversity at the population level.
Some inbreeding is, in the long-term, also a consequence of selection or, as in the case of yak now in the USA and Canada, from having a small base population from which the expansion in numbers has occurred. Inbreeding causes a deterioration of reproductive performance, growth, survival and general vigour. Inbreeding should, therefore, be avoided as far as possible. There are useful reports of yak breeders' groups where active measures are now taken to avoid this long-standing and risky strategy of close breeding. In this situation, molecular information could have a role in avoiding mating together of animals that are particularly alike in their DNA profiles. This, however, is only the theory. It is unlikely, nor cost-effective, to provide the necessary infrastructure for such DNA profiles, or the scale of animal identification that would be required. Conventional procedures for identifying individual animals leading to the development and use of pedigrees would seem a more immediate strategy for controlled breeding practices than the use of DNA profiles without pedigree information. Moreover, the pragmatic approach to the practical reduction of inbreeding is to rotate bulls among a number of herds, so that no bull is ever likely to be mated to his own daughters or other close female relatives.
Setting aside the social, cultural and economic constraints applicable to any improvement programme, there are opportunities for the improvement of yak production by both genetic and non-genetic routes. It is important to try to achieve such improvements to enhance the income of the herders and to secure a future for yak production. Most genetic improvements in yak are likely to come, within the immediately foreseeable future, from traditional animal breeding methods. However, the contribution that molecular genetics may be able to make has also to be considered.
Molecular genetics is both modern and exciting, in so far as it lays bare the underlying pattern of heredity. It is attractive to researchers and relatively successful in attracting funding. Until the time, however, when far more genes are identified and far more is known about the association of particular genes with particular aspects of animal performance, the molecular approach will not add significantly to what the traditional geneticist, animal breeder and herder could achieve now in the yak. Immediate benefits from using genetic markers are more likely in the pursuit of maintaining genetic diversity and assisting conservation programmes, and, where necessary, in parentage verification.
In conclusion, the yak industry would be badly served if the attractions of molecular genetics, as an academic pursuit in the search for knowledge and some of its potential uses, were to detract from the overriding need to provide funding for mundane but essential breed comparisons, crossbreeding trials and selection schemes in relation to genetic improvement, and for the non-genetic routes to improvement.
Stephen C. Bishop was funded by Competitive Strategic Grant (CSG) from Biotechnology and Biological Sciences Research Council (BBSRC) in UK.
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