CHAPTER 14
ADAPTATION OF LIVESTOCK TO THEIR ENVIRONMENT

by

J.M. Rendel

CSIRO Division of Animal Production
P.O. Box 184, North Ryde,N.S.W., 2113
AUSTRALIA

Summary

There are severe environments, such as the wet tropics, where special adaptations to local conditions are necessary for animals to survive and produce successfully. There are four broad classes of adaptation which are desirable: to the local climate, to local diseases, to disease which depends for its success on special relationships between a virus or viroid and the genotype of the host, and, to methods of husbandry and types of production which have their own special requirements.

Where a local breed and its crosses are the only ones to survive, the local breeds will be preserved. There is danger in localities in which highly improved breeds can survive that they will predominate and push other breeds out of existence.

A breed which becomes monotypic is an open invitation to a specialized disease to evolve and wipe out large sections of the breed. The problem facing us is one of preserving nascent breeds, based on highly productive ones or on crosses, from being swamped by highly product­ive breeds before they have had time to adapt and improve.

14.1 Introduction

It is generally recognized that the number of breeds has been shrinking rapidly in all major domesticated species. There are a number of reasons for this. One important reason, which has a bearing on adaptation of animals to their conditions of life, is the increasing independence of animal husbandry from foraging on native pastures and browses. With the introduction of improved pastures, supplementary feeding, housing and veterinary services the environments of the world have come closer together, and highly productive breeds, bred in most cases for maximum production by the individual, can be of use whenever feed and perhaps housing is available. Highly productive breeds have tended to sweep across the world with an accompanying rise in productivity which is hard to partition between the influence of the productive breed on the one hand and the improved methods of management that go with it on the other, From time to time, where the need to be able to cope with local climates and diseases exists, the improvements expected from introduction of produc­tive breads have not been forthcoming. The failiires have then been approached from two sides. Eradicate tsetse fly or introduce resistance to trypanosomes. Provide shade or breed for heat tolerance. Adapt the breed to the environment or the environment to the breed. The environments to which breeds have to be adapted will include the system of production in which the animal is expected to perform.

Some breeds have shown themselves to be remarkably adaptable. The White Leghorn chicken and indeed other breeds of chicken have spread to all corners of the world. Their capacity to do so seems likely to be due to uniformity of husbandry conditions. The Merino sheep which thrive near the snowline in New Zealand and in near desert conditions in Australia as well as in Russia and China where they are often housed for most of the year are limited mainly by their failure to reproduce in very high temperatures, their susceptibility to fleece rot in wet climates and their dependence on reasonably short pastures. Although breeds like the Merino are widespread, it has been difficult to show that they have become adapted in any special way to their new environments, at least in recent times. The environments inhabitated by the Merino in Australia are varied as to temperature, vegetation, mineral deficiencies and humidity and breeds might have been expected to have become adapted to habitats of characteristic kinds. It has been demonstrated (Dunlop, 1962, 1963) this has not happened in five major strains of the Australian Merino. Three environments were chosen - The New England Tablelands, deficient in sulphur, lying between 2,000 and 3,000 feet with 25-30 inches of rainfall and cold in winter; the plains of Deniliquin, which are low-lying and hot and can be irrigated; and the dry, hot inland of south Queensland near Cunamulla where sheep graze mainly Mitchel grass dependent on a variable rainfall of about 12 inches.

No important gene environment interactions in wool production were found between these five strains. There was a slight tendency for fine wools to do better at Armidale. No important interactions were found for fitness characters either, once more fine wools had a slight tendency to breed and survive better at Armidale. Dunlop points out that this does not mean that interactions between gene and environment do not exist, but that if they do they have not been used in establishing these strains. He suggests the strong variation from year to year at any one station in rainfall in particular may operate against selection in one direction.

Adaptation to local conditions will not be automatic unless the breeding system and climate are such that natural selection in the local habitat can play a part. Though in the past introduced species such as creole and criollo cattle have adapted over the course of 200 years or so to a highly adverse environment, when there is strong selection for production and little pressure on adaptation the evidence of adaptation is not there.

Experimental flocks in which rams instead of strains could be compared after selection in different environments were then set up to see if gene environment interaction with respect to production characters were in fact present. Latest results suggest that inter­actions with respect to production do exist. To date, the Merino breeders have not made use of these interactions; perhaps because most of the studs are in similar environments; perhaps because the strains have been adapted more to particular uses (fine, medium and strong wool growers) than to local conditions, perhaps as Dunlop suggests because variab­ility is a characteristic of looal conditions. Since the production of wool by an individual sheep has doubled over the past 30-40 generations in the Merino flock, it may well be that the decision, assuming it was a decision, to concentrate on production per head in this rather versatile breed was the correct one. Pigs and horses are two other species which seem to have been developed for special uses rather than for particular environments. In these two species, the intensity of management and housing may well be responsible for absence of major local adaptations.

The number of breeds of cattle in temperate climates has reduced drastically in the recent past suggesting that adaptation has a minor role within temperate regions, but attempts to introduce the predominant temperate breeds into the tropics and subtropics were far from satisfactory in the early part of this centery. Some degree of adaptation to hot climates has been found necessary in all but the most elaborate systems of manage­ment. Even where production of imported breeds has been better than that of local breeds, survival has not, and herd replacement without regular "breeding has been a problem. It is in the cross between the temperate and the locally adapted animal and the strains extracted from the cross that the extra productivity of the temperate animals can really make itself felt in the most severe environments.

In general, improvement of breeds, despite some notable exceptions where attention is paid to economy of gain, has concentrated on increasing the total production by the indiv­idual animal. The great success of this exercise in the more favoured environments has swamped advantages coming from adaptation to other particular environments except where environments are extreme and has resulted in highly productive breeds from one environment spreading to another. The tendency for high productive ability to swamp adaptation to local conditions has been reinforced by improved husbandry.

In order to consider how adaptation to local conditions bears on the conservation of animal genotypes, it is necessary to include productivity in the picture. Adaptation to local climate and disease often takes place in ways that are inimical to productivity. In addition, local husbandry practices are expected to set an optimum level to feed consumption and the amount used for maintenance and production, which have to be balanced for maximum profitability. These interactions are most easily expressed with reference to this equation:

A = G + O + P + S + H

A is appetite and measures what the animal eats as total digestible nutritents,
G is what it costs to grow the animal to the point at which it becomes a productive machine. Once the animal is grown, G disappears. G and P are interchangeable when it is meat animal one is concerned with.
0 is the cost of operating and repairing the machine,
P is the cost of production of meat, milk, eggs, etc.
S is the surplus of A over G + 0 + P and is usually laid down as fat,
H is the heat generated by the whole operation which in cold climates suffices to keep the animal warm. It is not difficult to dissipate a surplus in cold climates. In hot climates, the surplus is not only bigger for the same intake of feed since there is less required to keep warm, but hard to dissipate.

Where high quality feed is fed ad lib the animal's appetite A determines how much is eaten. Appetite is closely related to basal metabolic rate. Selection for production on ad lib feeding results in animals with large appetites, a large production machine and a high maintenance requirement. The breed relies for efficiency on the large amounts that a single individual can process. If S is small it is because P is large as also is 0. Since 0, though it varies with A, is to some extent independent of it, efficiency is also depend­ent on A being able to saturate 0 and P without increasing S. It also depends on success­ful disposal of H. Where A is limited, efficiency comes from a proper balance of 0 and P. Ideally, 0 and P must divide A exactly between them. If 0 is too high, P must suffer. Falconer's classic experiment with mice is an example. Peed may be limiting both because it is sparse and takes time to collect and because it is of very low quality and takes space to accommodate during digestion. One would not expect an animal selected for very high production on unlimited supplies to produce economically when fed a restricting diet. The elimination of heat is not a problem in temperate climates, but in hot ones it is. If heat production and absorption become more than can be eliminated animals lower their feed intake. This is expected to reduce efficiency of highly productive animals dependent on maximum feeding.

Although when a highly productive breed is introduced into a new environment, there should be an advantage in it becoming adapted to local husbandry conditions, it can have an advantage over breeds that are adapted to live in the new environment if these have not been adapted to produce at a high rate as well as to live and breed. Although the introduced breed may not be fed at the optimum rate, because it is developed to produce, it may still outproduce the local breeds. This should not be taken to signify that adaptation is not important.

When the dispersal of heat becomes difficult as it does in hot and particularly hot, humid climates, something has to be done about heat load. It appears that it is genetically easier to reduce basal metabolic rate and appetite and hence heat production than to increase the efficiency of active cooling. Animals that live in hot climates and those that have been selected in them certainly have lowered basal metabolic rates. Selection for production in these conditions has led to a lowered 0. That is to say, where appetite is restricted to conserve heat production, maximum production comes from the right 0, not a large 0. Unfortunately, seasons are not uniform, so in grazing conditions, selection for production will favour a different 0 one year from that in the next for two reasons: partly because temperatures differ and partly because the quality and quantity of grazing is different. A lower 0 also preserves an animal against a shortage of nutrients in climates with variable rainfall providing an unreliable feed supply. A good protein reserve moreover has been shown to favour an animal's power to resist disease.

Elevated body temperatures in cattle have been shown to reduce an animal's capacity to resist disease by damaging their immune mechanism in some way. They "become unable to mount a secondary response to an antigen and so never develop a proper immunity to diseases to which adapted animals rapidly 'become immune. Levels of hormones in the blood fall, choles­terol levels fall, the blood picture changes, tissue wastage takes place as shown by raised creatinine levels. It has been shown that reproductive rate is directly proportional, within unadapted "breeds of "beef cattle, to the extent to which body temperature is raised. So quite apart from the energy relations between feed intake, basal metabolic rate, production and heat dissipation, it is essential that an animal not protected from heat be able to keep its body temperature within physiological limits.

It is quite clear that there are environments which require adaptation. Heat combined with humidity form one set of environments. Altitude might be another, though whether areas of extreme altitude are very large from the animal husbandry point of view is questionable. Grazing as against grain feeding is another contrast. It may prove desirable to subdivide the environment further with respect to temperature. Environments in which animals have to use intake over and above that required for 0, G and P to keep warm may form another group.

Apart from general defence mechanisms against disease — mechanisms of the kind that break down when animals cease to be able to maintain body temperature between normal physiological limits - there are two sorts of disease resistance from the geneticist's point of view. The first is polygenically determined, seems to have been established by selection and is against local diseases. Familiar examples are the resistance to the tick, Boophilus microplus, present in Indian cattle but not in European cattle. The multi-host ticks of Africa are a different problem. Some sheep are more resistant to intestinal parasites than others. Harayana cattle are resistant to Anaplasmosis, European cattle are not. N'Dama cattle are resistant to trypanosomes. The most impressive demonstration of this phenomenon of resistance to local disease was the disastrous result of the spread of European diseases to human populations which had never been exposed before when Europeans began to travel all over the world and the contrast between relatively low mortality of Europeans infected with bubonic plague which flourished in Europe between 600 and 1700 AD. by comparison with the almost one hundred percent mortality that followed its first intro­duction into East India in the 19th century. One of the best documented accounts of the acquisition of resistance to a virus disease is that of the resistance of the Australian rabbit population to myxomatosis. After 27 years, the resistance is nothing like as strong as the resistance of the sylvalagus rabbit to myxoma virus but it is clear that the Aust­ralian rabbit population has come to terms with the virus and will not be exterminated by it, whereas when first introduced mortality was of the order of 99%.

Local diseases, particularly in the tropics, have proved a major drain on both survival and productivity. Adaptation to these is obviously an advantage and in some cases a necessity. It is not easy to make an estimate of the effect of adaptation on production. It is meaningless to do so without some measure of the degree of the environmental stress. Some examples from the work by CSIRO at Belmont are given here.

14.2 Temperature

The degree of stress in this case is measured by rectal temperature. The effect of raised rectal temperatures on the number of calves born to 100 cows which have been mated for 7 weeks can then be expressed as the regression of calving per cent on rectal temper­ature measured in °C. In a herd of Hereford/Shorthorns, rectal temperatures lay between 38.5°C and 40.5°C. The rate of calving fell 20% for each rise in rectal temperature of 1°C so the total effect of rectal temperature differences was 40%, calving rates being directly related to temperature and running from 30% to 70%. The advantage of the Brahman X and Africander X is not in its reaction to raised rectal temperature but in the far more severe conditions required to raise it.

Rate of gain was also shown to fall with increased rectal temperature. Animals of 180 kg liveweight gained weight at a rate which fell 0.37 kg/day for each rise of 1°C in rectal temperature. Rate of gain lay between the limits of 0.37 kg/day and 1.11 kg/day.

14.3 Infectious disease

B.I.K. ('bovine infectious keratitis) is an infection against which Zebu cross cattle are far more resistant in the tropics than Bos taurus» They are less often and less severely attacked. Calves of different ages from 3-15 months were scored for infection with B.I.K. and weighed. Presence or absence of infection was the indication of stress. The weights of those animals infected and those not infected at time of weighing give some idea of the effect of this disease. Again Brahman cross and Africander cross gain from not being infected, not from reacting less to the infection.

14.4 Tick infestation

Brahman and Africanders are resistant to infestation which they throw off to a much greater extent than Bos taurus. The degree of stress is taken to be the number of female ticks that reach the last instar and attain mature size. This is counted on one side and expressed as a total tick load by multiplying by 2. HS had 176 ticks, AX 60 and BX 54. When initial weights were 167, 193 and 198 kg respectively, it was found that over 27 weeks the rate of gain was reduced by 0.23 kg a tick a year when animals of the same breed were compared (regression of rate of gain on tick numbers within breeds). When the differ­ence between breeds was compared ticks accounted for 0.28 kg/tick/year. Estimates have been made by other people and vary from 0.3 kg/tick/year to over 1 kg/tick/year. The effect is not independent of rate of gain. When feed is good, the effect of the tick is reduced. Worm infestations have been found to have little or no effect on Braham X and to account in one trial for some 25% - 40% of the differenpe in growth between BX and HS.

The second class of diseases are those which depend on the relationship between the pathogen and the host, often one gene being responsible for the difference between resist­ance and susceptibility. In the absence of a favourable relationship between the two, the pathogen will not grow. Of course the pathogen can adapt to the host it it is able to mutate. The influenza A. virus is an example. Strains adapted to humans or dog kidney cells or the chorio-allantois of the chick readily infect their own cell type, but not any other. However, populations from one can adapt, presumably by mutation and selection, to an alternative host cell. Extensive studies of this sort of resistance have been made in mice where large numbers of pure lines make the study of this phenomenon easier to analyse. I have listed a small sample of 8 cases reported in the recent literature. The danger of this relationship between a single gene and a pathogen is that if a breed becomes too monotypic, it runs the risk that a pathogen will become adapted to it and sweep through the whole breed. This is of much greater danger in plants grown from selfed stocks or clones than it is in animals. An effective breeding population of five hundred should have sufficient genetic variation in it to shelter some individuals that will be resistant where others are susceptible. But one can always be unlucky and even in animals the possibility, since it exists, should be guarded against. Leucosis in European dairy cows may turn out to be an example.

Breeds of livestock should stem from a population whose effective breeding size is not reduced below 500. Preferably it should be much larger. The relationship between the effective population size of the seed stock and the size of the total commercial population is also important, since the latter simply multiplies up the former and what might be a small occurrence in the seed stock might magnify into a major disaster in a section of the commercial population coming from the sensitive section of the seed stock.

Diseases, resistance and susceptibility which depends on a single gene.

In summary the animal breeder has four different sorts of tasks ahead of him.

1. Even in the best of husbandry conditions, in which selection for maximum production is still increasing total production by the individual animal, economy of production can only be achieved by a nice balance between appetite, maintenance and production. When there is any limitation on feed, this balance becomes increasingly important.
2. In severe climates, the energy balance between production and maintenance must take account of the heat balance between the animal and its environment which is also important for the proper function of the animal as a whole.
3. There are local diseases to which local livestock must be resistant.
4. In all environments, one must avoid allowing a population to become a happy hunting ground for pathogens whose attack depends on the presence of the right allele at one or two loci.

As far as 1. and 2. are concerned, having settled on the foundation stock to be used, there seems only one way to go about adaptation and that is to select for optimal product­ion in the conditions of husbandry and climate that are going to prevail. Selecting for economy of gain does mean paying attention to feed intake as well as output of product and it is not going to be easy, so it will probably be done on a small scale in the first instance.

In 2. and 3. the choice of foundation animals is likely to be a cross. It is true that Creole and criollo cattle introduced into the tropics from Europe have eventually adapted and one could start with locally adapted cattle and select for production. Mahadevan has set out the arguments against this course. It is also possible to select in productive breeds for adaptation to climate and disease. It is the time scale that will influence the choice in favour of a cross "between a productive and an adapted breed.

The time scale on which changes can be made is still measured in terms of the generation interval. If we take 10 generations as a reasonable average time taken to make a signif­icant change (the exact number depends partly on gene frequencies and partly on your idea of what is a significant change) then it will take 10 years to modify a chicken population and 50 to modify a dairy cattle one. Since the constructive breeding will be done in a few small populations, time must he added for spreading the results to the population at large. Even modern techniques with the present organization of livestock breeding can often take a long time. There are obvious advantages in making crosses between breeds that complement each other. Adequate adaptation to temperature can be achieved by one cross in cattle. To put it another way, it would take at least ten generations probably more, to achieve the heat tolerance of a Braham x Hereford cross by selecting in a pure bred Hereford herd. Much the same can be said of tick resistance.

14.5 What can be done by selection in beef cattle in 10 years

The table shows what has been done by selecting in a herd of Bos taurus for rate of gain at pasture in a severe environment. Notice that in the absence of stress, when worms and ticks are removed and the animals stalled in the shade and given good quality feed ad libitum, the selected animals grow more slowly than the unselected controls. On the other hand, when at grazing under stress, the reverse is true, but ten years of selection leaves the selected group when at stress far behind the Braham cross.

Adaptation, if that is the right term, to 4. is a matter of maintaining a suitable population structure.

In the light of the four tasks outlined, preservation of genetic resources in breeds of livestock embraces much more than the preservations of locally adapted breeds in areas where diseases of a virulent kind and severe climates make the maintenance of a population introduced from outside the region difficult. Since there are still many areas in which the production of adapted animals is low and since it is still true to say that the main source of highly productive animals is in the favoured environments it will be of advantage to introduce these into the severe environments to lift production. Since it is much quicker to adapt them to the severe environment by crossing them to local adapted breeds than to select in either the adapted breed for production or in the productive breed for adaptation, it will be highly desirable to make certain for several decades to come that local adapted breeds in difficult environments are preserved, so that the crosses can be made.

But we need to go further than this. It is also necessary to preserve the nascent locally adapted breeds with improved production from being swamped by highly productive but unadapted genotypes. I think this is going to be a quite general requirement in all environments, not only in the more difficult ones. Once the present phase of animal improvement which has concentrated to date on maximum output by an individual begins to look for maximum product­ion from a given resource of feed instead, the importance of adaptation to local management, climate and disease will increase. Since these adaptations will be important parameters in maximising feed utilization, it will be necessary to preserve nascent breeds from being swamped by huge monotypic enterprises in more favoured environments. Sooner or later we shall have to regenerate genetic diversity in the form of new breeds to replace many that have been lost and I think we are concerned with more than the preservation of existing potentially valuable genotypes. We should be trying to preserve the possibility of genetic diversity and the possibility of developing it for the best use of local conditions. Once animals with very high oonversion rates are common place, and they are rapidly becoming commoner as methods of selection devised by the animal scientists become more widely used, further advances towards economic production can only come from adapting breeds to local conditions of husbandry, climate and disease, a proposition which is already true for the severe environments of the world.

Local adaptation is equally necessary for local disease, local management and local climates. So long as there are or seem to be large economic advantages in importing breeds or genotypes from favoured areas where production is very high or from breeds developed on a large scale by the best methods of selection for high production under particular systems of management, it is going to be impossible to reduce the trend towards monotypic species. The only possibility of preserving the opportunity of genetic diversity, and specialization for local conditions, is through institutions such as colleges of agriculture and govern­mental research organizations.

When it comes to diseases which depend on a special genetic relationship between the host and the pathogen, a relationship which may be determined by one or two gene loci, I find it difficult to assess to what extent monotypic breeds are at risk. Clearly the clone is the most exposed population. But if in a sexually reproducing breed a large number of progeny always stem from one sire there is a chance of having a large fraction of a population susceptible, to a disease depending on a single gene difference should the disease ever arise. I do not think there is any way of protecting oneself against intro­ductions of diseases from other species such as the introduction of myxomatosis into the oryctolagus rabbit or for that matter green monkeys disease into the human species. However, plant breeders have run into difficulties with diseases in which single genes determine susceptibility or resistance and I believe animal breeders should look more closely at the problem. It is one which I do not feel competent to say more about than that it is possibly there.

It is my opinion that at present the most urgent need to preserve existing adapted genotypes and the opportunity for genetic diversity and specialization is in those places in which environments are most exacting, environments in which it is already known that the highly productive breeds cannot thrive. In other areas, the danger is that the trend towards monotypic species will lead to monotypic methods of production.

14.6 References

Frisch, J.E. and Vercoe, J.E. 1969. Australian Journal of Agricultural Research. 20: 1189-95.

Vercoe, J.E. and Frisch, J.E. 1970. Australian Journal of Agricultural Research. 21: 857-63.

 Falconer, D.S. and Latyszowski, M. 1952. Journal of Genetics 51: 67-80.

Turner, H.G. Chapter in "Introduction to Environmental Physiology". Editor A.B. Slebodzinski. Panstwowe Wzdawnictwo Nankowe, Warsaw.

Adaptation du betail a son milieu
Résumé

En partie par suite de l'adoption croissante de méthodes uniforms d'exploitation et en partie grâce au sucoès des méthodes de sélection pour la production au cours de ces dernières déoennies, quelques races très productives de bétail sont aujourd'hui large-ment diffusées dans le monde entier. Leur capacité de production, même si elles ne sont pas particulièrement adaptées au milieu local, leur permet souvent d*avoir des performances supérieures aux races locales, du moins en apparence.

Dans certains milieux particulièrement défavorables, comme lee tropiques humidS ou les hautes terres du Pérou, une adaptation spécials au milieu local est nécessaire pour que les animaux survivent et produisent bien. Dans ces milieux, il importe de préserver des génotypes locaux Men adaptés pour les croiser avec des races très productives, afin de pouvoir combiner la productivité de ces dernières avec 1'adaptation au milieu de la race locale. Quatre grands types d'adaptation peuvent être distingués. L'adaptation au climat local, qui consiste presque exclusivement en une adaptation à la température ambiante, ne pose un pioblème que dans les climats extrêmement chauds ou extrêcmeraent froids. Dans le premier cas, la difficulté consiste à éiiminer la chaleur engendrée par la production dans un milieu qui produit lui-même de la chaleur au lieu de l'absorber; dans le second cas, le problème consists à éviter la consommation d'une trop forte proportion des ressources pour maintenir la température corporelle au niveau physiologique normal. Dans certains climate interviennent d'autres facteurs, comme le rayonnement solaire, la faible pression atmosphérique et l'humidité, qui peuvent jouer un rôle important en certains endroits et pour certaines espèces.

L'adaptation aux maladies locales est souvent importante. Des maladies comme les infestations de tiques, l'anaplasmose et diversea infections à virus et à bactéries reéiuisent la productivité, font baisser le taux de reproduction et compromettent la survie des animaux. Les races locales possèdent souvent une résistance intrinsèque à ces maladies, alors que cette résistance est absente chez les races introduites. Pour rendre une population résistante on peut reoourir à la sélection, mais le processus eBt lent. Les produits de croisement ont souvent une résistance suffisante, sans sélection ultérieure.

Il existe un second type de maladies - comme la tremblante ches le mouton - dont la manifestation dépend de relations spéciales entre un virus, ou un viroîde, et le génotype de l'hôte. Pour être en mesure de résister à leur8 attaques, la population considérée doit avoir la structure approprilée. Des maladies de ce genre apparaîtront nécessairement, Soit l'organisme pathogèns, soit l'hôte subiront tout au tard des mutations qui finiront par crier ceéte convergence, En pareil cas, la race doit posséder une variabilité génétique suffisante si l'on veut éviter des résultats désastreux. On l'a vu chez les plantes.

Enfin, les méthodes d'élevage et de production ont leurs propres exigences particulières. Jusqu'à présant, il n'y a guère de preuves concluantes qu'un génotype optimum pour une méthode de production ne sera pas le plus productif avec toutes les méthodes d'élevage. Je pense que cela est dû au fait que la sélection intensive a toujours été pratiquée dans des environnements qui sont optimums pour une forte productivité. Il n'y a pas d'exemple bien documenté de com-paraison entre des sélections d*intensité égale dans deux milieux trés différents, hors du laboratoire. L'expérience de Falconer sur les souris montre combien la selection peut être importante pour l'adaptation à des conditions particulières.

Je ne considèrs pas comme un problème sérieux les cas évidents où les produits de croisement entre une race adaptée et une race productive ou une race adaptée améiiorée sont les seuls animaux utilicables dans un milieu determine. Quand les races locales et leurs produits de croisement sont les seuls à survivre, les races locales seront préservérs, Dans les environnements où peuvent survivre des races fortement améiiorées, ces dernières risquent de prédominer et d'évencer les autres races. Comme il faut du talent et de l'argent pour créer une raoe fortement améiiorée, il faut s'attendre à ce que leur nombre soit faible, peut-être encore plus qu'aujourd'hui. En outre, tant que la productivité des races adaptéas restera médiocre, les races productives, male non adaptées, tendront fortement à les remplacer.

De cette situation dérive un dangeri la raoe tendra à devenir monotypique et elle sera alors très vulnérable à des maladies spécialisées qui risqueat de la décimer. Ce risque dérive du fait que son assise devient trop étroite. Cela s'explique par le petit nombre de grands élevagas qui pauveat se permettre de pratiquer les meilleuns méthodes d'amélioration génétique. II est à craindre que, à court terme, même si l'adaptation conbinée avec la forte productivité peut être payante, cette adaptation sera toujours limiée à un environnement et à una seule méthode d'élevage, étant donné que les races améliorées proviendront toutes d'un seul environnement. A mon avis, le problème que nous devons résoudre consiste à empêcher les races naissantes, obtenues par croisement ou dérivant de races très productives, d'être balayées par les races très productive avant qu'elles aient eu le temps de s¹adapter et de s'améliorsr. Je crois qu'il faut surtout nous attacher à créer une population animale aupérieure dans laquelle la diversité génétique et l'adaptation pourront être préservles contre la forte tendance à la domination d'une ou deux lignées fortcment améliorées, adaptées à un seul type de condition et à une seule forme d'élevage.

Adaptación del ganado a su medio
Resumen

Debido, an parte, a la creoiante adopción de métodos unificados de sxplotación, y en parte al ézito de los métodos de mejoramiento genético con miras a la producción empleados en estos últimos decenios, unas pocas rasas de ganado, sumamente productoras, están muy extendidas en todo el mundo. Por su capacidad de producción, es frecuente obtener de ellas aun en el caso de que no se bayan adaptado particularmente a las condicionas locales, mejores resultados que con las rates del país, o al menos así lo parece.

Hay Harios medios, oomo los tr�picos húraedos o las regiones más altas del Perú, donde los animales tienen que adaptarse espeoialmente a las oondiones locales para sobrsvivir y poder obtener de alios una buena producción, En esos medios es importante conservar los genotipos que se han adaptado a las condicionas locales, para cruzarlos con razas muy productives, a fin de combinar la productividad de estaa últimas con la adaptación al medio de la rasa del pais. Son deseables cuatro extensas clases de adaptación: la adaptación al clima local, que consiste casi siempre en la adaptación a la temperature ambiente, plantea un problems sólo en climas muy calurosos o muy fríos: en los primeros, la dificultad es eliminar el oalor generado por la producción en un medio que produce calor en vez de absorberlo. En el segundo, el problems es evitar la utilizacióin de una proporción demaaiado elevada de recursos en mantener la temperatura corporal a los niveles fisiológicos normales. Hay también otros factores, en algunos climas, corao la radiaci6n, la baja presión atmosférica y la humedad, que podrían ser importances en algunos lugares y para algunas especies.

La adaptación a las enfermedades locales es frecuentemente importante. Enfermedades como las producidas por las garrapatas, la anaplasmosis, y diversas infecciones virósicas y baoterianas se hanrevelado perjudiciales para la productividad, para el índice de reproducción y para la viabilidad de los animales. Las rasas locales tienen frecuentemente un poder de resistencia a esas enfermedades de que carecen las razas que han sido intro ducidas. Este poder de resistencia puede generarse en una población mediante selección, pero el proceso es lento: a menudo se obtiene una resistencia suficiente mediante cruzamientos sin necesidad de una selecoión ulterior.

Un segundo tipo de enfermedad es aquél cuya propagación depends de una relación especial entre un virus o viroide y el genotipo del huésped, como el prurigo lumbar de b s ovinos. Para que una población pueda resistir tal ataque tiene que tener la estructura adecuada. La existencia de esta claae de enfermedades es inevitable. Tarde o temprano ya sea el organiamo productor de la enfermedad o el huésped sufren una mutacitón, para poder adaptarse. La raza necesita entonces una variación genética suficiente para evitar resultados desastrosos, como se ha damostrado en las plantas.

Por úlltimo, hay oiertos métodos de cría y de tipos de producción que tienen sus propias exigenciaa especiales. Hasta la fecha, se tienen pocas pruebas convincentes de que un genotipo óptimo para un método de producción no serf el más productivo en todos los métodos de cría. A, mi juioio, alio se debe al hecho de que en los medios óptimos para una alta productividad se ha practicado sisapre una selección intensa. Ho existen, fuera del laboratorio, buenos ejemplos de comparaoiones entre una seleoción de igual intensidad en dos medios muy diferentes. El experimento de Falconer con los ratones musstra la importancia que puede tener la seleooi6n para ajustarse a un régimen particular.

No considero como un problema grave los caaos, obvios, en que los únicos animales posibles para una looalidad sean los obtenidos por cruzamiento entre una rasa adaptada y otra productiva, o los de una rasa adaptada mejorada. Cuando los únicos animales que sobreviven son los de la rasa local y bus cruoes, habrá que conservar esta rasa local. En las looalidades en que puedan sobrevivir rasas muy mejoradas existe el peligro de que predominen y ooasionen la desaparición de las demás rasas. Como las_rasas muy mejoradas requieren conocimientos y dinero, es de esperar que su número sea pequeño, tal vez incluso más pequeño de lo que es actualmente. Adsmás, mientras la productividad de las rasas adaptadas siga siendo baja, existirá siempre una fuerte tendenoia a reemplasarlas por otras rasas productivas, pero inadaptadas.

Los riesgos que esto puede acarrear son que la raza tienda a haoerse monotípica, con una manifieata invitación a que se desarrolle una enfermedad especialisada que haga desapareoer partes importantes de ella. Este riesgo se debe a la estrecha naturaleaa del caráoter que sirve de base a la raza. Esto está relacionado con el pequeño número de grandes empresas que pueden permitirse métodos genéticos óptimos de mejoramiento. Existe también el peligro de que, aunque la adaptación combinada, con una alta productividad dé buenos resultados, a corto plazo, como las razas mejoradas procederán, todas ellas, de un solo medio, no se conseguirá. nunca adaptarlas a más de un solo medio y método de cría. El problema que tenemos que afrontar es, a mi juicio, el de impedir que las rasas futuras, basadas en crusamientos o en razas sumamente productivas, sea aplastada por razas sumamente productivae antes de que hayan podido adaptarse o mejorarse. Creo que nuestro principal problema es el de crear una poblaoión ganadera cuya diversidad genética y adaptación puedan preservarse de la fuerte tendencia a la dominación de uno o dos linajes sumamente mejorados, adaptados a una serie de circunstancias y a una sola forma de cría.