CHAPTER 23
BREEDING PROGRAMMES FOR INDIGENOUS BREEDS

by

C.G. Hickman

Summary

Total levels of livestock production in adverse environments are mainly affected fey crossbreeding between indigenous breeds and high producing breeds from the temperate zones. Survival or adaptation genes, including those for disease resistance, are essential in hot dry or humid climates and for unreliable feed supplies* Regular genetic selection for adaptation is necessary for economic performance by indigenous breeds and for supplying hardy surplus stock for crossbreeding.

The importance of economic considerations in choosing optimum breeding programmes and policies is emphasized. The number of reproductive cycles per lifetime, is probably the most important trait for indigenous breeds. Breeding plans are presented for cooperative selection schemes for farmer-breeder associations with institutional herds. Breeding for heterosis is presented in the form of procedures for the maintenance of self-sustaining hybrids. Heterosis may best be achieved by reciprocal recurrent selection in institutional herds or flocks for identifying breeding stock for use in indigenous and exotic breeds. Applied conservation methods are called for in the form of global and inter-regional breed organizations and resource herds and flocks.

23.1 The need for breeding programmes

The question may be asked, why should indigenous or local breeds be organized and pro­moted by breeding programmes? The answer is related to the fact that indigenous or local breeds have developed an adaptation to their environment which enables them to produce and reproduce where imported breeds have serious survival problems. There are increasing examples of indigenous breeds being superior to imported breeds and the trypanotolerant livestock of West Africa are an outstanding example. Breed improvement of adapted breeds is justified as an attempt to maximize level of production in a region or country by genetically improving level of performance and at the same time raising the level of genetic adaptation. An alternative approach to maximizing level of production would be to import high-producing stock and subject it to genetio improvement for adaptation to the new environ­ments. A third method is to cross local and exotic breeds to establish a foundation for new breed development. Both of these alternative approaches require the same kind of breed­ing programme that would be designed for the original indigenous breeds. Breed improvement of the present indigenous breeds has a very important advantage over alternative approaches because these breeds can contribute substantially to crossbreeding programmes where heterosis is of major advantage.

The importance of breeding programmes for indigenous breeds has been highlighted by an economic analysis of breeding systems by Kasowanjete (1979)• A breed improvement programme for the native Malawi Zebu (MZ) was compared by simulation with upgrading or rotational crossing with the Charolais. These three systems were evaluated with and without the assist­ance of artificial insemination (Al). The six systems were compared by calculating the Net Present Worth (NPW) which is expressed as:

where,

Bt = benefits in each year

Ct = costs in each year

t = 1, 2...... n

n = number of years

i = interest (discount) rate (after Gittinger, 1972)

The results demonstrated that for beef production the breeding programme for the MZ without AI gave the highest NPW and adding AI to this within-breed programme gave the lowest NPW, The Internal Rate of Return (IRR) was also determined by finding the cash flow discount rate (i) which most nearly makes the NPW of the breeding system equal to zero. The IRR values as shown in Table 23.1 illustrate the highest return from the indigenous breeding programme (Mode] 1 = No AI, Model 2 = AI), This was based on simulation studies for nucleus, multiplier and commercial herds using economic values of local costs, returns and expected level of purebred and crossbred performance for Malawi.

Although breeding improvement of native breeds may not be the most economic way to maximize national yield of livestock produce in all countries or products, breed adaptation is being increasingly recognized as an indispensable part of national breeding and cross­breeding programmes. Aside from the economic potential from improving indigenous breeds, a fair evaluation of native stock is not possible without breed development. The use of local stock purchased from village livestock markets for comparison with imported stock, introduces a bias against the local breeds. Breeding programmes are an essential part of conservation of breed resources. The identification of a local breed type does not cons­titute conservation. The genetic potential of such breeds must be attained by a breeding programme that sorts out the culls and puts breed comparisons at a reasonable level of selective improvement for local breeds to allow fair assessments. Most local breeds would benefit tremendously and become more economic from massive culling campaigns. In addition to the neoessity of good breed development for evaluation and thus for conservation, there is also a national economic advantage from having an adequate supply of superior breeding stock available for small farms and villages.

The fact that genetic adaptation to environmental stress can be improved indirectly by selection for level of performance has been reported recently by Prison (in press). Very few data are available on how to improve adaptation genetically but there have been some speculations that random breeding is helpful if not essential to allow for the accumulation of adaptation genes. Frish's results and genetic theory substantially dispute this random-breeding concept. In his experimental results from selected and non-selected (control) populations maintained in natural range conditions in Queensland, Australia, the selected population shows a higher level of adaptation than does the random-bred population, which demonstrates that positive correlated responses in survival ability occurred from selection for rate of gain. The selection for rate of gain has endowed the population with greater rate of gain in body weight and feed efficiency, greater disease and parasite resistance, water evaporation ability (heat loss) and lower mortality, whereas the random-bred population, although exposed to the same environment for the same number of generations, remained at a lower level of fitness.

These results will likely be repeated and confirmed by other experiments presently being conducted to study the inheritance of adaptation. The importance of studies for disease resistance, such as trypanotolerance, is critical and will show whether by selection for level of performance, there is a positive response in immunogenetic ability or other disease protection mechanisms.

23.2 Special features of breeding programmes for indigenous breeds

Indigenous breeds or animals resembling these "breeds account for the largest portion of the world's livestock. Yet breed organization is minimal, if existent at all, for even the most well known native breeds. The very specialized breeds of the temperate and relatively lightly populated zones benefit most from the application of genetics. The reason for these rather paradoxical differences lies in the lack of resources where most of the tropical indigenous breeds exist. Personnel, technology and finance are generally in short supply to organize and promote even the better known indigenous breeds. Breeding programmes for these breeds must be at a level of minimal sophistication and the selection objectives and criter­ion must be carefully chosen to suit the meagre conditions on small farms and in villages. It is necessary to emphasize adaptability traits for breed improvement but at the same time it must be recognized that heterosis also has a higher importance in more favourable than in stressful environments. For example, the breeding method known as reciprocal recurrent selection (RRS) is of little significance where heterosis is of low magnitude but it could be a distinct advantage in breeding indigenous breeds for crossing with exotic stock. Like­wise the use of criss-cross or rotational crossing is relatively more important in harsh environments than where heterosis is not a major factor for performance or survival. In fact hybrid breeding methods could be assisted by blood-typing males to ensure the presence of certain genes or genotypes that are known to increase heterosis. An indirect measure of heterozygosity by individual performance testing for sire selection might be as important in developing countries as is progeny testing for additive gene action in temperate zones.

Figures 23.1 and 23.2 are diagrams of breeding methods that are useful for maintaining or improving heterosis. The use of these "self-sustaining" hybrids (Hickman, 1979) should be promoted for those livestock species that have low reproduction rates. Such mating systems require superior adaptation for indigenous breeds and a careful choice of exotic breeds for the regular production of hybrid bulls.

In these diagrams for hybrid breeding systems the use of more than one breed in an overall breeding programme is fundamental. The crossbreeding of dairy breeds that are adapted to harsh environments and to limited feed supplies with dairy breeds that are adapted to favourable environments and high feed intake, gives maximum production in the humid tropics where livestock production is concentrated. Figure 23.3 is a graph representing the general situation for the equatorial areas, although other areas could be included under the design­ation "marginal land".

Present results show that heterosis is necessary for maximizing performance especially for milk production and suitable parental breeds will be necessary for capitalizing on appropriate crossbreeding programmes. As indigenous breeds are improved by well designed breeding programmes and husbandry knowledge is gained, the importance of heterosis may change. In other words, if the environment can be improved by the elimination of disease, or of disease-causing conditions, or by the elimination of heat or climatic stress, then high intake breeds and breeding methods could apply where they were previously unsuccessful. That is to say, a large portion of the presently observed heterosis may be explained by the inability of the imported exotic to cope with the tough environment.

23.3 The use of standard testing procedures for identifying superior breeding stock.

Table 23.2 tabulates the possibilities for organized testing methods applicable to devel­oping countries. Artificial insemination is listed separately because it is not applicable or needed in most cases but can be considered as a cross stratification for the methods listed under 1 and 2. The following discussion on these methods will emphasize opportunities for testing that can be conducted at the community level. Farmer-breeder operations will be contrasted with research or college herds that require large investment and costly institut­ional administration.

Testing Stations. Figure 23.4 outlines the essential operations for breed development using testing stations. The main purpose of testing stations is to provide a uniform test environ­ment. This is particularly helpful where the environment effects of farms or villages cover up real genetic differences between prospective breeding animals. Other advantages are that a testing station need not be costly; it can be organized for community or area development and it applies to all kinds of livestock. For growth, carcass or draught traits a testing station can consist of simply a grazing area with the necessary weigh-scales, a livestock chute, veterinary requirements and livestock personnel. For milk or wool production or any measurements of reproductive efficiency the husbandry specialization and length of test period complicates the testing station concept. Aside from these limitations a testing station is unique among all testing methods in its general applicability. Whether perform­ance testing or progeny testing is envisaged a testing station operation can be useful. The latter differs primarily in requiring more animals than the former. Which is the same as saying that the test station works best for highly heritable traits whereas other methods are better for lowly heritable traits. Currently, testing stations are being included in plans for the breeding of trypanotolerant livestock in West Africa where it is logical to obtain measures of immunogenetic responses to standard antigen challenges during the period of testing for individual performance.

Home testing. This is a term applied to performance measurements taken in different herd or flock environments in contrast to the testing station concept. Home testing is difficult if not impossible in most developing countries. What opportunities there are, require the keenest knowledge of animal breeding, innovation and a special understanding of each partic­ular situation. Naturally, nomadic or transhumance operations are basically more difficult to organize into breeding structures than are sedentary production practices. Yet these two production systems have their own unique advantages. The nomadic system generally involves large herds or flocks which allow for intra-herd or flock ranking on producing ability. The sedentary systems, although usually of very small herd or flock size, allow for regular contact by extension of veterinary services, artificial insemination and/or exchange of breeding stock and organization into cooperative units. Most of the N'Dama cattle are managed under at least a partially nomadic system whereas the Shami cattle and goats are substantially sedentary. Both systems are being approached by "home testing" but in entirely different ways.

The N'Dama cattle, under the care of Foulani tribesmen are in projects designed for multi­plication and improvement and in one case there is a combination of home testing and station testing for the breeding programme. The home testing part of the operation consists simply of identifying superior cows and bulls. The measurements for this are very simply a record of longevity under heavy tsetse challenge conditions. Regular veterinary inspections or innoculation exercises would keep track of exceptional animals and try to obtain agreement to purchase male offspring or the animals themselves. The male offspring would then be subject to a testing station performance test to allow for selecting the best males for return to the private herds.

The home testing for sedentary systems, which are extensive in India, are hindered by lack of technology and the absence of applicable methods. Since herd or flock size is very small, methods for using village averages or for measuring individual feed intake or body condition must be established to enable useful estimates of breeding values. Unfortunately, specialists in the field of animal breeding have avoided such field opportunities in favour of breeding programmes for research station or college herds. The importance of these types of operations in developing countries is evident from the several attempts that have been made and the resulting contribution to breed development.

Where the testing station can be of help with individual performance appraisals, in home test, under a variety of meagre conditions, can be helpful in detecting superior fitness or longevity under practical conditions.

Institutional or commercial operations are of limited value in developing countries unless special arrangements are made for cooperation with farmer-breeder associations. The terms of reference for one such association in one actual case is given in Annex A. Very often attempts to improve indigenous breeds, by using so-called government herds, are fraught with failure because any selection gains are offset by inbreeding depression. Very infrequently, if ever, are government operations in breed improvement large enough to be effective.

A minimum of 2,000 females of the bovine species are needed if progeny testing is to be used and more or less this number is needed for other species depending on level of prolificacy. Poultry require the smallest number to avoid inbreeding and to allow sufficient selection but, even with poultry, government efforts do not appear successful in maintaining rare and adapted genotypes. Only the Australian Milking Zebu and the Jamaica Hope among the adapted dairy cattle breeds, seem to be maintained by government or semi-government support in sufficient numbers for breed improvement purposes.

In India the Federal Government, through the Indian Council of Agricultural Research and military farms, and several colleges and State Governments have maintained large herds and flocks for research and development purposes. Yet only recently have cooperative efforts with farmer-breeder operations appeared. Consequently, the famous zebu breeds have not improved in genetic quality over the years but hopefully the new Indian Bureau of Animal Genetic Resources will be effective in organizing at least minimum numbers for effective breed improvement.

The arrangement of matings in institutional herds should be such as to accomplish;

1. genetip relationship among other superior herds or flocks within the breed located elsewhere in the world and thus be a resource herd;
2. genetic relationship of the institutional herds or flocks with farmer-breeder herds or flocks in the area or region so that testing and the estimation of breeding values are -under practical conditions and numbers are large enough to enable effect­ive selection;
3. a measure of genetic change for ready application to the selection criteria and to the important component traits.

Table 23.3 illustrates a mating scheme (Hickman and Freeman, 1969) which has been applied successfully to different species and which is particularly applicable to developing count­ries and indigenous breeds. This method allows for the repeated use of groups of young males (or females) over years (or breeding periods) with a group being introduced each year. This allows easy measures of response to selection and a comparison of current with previous breeding stock. The stratification by age of the other parent (age of female parent) allows for the estimation of the effect of parental culling on current progeny performance.

Regarding cooperation with farmer-breeders one method that has been successfully used is to lease the older members of institutional herds to private operations with the agreement that all female offspring will be returned to the institutional herd (see Figure 23.5). This way a breeding flock or herd of ewes, cows or buffalo etc. can be divided between the institutional and farmer-breeder flocks or herds. By using all the institutional facilities for young females and leasing the older females, the total size of the herd or flock can be increased by three- or four-fold. The young females can be used appropriately for detailed performance measurements and the older cows give valuable information on longevity and general adaptability. The females that last the longest and reproduce most regularly in farmer-breeder herds or flocks would be highly rated as better adapted to the adverse envir­onment .

A genetic relationship among nucleus herds or flocks or breed populations makes breed evaluation possible over wide areas. The testing of Finnish Landrace, D'man or Romanov sheep, to cite a few examples, would be most effective if the whole population of each breed around the world were highly related. Similarly with other breeds or species. And if all regional populations could not be included, then portions of world populations should have relationship within these sub-populations which allow for comparison inferences. This regular interchange of breeding stock would establish reference breed populations consisting of several widely spaced and related resource herds or flocks. They then form a standard of comparison against which local breeds can be evaluated for the environment in which a reference breed is adapted. The results of breed evaluation then become referable to known populations and experimental results are applicable over wide areas rather than to isolated sample herds or flocks of a breed in a certain particular region. These comparison possib­ilities result from the mating arrangements given earlier as:

1. relationship among resource herds or flocks;
2. relationship among each resource herd or flock and local herds or flocks, and;
3. relationships over time to allow measures of response to selection.

It is suggested that these items he considered by this meeting as fundamental in stating the objectives for establishing a programme on the management of animal resources and breed conservation.

23.4 References

Kasowanjete, M.B.B., 1979. Evaluation of the three breeding systems: within - Malawi Zebu (MZ) breeding, upgrading MZ to Charolais (CH) breed, and rotational crossing between CH and MZ breeds for maximum and efficient beef production under Malawi conditions. Royal Veterin­ary and Agricultural University, Copenhagen.

Frison, J.E. (in press) Animal Production.

Hickman, C.G, 1979. The estimation and use of non-additive genetic variability in cattle and buffalo. The Indian Journal of Animal Genetics and Breeding. 1(1):1-6.

Hickman, C.G. and A.E. Freeman, 1969. New approach to experimental designs for selection studies in dairy cattle and other species. J. Dairy Sci. 52:1044.

Fig,23.1. CRISS - CROSS MATING. *

* Rotational crossing is the same as criss-crossing but uses three or more breeds.

Fig,23.2. REPEATED HYBRID MALE CROSS.

Fig.23.3. LIVESTOCK CLIMATOLOGY : Equatorial Areas.

Fig. 23.4. COMMUNITY BREEDING WITH A TESTING STATION.

Fig.23.5.INSTITUTIONAL - FARMER BREEDER COOPERATION.

Table 23.1

Internal rate of return; that discount rate (1) is

Breeding System

Table 23.2

Testing for pure breeding of indigenous breeds 

 Table 23.3

Overlapping young males - method of measuring response in selection

R�sumé

Leo programmes de sélection du bétail indigètne reposant sur lea hypothèses suivantes:

1. II exists un système effectif d'exploitation et aussi un inarché (ou quelque autre débouché) grâce auxquels le producteur peut tirer profit de see efforts.

2. II existe un service consultatif qui aide les éleveurs privés à résoudre les problèmes de sélection, d'alimentation, d'exploitation et de santé animale qui se posent au niveau de la ferme ou du village.

3. La valeur des races ou des systernes de selection peut être déterminée objeotive-nent, dans les conditions où les types sélectionnés seront appelés à produire, sur la base du rapport inputs/sorties.

4. Le développement des races et d'organisations de producteurs peut être encourage par la fourniture de oertaines quantités d'aliments, par les servioes de vulgarisation, et par le subventionnement des prix, etc.

5. II faut aussi présumer que la théorie de la génétique quantitative peroet de traiter 1'adaptabilité ou ses composantes comme n'importe quel autre caraotère quantitatif et que cette adaptabilité existe sous des formes différentes dans tous les environnenents.

Ces conditions sont jugées nécessaires pour encourager les efforts de sélection durables qui sont indispensables à une amélioration genetique effective. Si la sélection régulière d'animaux supérieurs ne deviant pas une pratique solidement Stabile, les avantages de 1'amelioration génétique risquent de ne jamais être entièrement exploités. Les critères de sélection doivent être uniformes au cours des années, car les changements d'orientation de la sélection peuvent annular des gains précédents. La technologic nécessaire pour la pratique professionnelle de 1'amélioration génétique profite à la societé en général, elle est le produit d'une structure sociale évoluée et c'est un facteur d'amélioration de la qualitié de la vie.

La communication présentée comprendra des graphiques illustrant des programmes de sélection prometteurs pour des utilisations non spécialiées avec une analyse du rapport inputs-sorties dans chaque cas. Différents systèmes sont envisagés:

- stations expérimentales
- essais au niveau de la ferme
- troupeaux appartenant à des organismes publics ou à des sociétés privées
- insémination artificielle et importation de sperme et d'animaux reproducteurs.

Toutes oes opérations sont partioulièrement effioaces quand elles sont menées par des associa­tions d'éleveurs. Pour juger de l'intérèt des differénts programmes de sélection, il faudrait prendre comme critère le coût du programme et 1'amélioration génétique totale de la production nationale. Si un troupeau appartenant à un programme public ou à une société privée (troupeau d'élite) s'améliore génétiquement au rythme de 3 pour cent par an, le cheptel des fermes qui ue proourent des reproducteurs mâles dans ce troupeau s'améliore beauooup plus lentement et ce taux plus faible n'est atteini qu'après un délai considérable. Cette "réponse retardée" est supposée égale à la moitié de la réponse du troupeau d'élite (o'est-A-dire 3/2 - 1,5) pendant plusieura générations. On suppose en outre que la fraction de la population animale qui re­colt des mâles du troupeau d'élite ne représents que 10 pour cent du cheptel national total. Le taur d' amélioration généique du cheptel national n'est done que de 10 pour cent de celui de la fraction utilisant des taureaux améliorés, soit 0,15 Pour cent par an. On peut également Pour obtenir une amélioration génétique entreprendre une sélection au niveau de la ferme ou d'un ensemble de fermes en utilisant uniquement les taureaux issus des 10 meilleurs pourcent des vaches d'une grande population dans une sone determinée. Ce système pourrait être désigné sous 1'appellation "mére à taureaux". 1e taux d'amelioration génétique qui peut ètre obtenu en utilisant uniquemant des mâles sélecttonnés d'après les performances da la mère ne représents qu'una pat ite fraction du taux réalisable quand toutes las possibilites d'amélioration genStique sont exploitérs (comma dans les troupeaui des organismes "publics"). Mama si ce faible taux d'améioration na représente qu'un huitième du taux réalisable dans les populations fortamant spécialisées da reprcducteurs, où il peut atteindre 3 pour cent, le taux est da 3/8 = 0,37 pour cent environ dans la très grande population où seuls la forme plus simple de sélection peut être pratiquée. Si l'on part de ces chiffree arbitraires de 3 pour cent, 10 pour cent et un huitième, on voit alors que le systèms "mère à taureaux" donne un taux d'accroissement de la production nationale 0.37/0.15= 2,5 fois plus èlevé que le systèms reposant uniquement sur 1'utilisation, dans un nombre limité de troupesux privé s, de taureaux superieure provenant d'un troupeau "public" fortement sélectionné . Cela montre que les méthodes de sé lection les plus compliqué es et les plus poussées, etant donné " les effect ifs limitée de la population animals auxquels elles peuvent s'appliquer, peuvent ne pas être aussi avantageuses pour un pays qu'une methods plus simple maie ayant de plus larges applications. Un bien plus grand nombre de troupeaui peuvent être inclus dans un systèms de sélection plus simple) le rythme d'amélioration est plus lent, mais les bénéficiaires sont plus nombreux, de sorte que 1' aocroissement total de la production nationals est substantial, même s'il n'est pas maximum. Evidemment, il faudrait chercher à combiner les deux aspects - méthodes perfect ionnées et gros effect ifs - mais il est impossible d'appliquer 1' insémination artificielle, le contróle sur descendance, etc., a beau-ooup des races indigènes parce que les noyens financiers, les capacités professionnelles et les mo yens de transport font défaut.

Des taux d'amélioration intermédiaires peuvent être obtenus dans les stations expéri­ment ales ou avec le système du "troupeau d'élite ouvert". Ces méthodes consistent à évaluer avec soin de jeunes animaux choiais dans des troupeaui privés, à éliminer les sujets qui ne répondent pas à un standard minimum et à réintroduire les meilleurs animaux dans les troupeaui privé's, après evaluation. Les troupeaux appartenant à des organismes publics peuvent être utilisés de nombreuses faeons pour remplir ce róle de station expérimentale. En outre, en utilisant ainsi ces troupeaux on évite la consanguinité dont sont normalement viotimes ces petites populations isolées, tout en établissant de bona contacts avec les éleveurs privés aux fins de la vulgarisation. En fait, ces troupeaux peuvent faire l'objet d'une exploitation coopérative qui permet une large diffusion de la méthode dans de nombreuses comrnunantés.

L' nsémination artificielle est une technique qui peut se greffer sur de nombreux programmes de sélection de types different. Son principal avantage, pour de nombreuses races indigènes,réside dans le fait qu'elle permet d'utiliser largement des mâles provenant d'autres régions. De nombreuses races indigènes se oomposent de populations isolées dans laquelle la consanguinuité est courante, et les évaluations sont limitées à ces populations. L'inséminar-tion artificielle permet d'obtenir de grandes populations productive à partir de petites populations inefficientes.

L'insemination artificielle est utiliséepour introduire du matériel génétique exotique mais malheureusement il s'agit le plus souvent de races non adaptées (les races adaptées sont en grande partie ignorées). Les descendants ainsi obtenus ont la moitié des gènes d'adaptation de la population indigène originals mais, étant donné le fort potential productif des génotypes importés et le phénomène de 1'hétérosis, les produits de croisement ont des aptitudes supérieures, Quand l'hétéroise traduit dans les performances, il y a généralement un manque d'adapta-tion de la race importée, de sorts que les descendants de la génértion F1 et de générations rétrocroislés ont un ran dement décevant.

Bien étendu, il n'est pas nécessaire de recourir à 1'insémination artificielle pour im­porter du matériel génétique exotique. Dans le cas des ovins par example, animal dont le sperme est difficle à congelar ou à utiliser avec des taux de fécondation Slevés, des béliers importés peuvent être mis à la disposition das petites fermes et des villages. Etant donné les grandes différences de prolifioité entre les raoes indigènes d'ovins, l'importation de béliers de raoee adaptérs et prolifiques peut donner de très bons résultats imméaiats.

Les programmes de sélection même les plus simples peuvent done être extrêmement utiles. et ils n'exigent que dee connaiesances et une surveillance minimee. Lee méthodes d'ameliora­tion génétique plus sophiatiquées et plus rapidee sont attirantes, maie coûteuses et extréme-ment exigeantee du point de vue technique. Malheureusement, 1 •heritability de 1'adaptability générale n'a pas fait l'objet d-' étudaé appro fondies, et 1'on ne sait pas si une séction in­tensive pour la productivité en milieu défavorable accroitrait 1' adaptabil ité. Logiquement, elle devrait le faire. L'effioacité des facteurs immuno génétiques dont dépend 1'adaptation devrait être variable et susoeptible d' amélioration par la sélection. De même, on devrait pouvoir ren forcer la tolerance à la chaleur et la ràsistance aux in sect es en pratiquant une sélection génétique visant par exemple à augmenter la fréquence du type le plus efficace de glandes sudoripares et de caractéristiques de la peau. Mais il faut reconnaitre que la forte production est en soi un faoteur stressant, spécialement dans les environnements trèe chauds, et qu'elle contribuera de ce fait a 1' infertilité et aux troubles physiologiques. Dans chaque environnement, il y a un niveau moyen de production optimum du point de vue economique. Pour établir des programmes de sélection, il faut avoir quelques lumières sur les rapports entre 1'adaptabil itè et la capacitè alimentaire (niveau de production). Avec ces oonnaiesances, il est possible d'établir des programmes nationaux de sélection en vue de 1'amélioration génétique dee caractéistiques qui sont les plus importantes pour un taux d'exploitation maximum.

Resuaen

Los supuestos fundaaentales de los programs de oria de rasas indigenas son lo. siguientest:

1. Exists un sistema de aanejo pràctico y tanbién un mercado (o alguna otra salida) que peraiten al productor sacar buen partido de bus esfuersos.

2. Exists a disposioión de los ganader os privados un servicio de asesoria para resolver en la granja o en las aldeas los problemas planteados en materia de crianza, alimentación, manejo y eniernedades.

3. Pueden efectuarse evaluaciones imparciales sobre rasas o sistenas de orianza, toaando como la base la relación total insumo-producción en las condioiones en que ban de producir los diversos tipos de rasa.

4. El desarrollo de rasas y las organizaciones de productores pueden fomentarse nediante incentivos en forma de cupos de piensos, servicios de extensión, subsidies a los precios, etc.

5. Se supone asiaismo que la teoria génética cuantitativa peraite oonsiderar la adaptabilidad o sus componentes como cualquier otro carécter cuantitativo, y que esta adaptabilidad exista en diferentes foraas, on todos los ambientes.

Estas condiciones se consideran necesarias para alentar los programas de selección a largo plazo que son esenoiales para una mejora genética efectiva. Si la seleccion sistemática de animales superiores no puede ser tan permanente y regular como los hábitos sociales noramles, nunca podrán realisarse plenaaente los beneficios de la mejora genética. De igual modo, los criterios de selección deben ser constantes a lo largo de los anos, por-que los cmabios en la dirección de selección pueden anular los logros obtenidos. La tecnologia necesaria para la actividad profesional an la mejora de las rasas reporta un benefioio general a la sociedad y refleja una esclarecida estruotura social y una calidad mejorada de la vida.

La presentación comprenderá diagraaas de planes de cria prometedcres para condiciones no especializadas, con una estimación de insuao-producción para cada uno de ellos. Estos planes se basan sobre los siguientes procedimientos:

- estaciones de prueba
- pruebas en la granja
- rebaños o manadas institucionales o corporativas
- inseninaoion artificial e iaportación de seven, y animales de cria.

Todas estas operaoiones son eficaces al máximo cuando se llavan a cabo por asociaciones de productores. Los criterios para la evaluación de diferentes programas de cria hábran de ser el costo del programa y el aumento genético total en el nivel nacional de producción. si un rebaño (núcleo) institucional o corporativo mejora genéticamente a razón del 3 por ciento al año, tal población de granja que obtenga ganado de crianza macho de ese rebano mejorara según un proporción muoho más baja que el 3 por ciento anual y ese indice bajo sólo se aicanza despuéa de mucho tieapo. Durante varias generaciones,se supondrá que la respueata de retraso" es igual a la mitad de la respueata del rebaño núcleo (es decir 3/2 - 1,5). Adenás se supone que la sección de la población que reoibe machos del rebano núcleo ea só1o el 10 par ciento de la población nacional total. Por lo tanto, el indice de aejora genética nacional ea sólc al 10 por ciento del correspondiente a la seccion que eaplea los toros majorados, 6 sea. el 0,15 Por ciento anual. Alternativamente, la mejora genética pudiera derivar de una prueba efectuada en la granja ó de un programa de cria por una cooperat iva de ganaderó eapleando sóle toroe que provengan del major 10 por ciento de las vacas, en un población de diatrito muy amplia. Este sistema podria dencainarae " madre de toroe" o bien "madre de padres". El indice de mejora genética obtenido empleando solamente selección aasoulina sobre rendimiento de madres conatituye só1o una pequeña fracción del que se obtiene cuando se emplean todas las vias posiblea de mejora genética (coas que puede hacersa en rebaños instituoionales). Incluso si este bajo indice de mejora represanta sólo la octava parte del indice correspondiente a las poblaciones de oria altamente especializadas que coansiguen un indice de mejora del 3 por ciento, el indice es aproximadamente de 3/8 = 0,37 por ciento en la población muy grande en la que só1o puede aplicarse la forma de selección mis simple. Asi pues (eapleando estas oifras discrecionales del 3 por ciento, 10 por ciento y octava parte), con el sistema "padre de madres" se obtiene un incremento nascional en products que resulta 0,37 = 2,5 veces mayor que el obtenido con el sistema basado únicamente era la utilización de 0.75 su periores de un rebaño institucional altamente seleccionado an un número limitado de rebanos privados. Esto indica que los métodos más complicados completes de selección pueden reporter menos beneficio total a un pais que otro sistema más extendido pero más simple, dado el tamaño limitado de la poblaoión capaz de proporcionar estos altos nivales de perfeccionamiento._ Puede incluirse en un procedimiento de crianza menos complicado un nûmero mucho mayor de rebaños. El indioe de mejora es manor, pero los beneficios son generales, de modo que el incremento total del rendimiento nacional es importante incluso si no es máximo. Evidentemente, el objetivo deberia consistir en combinar la complejidad con el tamaño grande de población, pero para muchas de las razas indigenas del mundo no se dispone de apoyo financiero, direccion profesional ni medioa de transports que permitan aplicai los beneficios de la inseminacion artificial, prueba de descendencia, etc.

Indices intermedios de mejora pueden ser logradoa por las eataoiones de ensayos o con el método denominado eaquema de núcleo abierto". Estos métodos suponen la avluación precisa de ganado joven seleccionado procedente de rebaños privadoe, la eli­minatión de las reses inscritas en el registro genealógico que no alcancen un nivel mfnimo, y la reintroducción en rebaños privados de los mejores animalea después de la evaluación. Les rebañoa institucionales pueden emplearee de muchas formas para esta funcidn de bstación de ensayos. Es más, eapleando un rebaño institucional de esta manera se evita la endogamia habitual que afeota a esas poblaciones pequeñas y aisladas, y al mismo tiempo se facilitan los contactos con produotores privadoe, a efectos de extensión. Tales rebaños pueden manipularse de hecho en base comunitaria cooperativa, lo que permite una amplia ex­pansión del método a través de muchas comunidades.

La inseainación artificial (IA) es una téonica que puede superponerse a muchos tipos diferentes de programas de cria. Su ventaja principal para muohas razas indlgenas es que posibilita el amplio uso de machos procedentes de otras regiones. Michas rasas indlgenas se componen de poblaciones aisladas en las que la endogamia es comun y las evaluaciones se efeexuan solo dentro de esas poblaoiones. La IA permite obtener grandes poblaciones eficaoes a partir de poblaciones ineficaces pequenas.

La IA se emplea para importar ganado exótioo de cria, y desgraciadamente la mayor parte de asta aotlvidad se aplioa a razas no adaptadas (de las razas adaptadas se hace caso omiso con frecuencia). La descendencia resultante dispone de la mitad de su frecuencia genética para genes de adaptación que provienen de la población indlgena original, pero debido al alio potencial de producción de loa genotipos iaportados y al fenóaeno da vigor hibrido, lot orusaaientoB tienen rendiaientos auparioras. Sieapre qua al vigor hibrido rasuita evidente an los caracteres da rsndinianto, suala haber falta da adaptaoion an la rasa importada y, oonsiguienteaente, loa descendientes da reproduotore* de F₁ y las ganaracionas da orusaaiento da raircceso dan lugar a dacapcionas.

For supuesto, al sistema da IA no as nacasario para importer material raproductor exótico. Con los aviaos, por ejempe, cuyo seman no se congala fácilaente ni sa anplea con altos por-centajes da concepción, puadan faoilitarse Ios carnaros iaportados a pequeños agrioultores o a las aldeas. Dado al amplio aspectro da fertilidad antra rasas indienas de ovinos, caba obtener muchos banaficios inaediatos da la iaportación da cameros da rasas adaptadaB y proifficas. For consiguiente, incluso los programas da oriansa más simples son extremadamente útlies y sólo requieren ligerisiaos conocimientos da oria y supervisión. Los métodos más perfeooionadoa y rápldos da majora da rasas son suaamente atraotivos, pero caros y axigen el eapleo de téonicas da altislmo nivel. Desgraciadamante, no se ha estudiado extensamente la herencia de la adaptabilidad general, y no se sabe si la selección intensiva para produotividad an un ambiente duro aumentaria la adaptabilidad. Lógicmente debaria ocurrir asi. La eficacia de los factores de inaunidad genética que haoen posible la adaptación, debaria ser variable y mejorable por selección. Del mismo modo, la tolarancia al calor y la resistencia a los insectos debarian ser mejorables por selección genética, por ejemplo, aumentando la frecuenoia de los tipos más aficaces de glándulas sudoriparas y caracterieticas de la piel. Paro hay que aceptar qua la producoión alta por si aisma es un factor de esfuerzo y fatiga, especialmante en aabientes de temparaturas elevades y, por lo tanto, contribuirá a la infertilidad y la inoapaoidad fisio-lógica. ftura cada ambiente, exists un nivel medio de máxima economla de producción. Para or ganisar programas de criansa hay que tenor ciertos conocimientos de las relaciones entre la adaptabilidad por una parte y la capacidad o nivel producción, por otra. Con estos conocimientoe podrán organisarse programas nacional/de criansa para la mejora genética de los caractares que son de la máxima iaportancia para conseguir el máximo rendiaientc comercial.