1. ANIMAL GENETIC DIVERSITY AND THE ENVIRONMENT - THE LINKAGES

1.1 Change in Farming Systems
1.2 Impact on Genetic Constitution
1.3 The Importance of Population Size
1.4 The Present Status of Domestic Animal Diversity

Agriculture, in both developed and developing countries, is built on a number of basic resources. Some, like sunlight and air, are not depleted or compromised by agricultural activities. Others, like land and water, can be degraded or depleted by inappropriate use. They therefore form the focus of concern in the interactions between agriculture and the environment.

Livestock, which are often considered simply as a product of or an instrument in agriculture, are also a basic resource, on a par with land and water. In this context, the livestock resource also forms part of the environment, and is exposed to deterioration from many of the same causes.

Degradation of the livestock resource can take several forms, of which the most easily recognised is the depletion of genetic diversity. However, in many parts of the world, livestock populations can also be compromised in other ways. In many subsistence agricultural systems, particularly with communal land ownership, perennial feed shortage means that animals function at little above survival level. Throughout the tropical world for example, cows typically calve for the first time at four to five years, and thereafter at two-yearly intervals. Calf survival rates are often low. The net effect is a population barely able to maintain its numbers, with persistent under nutrition and distress for the animals and very limited capacity to generate productive output.

A series of case studies has been prepared to illustrate or support various aspects of the use or conservation of animal genetic resources. These are given in the appendix and are referred to as A1, A2 etc. Other references are given in numerical order.

1.1 Change in Farming Systems

Economic and demographic pressures cause changes in farming systems. These pressures are generally for intensification of systems. There are occasional exceptions to this. For example, a scheme (Regulation 2078/92) introduced under the Common Agricultural Policy of the European Union, provides financial incentives for the extensification of livestock farming. Among other provisions, it seeks to reduce stocking rates and associated fertiliser applications below certain limits.

However, the pattern of recent change in the different livestock systems has been, almost without exception, one of growing intensity of production. At a very broad level, this can be quantified by comparing annual rates of growth in the numbers of animals on the one hand, and of volume of production on the other.

Table 1 makes this comparison for cattle, buffalo, sheep and goats and dairy cows. For these species it is clear that, over the past decade, ruminant meat output has increased annually more than three times faster than animal numbers. Milk production from dairy cows has increased at more than twice the growth in the number of cows.

Table 1 Average annual rates of growth for the decade 1981-83 to 1991-93.

	Number of Animals	Volume of Production	Ratio in growth rates production/animals
	Annual percent growth		Ratio
Cattle (meat)	0.4	1.5	3.75
Buffalo (meat)	1.5	4.8	3.20
Sheep & goats (meat)	0.7	2.4	3.43
Dairy cows (milk)	0.4	0.9	2.25

Source: Agrostat

Because of difficulties of enumeration, the process of intensification cannot be quantified in quite the same way for pigs and poultry. Asia accounts for 42% of world pig production and almost 90% of that in the developing countries. Annual output increased at an average of 7% in Asia, but at a rate of 17.8% per year in landless farming systems (which in turn accounted for 29% of output in the region). A similar pattern is evident in poultry production, with overall annual growth rates of 9.6% in Asia, rising to 17.3% for poultry meat, and 18.5% for eggs from the landless systems (which account for about half of total production in the region).

The components of these increases in productivity cannot readily be quantified. However, in different systems, they undoubtedly reflect more intensive feeding patterns, with increased offtake rates, higher slaughter weights and milk yields, and reduced losses from disease.

Intensification has different effects at different levels of development of the economy. In agricultural systems which are emerging from subsistence level into a market economy, the drive to increase saleable product often leads initially to an expansion in animal numbers. This can be seen, for example, in the Near-East and North African area, where in the period 1965 - 1986 a doubling of meat output from the cattle, sheep and goat populations was achieved almost solely by an increase in animal numbers¹. In other cases, both an increase in population size and in output per animal takes place. Beyond certain limits, this requires substantial imports of nutrients in the form of feed and fertiliser to the system. The best documented case is that of the Netherlands, where for 1989, it was estimated that the balance of nutrient inflow and outflow from agriculture left a net excess of almost one million tonnes of nitrogen, phosphorous and potassium in an agricultural area of less than 4 million hectares. Over 80% of this excess was credited to the livestock sector².

Changes such as this are the result of a long process of intensification. In other cases, changes are now being introduced in farming systems which had been stable for centuries. A case in point is the extension of cropping into what had been exclusively livestock grazing areas throughout the Sahelian zone in West Africa³.

These changes in farming systems can have a major impact on the genetic constitution of the livestock populations involved. Change in genetic constitution can in turn be an agent for change in the farming system. For example, the availability of cross-bred cows through AI can be the catalyst for an intensification of milk production systems.

1.2 Impact on Genetic Constitution

These linkages between farming systems and genetic constitution are shown in Fig. 1. Change in the farming system often involves changed breeding objectives for livestock. In traditional systems, number of livestock rather than output per head is often the main consideration. In these circumstances, traits related to survival in the face of nutritional, health and climatic stress predominate. As farming systems become more market oriented, volume and value of saleable product take over. In these circumstances, selection goals often change from multi-purpose use to much narrower targets. They shift to such traits as prolificacy, early maturity, individual growth rate and milk yield, and aspects of milk composition and carcass quality.

Selection for such traits has transformed the genetic constitution of many developed breeds. For example, in many non-market traditional cattle systems, cows first calve at four to five years and thereafter at two year intervals, producing perhaps 500 litres of milk per year. In modern highly developed dairy production systems, cows first calve at two years, and thereafter at yearly intervals, and produce up to 10,000 litres of milk per year. The genetic change in the animal has made possible the intensification of the system, while the evolution of the system of feed supply, management and healthcare has made possible the support for high producing genotypes. The net result is a dramatic improvement in efficiency of resource use.

In addition to the direct effect of selection on the genetic constitution of livestock populations, changes in breeding objectives can also promote inter population gene flow. Thus, in West Africa demand for higher body weight beef animals has led to substantial cross-breeding of larger Bos Indicus on the smaller Bos Taurus populations^A10. In China, local breeds of pigs are being crossed at unprecedented levels to imported breeds^A7. Such patterns of cross-breeding, over a few generations, can effectively lead to total replacement of the domestic pool of genetic diversity. In some cases, this can happen very rapidly, for example in some poultry systems, where domestic strains are simply bypassed by the importation of totally new genotypes.

The second route through which changes in farming systems can affect the genetic constitution of populations is through changes in breeding structure. In parts of Asia, the move from draft power to mechanical cultivation has meant that some cattle and buffalo populations are no longer dominated by working animals. The move to a market economy may lead to increased and selective offtake of animals from some pastoral systems, leading to changes in sex ratios and age groups. The introduction of hatcheries in poultry, artificial insemination in cattle, hybrid breeding companies in pigs, can all promote direct changes in the genetic constitution of populations, as well as speeding up inter population gene flow.

Changes in breeding structure, reflecting changes in the farming system, can also have a major impact on population size. The most dramatic effect is through competition between populations or breeds. This can lead to the rapid growth to dominance of more specialised breeds, and a parallel rapid decline in less competitive ones. It can also have an effect on population size in the system as a whole. With expanding demand, both animal numbers and animal productivity tend to rise. However, as can be seen in North America and Europe, limited growth in demand accompanied by rapid increase in animal productivity has produced substantial declines in dairy cow populations. Reduction in population size can effect the genetic constitution, mainly through reduction in diversity at the breed level, but also through the effects of inbreeding within small populations. Increasing population size is also a major direct factor in the interaction between livestock and the environment.

1.3 The Importance of Population Size

All animals within a population, a breed for example, share a common pool of genetic diversity. When population size falls below a certain level, this genetic diversity is affected in two ways. Firstly, certain genes may be lost from one generation to the next, since with restricted population size a restricted sample of the genes available is passed on. Secondly, because the number of individuals in the population is limited, the probability is increased that the parents of any new individual are related to each other. This means that the probability of the individual receiving the same genes from the maternal and paternal sides of its pedigree is increased. This is turn means that the genetic variability within the individual is reduced. It is a well established and universal fact that this reduction in variability within an individual (particularly a reduction in variability within each locus, or heterozygosity) is associated with deterioration in many physical traits, particularly those associated with viability.

These effects of small population size put the continued survival of a population in great danger. Conservation of bio-diversity at the breed level therefore requires particular attention to the maintenance of adequate population size in all current breeds.

The principal effect of reduced population size is the associated decline in intra-locus genetic variability within individuals (heterozygosity). This in turn is measured as the inbreeding coefficient. Many studies have shown that functional traits deteriorate at between 3% and 5% for every 10% increase in the inbreeding coefficient. In a random mating population of N individuals, with equal numbers of males and females and discrete generations, the increase in inbreeding per generation is a simple function of the population size: 1/(2N+1). However domestic livestock populations do not mate randomly, and usually have many more breeding females than males. In these circumstances, the “effective population size” N_e is usually much lower than the number of animals in the population: N_e = 4MF/(M+F), where M and F are the numbers of males and females respectively. In a population with a large number of breeding females, and a small number of males (as with artificial insemination for example) this formula means that the effective population size is approximately 4 times the number of males.

The impact of small population size on genetic diversity within the population is also affected by a number of other factors. These include:

· Generation Turnover: Long generation intervals, as in horses, can delay the negative effects of small population size and inbreeding.

· Current and prospective changes in population size: if effective population numbers have been declining and are likely to continue the downward trend, then calculations based on current size are likely to underestimate the risk.

· Changes in herd structure: in some small populations, the total number of herds may be quite small. The smaller the number, the greater the risk to the breed arising from the termination of any one herd.

· The extent of cross-breeding: while cross-breeding can remedy the effects of inbreeding, it also means that the genetic diversity within the population is being reduced.

A recent report on the status of breed diversity in Europe⁴, has proposed a classification of endangerment of breeds on the basis of arbitrary limits on acceptable inbreeding over a fifty year period. Because of the different generation intervals, this corresponds to different population sizes in each species. The five classifications are shown in table 2.

Table 2 Definition of breed status

	Max. acceptable inbreeding %
	Over 50 years	Per generation
Normal	< 5%	0.36
Potentially endangered	5 - 14	1.07
Minimally endangered	15 - 25	1.79
Endangered	26 - 40	2.86
Critical	40+

Source: Simon & Buchenauer, 1993

1.4 The Present Status of Domestic Animal Diversity

Global domestic animal biodiversity at the breed level comprises some 4,000 different breeds or landraces. This is approximately equal to the total number of mammalian species currently in existence. Because of the rapid changes in farming systems, the local breeds and types are coming under increasing pressure. Concern about the erosion of genetic diversity at the breed level has underpinned the establishment of a global programme on animal genetic resources in FAO⁵. The first initiative under this programme was to produce a World Watch List⁶ of breeds at risk. Working with rather general criteria, a first, and necessarily incomplete, list has been compiled. This includes a total of 2,719 breeds across the seven main species. These represent some 68% of the total. Of those breeds with adequate data for assessment, 27% were classified as endangered. This is believed to substantially understate the degree risk of breed loss, because undocumented breeds are often likely to be more vulnerable, and because a further 20 or so domesticated animal species have not been covered. The net conclusion is that there are likely to be at least 1000 breeds in danger of extinction.

In Europe, a recent inventory of breed diversity showed that less than half of the total complement of 864 separate breeds was judged to be “normal”. Some 158 or 30% were evaluated as “critical”.

These figures illustrate the reality that the erosion of biodiversity at the breed level is not simply a concern for the distant future, but an active ongoing process.