Helen Newton Turner
Breeding objectives
Selection criteria and measurement techniques
Techniques for genetic improvement
Genetic considerations are part of an improvement package for small ruminants. The whole package includes
1. Definition of breeding objectives,2. Choice of selection criteria, and decisions on measurement techniques,
3. Decisions on the techniques for genetic improvement,
4. Establishment or improvement of marketing systems, to ensure that increased production gives extra income as an incentive to improvement,
5. Improved management (including veterinary care) to ensure realization of genetic potential,
6. Development or improvement of an extension service, which will involve training extension officers to assist producers in implementing improvement plans.
These aspects are not necessarily in order of sequence of application; some are preferably concurrent. Only the first 3, i. e. the genetic aspects, will be discussed in this section.
The main objective is to maximize the output of the system per unit input. The output may be the economic return, or amount of product (food, fibre, skins, etc.) and its quality. The inputs are food, land, investment, labour, veterinary care etc., many items of which are on a per head basis, and so it is convenient to measure production too, per animal. The quality of animal products must be defined in relation to the requirements of the end user or target market. This market may be export, the internal market or home consumption. These objectives have been called "breeding objectives" but they apply whether improvement is sought by breeding or improved management.
The quantity of meat produced depends on the number and weight of surplus animals at age of sale. The importance of reproduction rate must be stressed in relation to surplus animals for sale for meat. Meat quality is poorly defined in most less developed countries. Urban markets in general require lean meat, but the fat tail is a delicacy in some countries, and there may be specific flavour requirements in some areas. Skins are a valuable by-product of the meat industry, but there is no clear definition of skin quality.
There are basically two types of wool: apparel and carpet. The quality of wool depends on average fibre diameter (fine for apparel, coarse for carpet wool), percent clean yield (including freedom from vegetable contamination), staple length, percent medullated fibres, kemp (shed fibres) and colour (white fetches a higher price on most commercial markets, but pigmented fleeces are sometimes preferred in cottage industries). Most small ruminants in Africa produce hair which is used locally, and there are no clear definitions of quality, apart perhaps from staple length and colour.
For milk the most important measure of productivity is yield per year. Quality is less important although goat milk may be desired because of its flavour, and sheep milk because of its high solid content.
Selection will be involved at some stage, whatever technique is chosen for genetic improvement. Where small ruminants are kept primarily for meat production, selection will be on the number and weight of offspring weaned per female per year. The number of offspring born can be increased by decreasing the number of females which fail to lamb (or kid), by increasing litter size, or by increasing the frequency of parturition. Females which fail to produce offspring after consecutive opportunities should be culled, but the heritability of such failure is low, and such culling may not have a marked effect on the reproductive performance of the whole flock. Litter size, on the other hand, will respond to selection. The acceptability of twins depends on the environment (particularly nutrition) and management system. There is considerable interest around the world in increasing the frequency of parturition, and there have been reports of success. There is little information so far, however, of the effect of more frequent parturition on total lifetime productivity. With the year-round reproduction which occurs in Africa, there is scope for research on the heritability of the parturition interval, and the effect of decreased parturition interval on lamb survival and growth.
For wooled sheep, selection is based on weight of fleece, but considering also fibre diameter and percent medullation. A long staple is required for most markets; in general, selection for increased fleece weight will also result in a longer staple, but it might be necessary to pay particular attention to staple length for indigenous Africa sheep.
Selection of animals for milk is in terms of quantity of milk produced per year, which is a function of quantity of milk per lactation, lactation length and parturition interval.
Techniques for genetic improvement include selecting between breeds (or strains), selecting within breeds and crossing. Crossing includes grading-up, developing new breeds and exploiting heterosis. It should be noted that even if crossing is chosen as a technique, selection is usually involved as well.
Selection between breeds. Many international meetings over the last few years have stressed the need to look more closely at indigenous (also called local or native) breeds, because of their adaptation to their environments, instead of assuming that importation of a so-called "improved" breed is a short cut to increased production. Selection between breeds will therefore mean comparing indigenous breeds, as well as comparing an indigenous with an exotic or its cross, as in the past. Selection between breeds must be based on performance data collected from groups run in the same environment. Comparisons in the past have usually been on experiment stations; there is a great need for them to be made in the environment where the animals will run. Techniques for field evaluation will be discussed at a later session. Comparisons MUST be carried through to give realistic estimates of lifetime production including not only measures of products such as body, fleece and milk weights hut also reproduction, mortality and morbidity rates.
Selection within a breed. Selection is usually done within a flock, i.e. among animals of the same age which have been run together. Genetic progress through selection (selection response) depends on heritability, selection differential and generation interval.
Heritability is fortunately moderately high for many of the characteristics with which we are concerned. Body weights later than at weaning and fleece weight can be selected on a single measurement for both sexes. For sex-limited characters such as weight of offspring weaned or milk production, selection of males must be on performance of female relatives. The most useful female relative is the dam; selection on records of half-sibs or progeny lengthens the generation interval and so lowers the annual rate of genetic progress.
Selection on number (or weight) of offspring weaned per female at a single parturition per year should preferably be based on more than one record. Selection of both ewes and rams can be based on the dam's lifetime record using the formula for heritability of a repeated record, namely:
Dam's ranking coefficient = 
Where h² is heritability of number (or weight) of offspring weaned per ewe mated; t is repeatability; k is number of matings; PD is number (or weight) of offspring weaned per mating for all matings of the dam under consideration; and PD is average number (or weight) of offspring weaned per mating for females in the same flock, up to and including the dam's present age. This formula is most readily used in flocks where full records are kept. Where they are not, it is still possible to devise a system of recording a female's performance at each parturition (at least for numbers of offspring) by means of ear-notches, eartags or tattoo-marks. The formula was devised for once-a-year lambing. Its advantage is that younger dams are placed on the same footing as their elders. It could probably be extended to cover year-round breeding, if estimates of the heritability of the interval between parturitions are available.
In areas where twins are acceptable, females which twinned consistently could he marked, while another mark could be placed on females which consistently had a short interval between parturitions. Where twins are not desired, a shorter interval might be used as the selection criterion, though, as already mentioned, more information is needed about the effect of shortening the interval on lifetime production.
The selection differential depends on the accuracy of selection (i. e. measurement rather than eye appraisal), the proportion of animals saved and the variability of the character concerned. When flocks are small, the proportion of animals saved will be high, and the selection differential lowered, particularly for rams; progress through selection will thus be very slow. Further, if rams are always chosen from within the Bock, inbreeding levels become high. Ways of overcoming this problem include establishment of a central sire-breeding nucleus of larger size, either by government or private ownership, establishment of co-operative breeding schemes, and the use of ram circles and ram exchange.
A central nucleus should contain at least 300 breeding females, preferably more. In it selection on measured performance would he practiced, so that response to selection would be predicted. Males from the nucleus would be distributed to small flocks. Whatever rate of genetic progress was achieved in the nucleus would also he achieved in the flocks which consistently drew their males from it. This progress could be 1-2% per year for the suggested selection criteria. If this seems small, remember that it is cumulative, so that after 10 years production would be 10-20% higher. Since indigenous breeds have become adapted to various harsh environments throughout Africa, it would he logical to choose among the breeds within each region, then establish a sire-breeding nucleus for each. The question of the population of commercial flocks which could he influenced by a nucleus of a given size will be discussed later.
Cooperative breeding schemes have become popular in New Zealand and Australia for both sheep and cattle. They involve the establishment of a ram-breeding nucleus through the cooperation of a number of flocks, each of which contributes its best ewes to the nucleus each year, receiving rams in return. Suppose a central nucleus of 300 breeding females could he established by a contribution of 5 females from each of 30 cooperating Hocks in each of 2 years. These flocks might consist of 25 breeding females each. The annual contribution thereafter would depend on the length of life of females in the nucleus and on its reproduction rate; calculations have shown that genetic gains in the nucleus are maximized if half its replacement females come from within itself, half from the cooperators. Annual contributions after the first two years would thus be of the order of 1-2 ewes from each cooperator. The nucleus would be able to supply sires to smaller flock as well as to its cooperators. Details of such schemes could be worked out if they were thought feasible. Selection in the nucleus would be based on measured performance, but the contributed females could be selected visually. Record-keeping would be required only in the nucleus.
Ram circles, as operated in Scandinavia and Ireland, require backing from extensive record-keeping and data analysis in all flocks involved. In the absence of such infra-structure, they are probably not adaptable to African conditions at present.
Ram exchange is more feasible. If several small flocks are normally herded together so that their general environment can be regarded as the same, then the females' reproductive performance can be recorded provided they are identified by ear-notch, eartag or tatoo. Males can be selected from the combined flock on a combination of their own weight (age adjusted) and their dam's reproductive performance. This would help to increase the selection differential, and if rams were rotated among similar flocks, inbreeding would be lowered.
Crossing. Crossing may be between indigenous breeds, or between indigenous breeds and exotics. Very often in the past it has been assumed that exotic breeds from temperate areas, with high production in their homelands, will automatically raise productivity when crossed with local breeds in tropical areas, and very often the introductions have failed. Crossing may have a place, but is not necessarily a rapid highway to success, and introductions should be carefully evaluated in the field before widespread crossing is undertaken. Such evaluation would include comparison with indigenous breeds.
Grading up involves replacement of one breed by another, with continual back-crossing, while development of a new breed means crossing of two (or more) breeds, then at some stage mating inter-se, with selection. Again evaluation in the field is needed to assess the new type of animal.
Exploitation of heterosis is a technique widely used in Europe and North America, particularly with cattle. It is useful if a cross between two (or more) breeds has higher production than the superior parent. This is sometimes the case, and the technique is used in Australia for lamb production. Merino ewes are crossed with Border Leicester or Dorset rams, the crossbred ewe progeny then being mated to a meat breed sire such as Dorset or Suffolk. The crossbred ewe has a higher lambing percentage than either parent, and the terminal sire gives fast growth rate to the lamb. Use of such a system means there must he a continual supply of the parent breeds. Estimates of heterosis for sheep and goats vary widely, and more data are required before such schemes could be recommended for small ruminants under African conditions. There could be a place for stratification of the industry, with crossing of two indigenous breeds to develop a system similar to the Merino x British breed crossing used in Australia, but research is needed.
Gene distribution. It is instructive to see how long it takes to distribute superior genes once a source is established. If males are being distributed from a selected central nucleus of an indigenous breed, any flock drawing those males will make the same rate of genetic progress as the nucleus. The number of males available annually will depend on the size of the nucleus and its reproduction and mortality rates. A nucleus of 300 breeding females might have 80 males available for distribution annually, and a nucleus of 500 breeding females, 130. If the average life of a male in the field is 3 years, then males can be distributed to 3 sets of flocks in rotation. In this way the numbers of females reached can be calculated as (average male life) × (males available annually) × (male: female ratio). Some examples are in Table 1. If artificial insemination could be used, the numbers would of course be greater.
Table 1. Numbers of females mated by sires from central nucleus
|
Size of central nucleus (females) |
Male: female ratio | ||
|
|
1:10 |
1:20 |
1:30 |
|
300 |
2,400 |
4,800 |
7,200 |
|
500 |
3,900 |
7,600 |
11,700 |
If exotic or synthetic breeds are being distributed, the first decision to be taken is the level of the new distributed genes required in the target flocks. Assumptions can be made about reproduction rates and death rates in the village flocks and, using these, calculations can be made of the length of time taken for the required level of genes to be reached in a village flock. At that point, males from within the target flocks can be used to maintain the level, though with some rotation of rams to avoid inbreeding. When a given set of flocks becomes self-maintaining, distribution of males can be switched to another set. Calculations along these lines have shown that, 10 years after establishment, a nucleus of 500 breeding females could raise 4, 500 females in village flocks to 50% level of introduced genes, provided males with 100% of the genes were distributed. Again, with AI this number would be greater. These calculations have been made to show that even if a first cross between an exotic breed and an indigenous shows a great superiority over the indigenous, distribution of genes through a population takes a long time.
