The early part of this course focused on the effects of the tropical environment on animal production. In this unit, another important factor controlling performance of farm animal will be examined. Genetic factors relate to the influence heritable characters from parents and ancestors have on the performance ability of an animal.
Genetic factors differ from environmental factor which do influence performance of animal arising from the external environment rather than from inherited. The two principal tools (genetic and environmental factors) are available for livestock owner to use for improving animal performance.
For each to be used successfully, the other must be properly taken care of. If maximum improvement is to be derived from selection, animals should be reared in most favourable environment. This unit will offer basic principles and techniques used for animal breeding for all important performance traits.
Methods of Genetic Improvement
The farmer has two main ways in which he can attempt to raise the performance of the animals. He can either improve their environment or try to change their genetic make-up in order to increase their genetic potential. The various traits or characteristics of a pig for example are genetically controlled and inherited through genes, which contain the basic hereditary material.
These genes can be manipulated to achieve genetic improvement by either increasing the frequency of favorable genes or combinations of genes by selection, or by introducing new genes into the population by crossbreeding with other breeds or strains.
1. Selection for Improvement
Genetic traits can be divided into simple traits governed by a single pair of genes, such as shape of the ears or coat colour, or complex traits, controlled by many genes, which include the performance traits such as growth rate, feed -conversion efficiency and carcass quality. With
simple traits genes are normally dominant and recessive. If present in the heterozygote (i.e. a mixture of dominant and recessive genes), the dominant gene suppresses the expression of the recessive gene.
Recessive traits will thus only appear when two recessive genes come together in the homozygous form this means that occurrence of a trait in a breeding programme can be predicted. As an example, if a recessive trait was desirable (e.g. prick ears) then only prick -eared animals would be used as parents. This pattern of inheritance was first discovered by an Austrian monk called Mendel, in his classic work with green peas, and is therefore known as Mendelian inheritance.
In the case of complex traits, the situation is entirely different. If we take growth rate as an example, within a given environment the individuals, who possess a greater frequency of genes favouring growth rate will exhibit superior growth rates compared to the rest of the population.
Thus, if only animals with superior growth rates are selected as parents, this will increase the frequency of the favourable genes in the next generation. This may be illustrated in a population of pigs where selection is being considered according to growth rate, and only the animals growing faster than 750g/day would be used as parents.
The major factors which affect the efficiency and genetic progress in selection programme are as follows:
• Definition of objectives
It is of paramount that selection objectives are clearly defined before a breeding programme is embarked upon, and that they are not subject to constant change. This is particularly important in tropical conditions, where traits such as adaptability, coat colour and ability to produce on low-quality diets may be more critical than growth rates or carcass characteristics.
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Priorities may be completely reversed, e. g. fat pigs are preferred to thin pigs in some situations, The number of traits in a breeding programme must be kept to a minimum, as the more traits are selected for simultaneously, the slower the progress for each trait.
• Selection differential
This is a measure of the superiority of the selected parents over the mean of the population from which they are derived; the bigger the differential, the greater the genetic progress. Clearly, the larger the variation in a heritable trait in a population, that is the scope for a bigger selection differential, the faster the rate of genetic progression.
• Heritability
Heritability is a measure of the proportion of the superiority of the parents above their contemporaries which on average is passed to the offspring. More precisely, the heritability indicates the proportion of the total phenotypic variance that is due to additive genetic effects.
Conversely, a heritability of 100 per cent indicates that the trait is totally heritable whereby differences in environment between animals will not affect the phenotypic variation of such a trait. In general in the pig, reproductive traits tend to be of low heritability, growth traits of medium heritability and carcass traits of relatively high heritability. Genetic progress will always be greater when selecting for traits of higher heritability.
Generation Interval
This is defined as the average age of the parents when their offspring are born, and represent the time interval between generations. The shorter the generation interval, the more rapid the genetic progress, and if young boars are mated with gilts and replaced by selected progeny after one
litter, the generation turnover can be as short as one year.
Even in the normal situation where it is assumed that progeny born in first-to-fifth litters are equally likely to be chosen as replacements, pigs with an average generation interval of 2-2.5 years still have a great advantage over other domestic meat-producing species such as sheep (3-4 years) and cattle (4-5years).
Accuracy of measurement of units
The success of a selection programmed is entirely dependent, in the first instance on the accuracy of the records that are used. In this case important traits are easy to measure, e.g. live weight gain, but in others it is more difficult to be accurate, e.g. feed -conversion efficiencies and carcass measurements. Before embarking on a selection programme, it is essential to ascertain that the traits involved can be accurately measured.
Estimating the response to selection
Once the values for heritability (h2) and selection differential (SD) and generation interval (GI) have been determined, the genetic gain per year can be estimated.
2. Techniques and Processors for Testing Performance
Progeny-testing
As the boar has a relatively large influence on the characteristics which the next generation of a pig herd will inherit, testing systems have tended to concentrate on improvement of boars. Boar progeny-testing systems have been in operation throughout the world for a long time and are based on measuring the relative merit of a boar’s progeny from several sows. This obviously gives a true indication of what a boar may be able to contribute towards the genetic improvement of a herd.
However, in addition to being very expensive progeny-testing takes a long time in order to accumulate the data required to evaluate a boar, which makes him relatively old before his potential is known. This reduces his useful life. Consequently, as a routine system, progeny-
testing has largely been superseded for traits of higher heritability by the performance test.
Nevertheless progeny-testing is useful for assessing traits which do not lend themselves to performance-testing, e.g. sex-associated and slaughter traits.
Performance-Testing
The basis of a performance test is that an animal own performance taken as a measure of its genetic merit and with traits of high heritability used as a guide to how its progeny will perform. Thus the better individuals are selected from within group of contemporary animals that have been treated similarly.
The value and accuracy of a performance test can always be checked by running a subsequent progeny test seeing if the result agrees with the merit order of the Performance test.
Performance test can be carried out at central performance-test station, where the environment standard for all animals tested. If facilities are adequate, testing can be carried out on-farm for within-herd comparisons.
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The traits and selection criteria which are used in a test will obviously vary between countries according to their relative importance. They will also vary between countries according to their relative importance they will also vary within that country depending on the use for which the pig is required.
The criteria for selection of a boar which is to used for generating gilts for commercial breeding, for example, will be different from those of a boar which is to be used as a terminal sire for the production of slaughter stock. Nevertheless, various combinations of growth rate, feed-conversion efficiency and back-fat thickness are the traits which generally form the basis for selection.
Selection Methods
Selection methods also differ and the two mostly commonly used are independent culling levels and the selection index.
Independent Culling Levels
In this method, a level of performance is set for each trait, and if a pig fails to reach the desired standard in any trait it is automatically culled. It can be likened to an examination system where if you fail any subject, you have failed the total examination.
A major weakness of this technique is that if a pig has outstanding qualities in some traits say growth rate and feed conversion efficiency, and just fails to reach the standard on conformation, it is culled. The genes for the outstanding traits are therefore lost. This method is the main system used for judging merit in pedigree breeding schemes.
The Selection Index
In the index method, the traits to be selected for are combined for one animal into a total score or index. Each trait is normally weighted according to its economic value and heritability, so that the highest -index animal should yield the highest financial return. These economic weightings in the index can be adjusted as economic circumstances change.
The advantage of the index method is that exceptional performance in one trait can balance out a weakness in another and if two traits are correlated so that improvement in one leads to a simultaneous decline in another required trait this can be allowed for in the weighting.
3. Improvement Programmes
As an example of how testing and selection procedures can be integrated into a genetic improvement programme, the national pig improvement programme in Zimbabwe is described. This programme is based on a pyramidal structure A small number of registered nucleus breeders assessed to be the best herds genetically in the country, send their pigs to a central performance – test station.
Approved boars and gilts pass out to multiplication herds, assessed to be the next rung down in terms of genetic merit, where their progeny is performance tested on the farm.
Animals which pass this test are then available to commercial producers, thus ensuring a constant supply of quality replacement stock to commercial herds.
At the central test station, pigs are housed in pens of two and fed individually and their performance is measured from 35 tropical indigenous and an imported exotic breed. For this reason, the level of heterosis will always be highest in the first (f) cross, and decrease in subsequent crosses as the genetic differences decline. However, the overall economic gain which can be obtained from heterosis effects can be cumulative within a cross- breeding system.
Thus for litter -size at weaning, for example, if the first cross dam gives a heterosis improvement of 11 per cent, and there is a further 6 per cent to be gained from her progeny’s performance, the cumulative effect is 17 per cent, or more than one additional pig weaned per litter. There are a number of different cross – breeding strategic which can be used, all of which harness different amounts of heterosis.
Of these two breeding systems, crisscross system is probably the most appropriate for the small- scale producer in the tropics. If a third pure breed is available this can be extended to a triple crossing system using a sire selected from each of three breeds in rotation.
Maximum heterosis is obtained when the genetic diversity, between the parent breeds is large, and there is therefore considerable potential for cross between exotic and indigenous tropical breeds.
The best use of such crosses is likely to be in semi-intensive systems of production where the hardiness and foraging ability of the indigenous animal will be complemented by the growth and improved performance of the exotic. Large numbers of crossbred, animals can be found on these systems.
The two main problems are the maintenance of exotic boars for generating the cross-breed, as they often fail to survive due to disease and management hazards, and the lack of breeding control, resulting in indiscriminate interbreeding.
One possible solution under such condition is the use of institutionally managed boar -holding centre, such as those in some parts of northern Nigeria, where indigenous sows from small- scale farmers are kept to be mated with pure-bred exotic boars.
Alternatively, private farmers with intensive production units could be encouraged to produce cross- bred progeny for sale to small scale farmers. Similar methods could be use for the generation of exotic x indigenous boars for use on indigenous sows in small -scale production systems.
The benefits of heterosis in female reproductive traits can also be utilised in boars. Cross- bred boars have been shown to have greater libido, larger testes and higher sperm counts than their pure- bred contemporaries, leading to more reliable breeding and improved conception rates. This enhanced reproductive fitness is obviously likely to be a major advantage in extensive and semi-intensive production systems in developing countries.
The harnessing of heterosis is maximised in the genetic improvement schemes run by the private breeding companies in the developed world. Each company maintains a network of super- nucleus and nucleus herds in which superior genes are concentrated.
These herds are subjected to strong selection pressure with rigid and comprehensive testing of stock for important traits. Selected purebred stock from nucleus herds is then used to populate multiplier herds where improved lines and breed are crossed. The resultant first cross or hybrid gilts are sold to commercial producers.
4. Cross Breeding and Artificial Insemination
In the previous section, methods of pig improvement have been based on selection within breed. There is, however, another means by which the producer can attempt to genetically improve the performance. This involves cross- breeding which is the:
• Exploitation of the phenomenon of heterosis, which occurs whentwo breeds, which are genetically different, are crossed. If the cross- bred individual shows an improvement in performance above the mean of both parents it is said to exhibit hybrid- vigour or the ability to combine in one or more individuals the desirable characteristics of two or more existing breeds.
The higher levels of heterosis tend to occur in traits which are of lower heritability, e.g. reproductive traits.
Artificial insemination
Artificial Insemination (AI) involves the collection of semen from a boar and then the introduction of semen into a sow or gilt at a later stage by means of a catheter. This contrasts with natural service where a boar mounts a sow and introduces his semen.
The major advantage of AI is that is allows for the wider use and distribution of boars of high genetic merit. The ejaculate from one boar can be extended to inseminate up to 25 sows.
Recent advances in methods of boar semen storage make it possible for developing countries in the tropics to the very top genetic stock from developed countries (e. g. in the UK, only the top 5 per cent of boars performance-tested by the Meat and Livestock commission are eligible for entry to AI studs). This calibre of genetic material would not otherwise be available to developing countries.
Other benefits of AI are:
• It overcomes the need to purchase, house and feed a boar. This isparticularly pertinent to the small-scale producer who cannot justify keeping a boar for a small number of sows, and who cannot afford a boar of good quality. The effective use of AI is especially relevant where small-scale producers are involved in group or co-operative pig development schemes, and their units can be serviced from a central boar-holding centre,
• It prevents the transmission of disease from farm to farm by the sale and purchase of boar and on-farm reproductive disease cannot be spread by boar-to-sow contact.
• It overcomes the practical problems of differences in size of males and females. On occasions, this problem can severely limit the use of boars of high caliber.
• It reduces the risk to stockmen of handling boars for natural service.
Techniques
a. Semen collection
Although various artificial vaginas and electro-ejaculators are available, they are not necessary for successful semen collection. Boars can easily be trained to mount a dummy sow device or an oestrous sow, and firm pressure on the penis by a gloved hand causes ejaculation to occur. The first low-sperm fraction can be collected through a filter funnel, which removes the gelatinous fraction, into a warmed (300C) bottle.
A drop of semen can be observed under a microscope to check its fertility characteristics and, if desired, the semen can then be diluted. A number of diluents and extenders are available, and the individual doses are normally stored in 50 ml plastic bottles for up to three days at 150-200c. the number of spermatozoa used under commercial conditions for one insemination normally varies from 1×109 to 3×109 .
b. Insemination
This involves the insertion of a rubber spiral catheter into the sow’s vagina, and then rotating it in an anti-clockwise direction until the tip locks into the cervix. The bottle containing the semen dose can then be attached to the other end of the catheter and the semen runs in under gravity slight pressure. The insemination process may take up to 15 minutes.
c. Heat detection and timing of insemination
It is very important note that higher conception rates are achieved with AI approach than those that occur with natural service. Accordingly, accurate heat detection must be carried out, preferably using a boar twice a day in order that the timing of insemination is correct.
To overcome inaccuracies in the detection of the start of oestrus and the natural variations in time of ovulation two inseminations approximately 12 hours apart are recommended.
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More recently, devices have been developed which measure the electrical resistance of the vaginal mucosa. As this varies in relation to hormonal events, it can be used to predict more accurately the timing of ovulation and hence the optimum timing of insemination.
System of semen storage
(a) Frozen semen
In contrast to bull semen, the nature of boar semen renders it susceptible to damage by the freezing and thawing procedures, and as a consequence only relatively recently have successful techniques for freezing boar semen been developed.
Frozen semen can be obtained in either pellet form or in straws, and acceptable conception rates obtained, this has provided a major breakthrough for the introduction of superior genetic material into developing pig industries.
(b) ‘Long-life’ semen
Extenders have now been developed which allow for the storage of fresh semen for up to seven days without any marked loss in fertility. This allows for fresh semen to be sent by air around the word and used to impregnate sows successfully in the country of destination
5. Some Inherited Defects to be Avoided
There are many anatomical defects for example in pigs which can affect performance. The majority of these are genetic in origin and therefore likely to be inherited from generation to generation. Tests are conducted to detect defects.
These defects may be simple (i.e. controlled by one pair of genes) or complex (controlled by several pairs of genes). At the end of the test, the feed conversion efficiency is calculated, and the fat thickness is measured by an ultrasonic machine.
Pigs are assessed on the basis of an index derived from the relative economic values of feed conversion efficiency and fat thickness; animals which do not pass the index are culled. In addition, animals with any abnormal sexual characteristics, blind or insufficient teats, or other defects such as weak legs or undesirable body conformation are also culled.
On – farm performance tests are then carried out on the sons and daughters of approved centrally – tested sires. Pigs are inspected for any weaknesses and, if they attain certain performance standards in terms of growth and fat thickness, they are made available for sale.
The potential value of regular performance -testing can be seen from the results of a control landrace herd, in which both boars and replacement gilts were selected on the above scheme over eight generations. The improvement in feed – conversion efficiency represents a considerable saving in costs.
Simple defects
The can be effectively eliminated from a herd by culling. The main defects in this category are:
• Congenital tremors in this condition, piglets are born with rhythmic tremors of the head and limbs. It occurs at a frequency of about 0.1 percent in the population. The recessive gene is carried by the dam, and in carriers 50 per cent of male piglets are affected. Dams of affected litters should be culled.
• Club foot this defect causes a deformed and swollen foot. It has a very low incidence and is restricted to the landrace breed. Both male and female parents should be culled.
Complex defects
These are more difficult to eliminate, but greatest response will be achieved by culling boars. However, the incidence of these defects is generally relatively low, so it may be more economic in the long run to keep a good boar which is a carrier of a known defect rather than replace it with a genetically inferior boar which is not a carrier.
If good records are available, the cost of the defect can set against the gain in pig performance attributable to the carrier boar. Such a boar should only be used for the production of slaughter stock and none of his progeny should be retained for breeding.
The main defects in this category are:
• Scrotal hernia: A weakness in the body wall allows part of the intestines to pass out into the scrotum. It occurs at a frequency of around 0.7 per cent and has an estimated heritability of 15 per cent.
• Umbilical hernia: A similar condition which occurs at the site of the umbilicus. Found at a frequency of around 0.6 per cent.
• Imperforate anus: The incidence of this condition in piglets is around 0.3 per cent. Mortality in male piglets is always 100 per cent, but often around 50 per cent of females survive as the faeces are voided via the vagina.
• Splaylegs piglets are born with either the front or hind legs splayed, sometimes both, and are unable to stand. The incidence is around 1.5 per cent and can be worsened by nutritional
deficiencies. If piglets survive for three days, the condition often tends to disappear.
• Hermaphroditism: In this condition, females tend to exhibit male characteristics. Incidence is estimated at 0.07 per cent.
• Cryptorchidism: Also known as a ‘rig’ pig, one testicle in the male fails to descend into the scrotal sac. Found at a frequency of 0.22 per cent.
• Female genital defects, including ‘inverted nipples’. These occur at a frequency of around 0.15 per cent.
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