This is carried out with meat producing animals where the characteristics wanted in the offspring can be seen in the sire. It is based on the phenotype, the performance and appearance of the animals being tested, and it is a measure of the actual performance of an animal.
Performance Testing involves the study of an animal’s performance to assess its suitability for use as a breeding animal. It consists in measuring the differences between animals for a certain trait such as daily livemass gain, under standard conditions of feeding and management. The animal under test is reared and its performance is measured, recorded and assessed.
Due to this type of testing being based on phenotype, it can be accurately only with traits that have a high heritability, traits such as:
|Daily Livemass Gain
|60 % heritability
45 – 60 % heritability
50 – 60% heritability
It follows, therefore, that it is most accurate with beef cattle and pigs:
- Heritability of livemass gain is 60%. This means that beef bull calves making fast daily livemass gains should sire calves that will do the same, especially if the calves are reared in a similar environment to that of the test animal; and
- Efficiency of Livemass Gain (Food Conversion) is 45 – 60% heritable, so that the animal which is a good food converter should breed offspring that are good food converters.
Research work has shown that the genes that produce rapid growth also cause more efficient utilisation of the food consumed by the animal. There is a good correlation between rapid livemass gain and efficient livemass gain, and this means that bulls selected for rapid livemass gain should also show efficient livemass gains.
It does not follow always that the best performers will breed the best offspring. A closely bred animal will, when mated with unrelated stock, breed better performers than itself. An animal produced from a wide outcross within a breed may perform well because of some hybrid vigour, but will breed poorer performers than it.
Performance Testing is carried out in many parts of the world by both private farmers and official organisations in dairy, beef and pig production.
Points to consider about this type of testing are:
- The high cost of special testing station.
- The heritability of the traits being studied during each performance test; and
- The effect of the environment on the animal being tested. Stock is best tested under the environmental conditions which their offspring will encounter e.g. ranch bulls should be tested under ranching conditions.
Sibs or siblings are animals that are closely related e.g. brother and sister. Sib testing is a method of performance testing for sex limited traits such as butterfat or milk yield. These traits are sex limited because only females can produce milk – you cannot milk a bull. When sib testing for milk yield or butterfat, groups of sibs (full sisters) are performance tested and the brother of the best bunch is selected for breeding.
The genotype of a parent or parents is assessed by looking at the performance of phenotype of the offspring. It is the best way of testing the breeding qualities of an animal, and while it is effective when used to test any traits, it is of most use in testing for traits with low heritability. It is most effective for testing sires because they can produce many offspring relatively quickly, and it is used to test dairy bulls, beef bulls and pigs.
In progeny testing, the performance of all the offspring of a sire are recorded and evaluated. If this is done in a large herd, differences in the performance of individuals must be due to breeding differences, but comparing the performances of a bull’s daughters that are in different herds can be misleading because these differences might be due to a different environment – different systems of feeding and management. In the U.K., the Milk Marketing Board which operates a country wide Artificial Insemination service has tried to overcome this problem in the progeny testing of its bulls, by using the contemporary comparison system of evaluating the breeding merits of a bull. This system is explained below.
The other main problem with Progeny Testing is the long time period between using a bull for breeding and completing the performance test of his heifers. Many bulls selected for A.I. service have produced daughters that show no real increase in milk production over the family average of the bull.
CONTEMPORARY COMPARISONS AND RELATIVE BREEDING VALUES (R.B.V.)
The objective of progeny testing dairy bulls is primarily to find out which bulls consistently produce the best dairy replacements. Farmers may carry out limited progeny testing in their own herds, but in the majority of cases they are limited by herd size and the number of bulls which they can use. To overcome this some farmers group themselves together and transfer their bulls from farm to farm amongst themselves, thus benefitting from an increase in size. In addition, the A. I. Service provides a useful means of progeny testing bulls.
It must be remembered that about 80% of the milk yield difference between herds is due to feeding and management, and only 20% to breeding. Thus within a single herd the level of production of a group of daughters by one bull is probably more a measure of the farmer’s husbandry ability, than his ability as a breeder.
THE R.B.V. AND CONTEMPORARY COMPARISON METHOD
The contemporary comparison method is a technique employed to assess the breeding merit of a bull, and consists of a comparison in terms of milk production of a bull’s heifers, with the heifers of other bulls milked in the same herds at the same time. This method aims to overcome the problem of variation in management practices from farm to farm.
The contemporary comparison is a useful guide to breeding value but it is known from experience that, with regard to milk yield, the progeny of a bull is very variable and a comparison based on a small number of daughters, may not accurately reflect the performance of all the bull’s daughters. Furthermore where the bull’s daughters are compared with a large number of other heifers, the comparison is more valid than where they are compared with only one or two others. In order to take this into account a weighting system is used. The larger the weight, the more accurate the estimate of the bulls breeding value. The weight is calculated from the following formula:
(number of heifer daughters of bull under test) x (number of contemporaries) divided by
(number of heifer daughters of bull under test) + (number of contemporaries)
e.g.: If a bull has 10 daughters in a herd with 25 heifers in milk at the same time the weighting is:
|10 x 15
10 + 15
The contemporary comparison shows what the bull has done in the past, but the main question is what are his future daughters going to be like? The Relative Breeding Value (R.B.V.) is employed for this. A bull is given an R.B.V. on the basis of the extent to which the performance of existing daughters is a guide to the performance of future daughters. Thus if a bull is given a rating of 100 it means that his future daughters are likely to be no better and no worse than the breed average. If the rating is 120 then on average he is expected to increase yields by 10% (i.e. sharing with the dam). If his rating is 80 he would be expected to reduce yields.
It is obvious that all daughters considered must be in milk recorded herds and production records compared should be for the same year. It will be equally obvious that the more daughters compared the more accurate the assessment. The formula used is as follows:
|(2b x (Daughter’s Av. Yield – Cont. Av.) + 0.2 (Cont. Av. – Breed Av.) + Breed Av. X 100) / Breed Average
Explanation: B is the regression coefficient. Thus if a bull has only five daughters with 450kg above their contemporaries, one could hardly predict that his future daughters yield would be higher to the same extent, as one could if he had 20 daughter production 450 kg above their contemporaries.
The formula to calculate b is n. / N + 12 Thus with 20 daughters, b is 20 / 20 + 12 = 0.62
2b is used because if the bull has been mated with a random sample of dams, his own value must be twice the amount he has lifted or lowered production (i.e. dam has half influence).
0.2 used is equivalent to 20% and it will be recalled that only 20% of the difference between herds in yields is inherited, so that bulls should be credited or debited with 20% (Contemporary average – Breed average).
Example: A bull has 35 daughters with a yield average of 5000 kg of milk and the average of their contemporaries is 4750kg. The breed average is 4500kg.
|35 / 35 + 12
|(2 x 0.74 x (5 000 – 4 750 + 0.2 (4 750 – 4 500) + 4 500) / 4500 x 100
This bull would be expected to raise production by 12 ½ %. Twelve and a half percent of 4500kg, which is the breed average, is 562.5kg.
The method has its weaknesses, but is still very valuable especially where an adequate number of daughter and contemporary records can be obtained. A particular disadvantage is the time taken to test a bull. Young bulls are now mated to 300 cows and then laid off until proven. This gives enough heifer calves likely to complete a first lactation.
When cows in a herd are selected to be bull mothers, the following formula to obtain the R.B.V. of the dam is used:
|100 x h x (Av. Yield – Contemporary Av.) + 0.2% (Contemporary Av. – Breed Av.) + Breed Av. Breed Average
The heritability is h and varies with the number of records making up the records of the cow. h is
0.3 for one record and rises to 0.5 for 6 records.
If the R.B.V. of the bull is 117 and that of the cow 106, one could expect a son of this match to have an R.B.V. of 111. This would not be so in every case, but a high percentage of bulls whose parents had these values would have 111 as their R.B.V.
PROGENY TESTING OF BEEF BULLS IN U.K.
In the U.K. more than half of their beef comes from beef inseminations of dairy cows. It is therefore essential for A.I. centres to provide semen from proven, performance tested bulls. For this reason progeny testing in the U.K. of bulls that are to be used on dairy cows is now being conducted on their crosses with dairy cows, not just on pure beef animals, as in the past.
TESTING PIGS IN U.K.
The pig Industry Board built special testing stations to house a total of 2 400 boars at a time, allowing them to performance test 6 000 boars each year.
Their principal test objectives were twofold, namely:
- Improvement of certain carcass qualities, as expressed by a combination of total lean, distribution of lean in carcass and eye musclearea; and
- Economy of production, as expressed by a combination of growth rate and food conversion ratio.
The procedure adopted combines both progeny and performance testing in the following manner:
- Six litter groups constitute a progeny test on one boar;
- A litter group consists of two boars which are performance tested, and a castrate and gilt, both of which are slaughtered;
- The tests on the boars start when they reach 25kg liveweight, and are normally completed when they reach 90kg or, for heavy hogs, 120kg. Growth rate and food conversion are measured and used to assess their economy of production. Backfat thickness is measured at several points by ultra-sonics and from these measurements an estimate of the lean content is made; and
- Results from the test on the two sibs are used to supplement this information. In addition the lean content is made by measuring their economy of production and their carcasses are examined in detail. Eye muscle is measured, and the distribution of lean meat assessed. A carcass score is then compiled.
METHOD OF SCORING
Boars are assessed on the basis of “breed contemporary comparison”. This means that they are compared with boars of the same breed being assessed at the same time at the same station.
The boars are given a score on points, but no absolute figures of performance are given. This is because it is realised that there is bound to be a difference in performance between pigs at the different testing stations, and at different times of the year. Differences in environmental conditions are known to affect economy of production and carcass quality.
At the end of the test every boar is given a points score for economy of production and for carcass quality. The average score for each factor is always kept at 50 points, so that if a boar has 50+ points for a factor (or 100 points for both factors) it is better than average for that factor. The extent of this superiority is indicated by the points.
This method of combining different measurements and assessments in this way gives the best indication of a boar’s genetic potential and is known as the selection index.
There is some evidence that the Arabs used Artificial Insemination on mares as early as the 14th Century, but the first scientific research about A.I. was recorded in Italy in 1780. Approximately in 1928 the Russians began to inseminate cattle and ten years later were breeding over one million cattle by A.I. In 1938 A.I. started in Denmark and today nearly all the cows in that country are bred by A.I. However, the real growth of A.I. began after the Second World War and increased with great rapidity in America and Europe, particularly in the U.K. where the Milk Marketing Board operates a service that covers the whole country.
There are a number of advantages of A.I. over natural service, and these are:
- Herd improvement by the use of the best bulls available in a breed. The semen of good, proven sires is available to both pedigree breeders and commercial farmers on a world-wide basis. There is no longer any excuse for farmers to use inferior and ‘scrub’ bulls in their herds, and the genetic improvement of cattle in America and Europe is due to the use of A.I;
- Bulls used by the A.I. services are tested for health and disease. With natural service there is always some risk of spreading disease, particularly venereal disease through the bulls being used. With A.I., this risk is eliminated;
- Using A.I. is cheaper than the cost of keeping a bull on the farm. A bull is unproductive and can be replaced by a profitable cow if A.I. is used. Furthermore, large herds have to keep more than one bull, and many bulls are culled from the herd, not because they have come to the end of their useful lives, but because they have become too closely related to the heifers in the herd. By using A.I. the farmer or breeder can carry out a flexible breeding programme, close breeding, line breeding or crossbreeding; and
- A bull is a dangerous animal and requires good safe housing and great care in handling. A. I. eliminates the risk of keeping these dangerous animals on the farm. In the past, many people have been killed by bulls.
- The technique of A.I. consists of two main operations:
- Collecting, diluting, freezing and packing the semen from the bull; and
- Thawing out and injecting the semen into the cow at the right time for conception.
With each ejaculation, a normal bull produces 7 – 10 millilitres of semen, each millilitre containing 1 – 1 ½ thousand million active, mobile sperm. After collection, the semen is diluted to five times the original volute using a solution called an extender. A typical extender consists of egg yolk, sodium citrate, antibiotics, dextrose and glycerol. The diluted semen is packed into long thin containers called straws, each straw containing enough semen for one insemination; each straw is labelled with the name and number of the bull and the date of packing. The straws are then deep frozen and stored in liquid nitrogen at a temperature of -142⁰C, or liquid nitrogen vapour at a temperature of -125⁰C. Although semen will freeze at any temperature below 0⁰C, it is safe from injury only when it is below -40⁰C to – 65⁰C. As the temperature rises above the safe zone, ice crystals in the semen will enlarge and move, causing damage to the sperm and reducing their viability. Once a straw has been thawed out it must be used at once or thrown away. Each straw contains 0.5ml of diluted semen. By using deep frozen semen, one bull can produce enough semen to inseminate 30 000 to 40 000 cows a year. Using natural service, one bull might serve 50 cows a year.
The semen is thawed out and injected into the uterus of the cow using a special instrument consisting of a long plunger and a long, thin plastic tube. A cow is inseminated when she is on heat, and the plastic tube must be passed through the cervix and the semen placed into the uterus, so that it is available to fertilise the egg after ovulation has taken place. Insemination is a skilled process and should be carried out only by a trained operator. A good inseminator should achieve an 80% conception rate for the cows inseminated by him.
Although A.I. can be used with any type of cattle it is mainly used in dairy herds or small beef herds where the cows can be carefully observed and brought into a pen when they come on heat. This has always been a problem with large beef herds. However, a technique has been evolved using Prostaglandins, which are hormone-like substances produced in a variety of body tissues and which control the oestrus cycle of the cow.
If a number of cows are injected with a solution of prostaglandins on the same day, and if they are given a second injection 12 days later, all the cows will come on heat 15 days after the first injection and can be brought into a pen and inseminated by means of A.I. This means that A.I. can be used for large numbers of cattle in beef herds, and it also means that all the cows that conceive will calve down within a few days of each other making for easier management at calving. The following programme can be carried out commencing 40 to 60 days after the cows have calved down.
|First prostaglandin injection done by veterinary surgeon
|Second prostaglandin injection done by veterinary surgeon
|First A.I. of cows
|All cows given a second A.I.
|Days 32 – 40
|Observe for cows coming on heat – repeat A.I.
|Days 275 – 290
|Peak calving period
Any farmer wanting to synchronise his cows should discuss the whole programme with his veterinarian.
Using this technique, a fertilised egg is washed out from the uterus of the cow and preserved in special fluid. It is then transported and surgically implanted into the uterus of a different cow where it will develop and produce a normal calf. More than one egg can be implanted into the ‘mother’ cow so that she will give birth to twins or triplets. While A. I. makes use of one good parent, the bull, ova transplants make use of two good parents, the bull and the cow. The mother cows can be of any breed and provided they are well fed and managed, they will incubate the egg and produce the calf after a normal gestation period.