Many systems of crossbreeding are used by farmers for cattle, pigs, poultry and sheep, but to obtain the maximum benefit, the crossbreeding programme must be designed carefully. In the last lecture we discussed two examples of practical crossbreeding in sheep; the stratification system used in the U.K. which matches the parent breeds and their crossbreds to the environment, and which makes full use of heritability and hybrid vigour throughout the crossbreeding programme. The second example was the production of a new breed, the Dorper, by means of a crossbreeding programme, which produced an animal suited to a particular environment and with the commercial traits required by the farmers in a specific area. Some practical systems of crossbreeding used by farmers are discussed below, using pigs as an example.
TWO BREED OR SINGLE CROSS
The simplest example is to mate a boar from one breed (and from a strain noted for its superior conformation, performance and lean meat production) with a sow from another breed (noted for its fertility, mothering ability in addition to performance and carcass quality).
A typical example is the Large White Boar X Wessex Sow to produce the Blue pig.
There is some hybrid vigour, the young are well mothered and fairly uniform and they inherit qualities for performance and meat production from their sire. Unfortunately, two pure-bred herds must be maintained to supply the parents for crossing or the pure-bred stock must be purchased.
The system also fails to take advantage of the hybrid vigour from the crossbred sow since these are slaughtered and not bred from.
BACKCROSSING OR CRISSCROSSING
In this system two pure breeds A and B are mated as above to produce (AB). The (AB) gilts are then ‘backcrossed’ to a board from one of the two original breeds:
A X (AB) = AAB
The gilts from this cross are then mated back to a boar from the other of the two original breeds:
B X (AAB) = BAAB
This system results in about two-thirds of the inheritance coming from the breed of the boar last used and one third from the other breed. There is, therefore, some variation from one generation to another. If followed on a long term basis some hybrid vigour is retained, but some is lost after the first few generations.
This system obviously reduces the need for the continual introduction of fresh pure bred parents.
THREE OR FOUR BREED ‘ROTATIONAL CROSS’ KNOWN AS ‘CYCLICAL CROSSING’
Used for breeding purposes. The degree of decline of hybrid vigour is much smaller than with criss-crossing if pure bred boars are used each time, but it supposes a big herd to justify the boars. As in ordinary second crossing there is bound to be some variation in type but this depends on the parent types selected. One considerable advantage of cyclic crossing is that owing to the amount of heterozygosity and heterosis generated, a boar might be mated to his grand-daughter and, if still alive, to his great-grand-daughter without probability of trouble through inbreeding because of the outcross through other breeds.
Whichever system is used it is essential that the best stock available, particularly from the point of view of carcass quality and performance, are used.
ADVANTAGES OF CROSSBREEDING
A survey of the figures showed that the superiority of crossbreds was particularly evident on poorer farms. Crossbred litters were more uniform and crossbred sows had shorter farrowing intervals. Crossbreeding appears to have only small advantages for economy of gain and only slightly more advantage for speed of gain. The carcass quality shows a very small effect of hybrid vigour and may almost be said to take the average of the parental breeds. Thus the quality of the original stock in this context is very significant.
TECHNIQUES FOR THE IMPROVEMENT OF CROSSBREDS
Although crossbreeding brings about an immediate advantage in hybrid vigour this advantage is not cumulative. Selection procedures have to be used to achieve cumulative improvement. There are several ways of doing this:
IMPROVEMENT OF PUREBREDS
Those traits of high heritability and economic importance such as feed conversion, speed of gain and lean meat content, can be measured by performance tests and response to selection can be comparatively rapid in the purebreds. This improvement will be reflected in their crossbreds and is, therefore, a vital part of improving crossbred performance.
USE OF INBRED LINES AND LINE CROSSES
Two Line Cross Hybrid:
This system consists of inbreeding several lines and making test crosses to find one which nicks. It is difficult to recommend this system because of time, effects of inbreeding and cost. However, it is done by large farming companies producing hybrids for their own use and for sale to other farmers.
RECIPROCAL RECURRENT SELECTION
The system known as Reciprocal Recurrent Selection is based on the selection of pigs based on crossbred performances. It is particularly useful for traits of low heritability especially if they can only be measured on one sex.
This system is effectively a progeny test. It has disadvantages of a two year cycle of testing and selection. It requires fairly large resources and it is, therefore, possibly better to concentrate on purebred selection than employ this system.
The three cross breeding systems, Single Cross, Backcrossing and Cyclical Crossing can be used in the cross breeding of cattle, although the more complicated systems can be used only in large herds, or by large farming companies. This is because of the need to maintain 2 or 3 pure bred herds to supply the parent stock, and also because of the long generation interval with cattle (e.g. the same time between one calf crop and the next crop). An example of crossbreeding in cattle is:
Crossbreeding has been used with beef cattle to develop new breeds such as the Santa Gertrudis, Beef Master, Bonsmara and the Brangus. It involves the mating of crossbred parents to maintain the advantages of breed combinations at certain levels. The system is confined to larger organisations with access to trained geneticists.
In poultry breeding, hybrid hens are produced both for laying and boiler production by crossing two or three pure breeds that have been inbred and selected carefully for desirable traits.
The development of distinct breeds of cattle and sheep began in Britain. As the Industrial Revolution gained momentum, people moved into the towns and away from the countryside. This influx into the towns caused a strong demand for agricultural produce, particularly milk, beef and wool. Farmers began to improve their stock, and early improvements were based on a wide range of unrelated stock – several ‘breeds’. Once a suitable strain had been found, one that produced better beef or more milk, this was fixed by inbreeding. The farmers who owned these improved strains of cattle gradually joined together to form Breed Societies.
Breed Societies perform the following functions:
- They establish a breed type and the trademarks of the breed – colour, horn etc.
- Once it has been formed, the Breed Society prohibits the introduction of blood lines, or genes, from other breeds or types. Exceptions are the ‘breeding off’ of horns in the Hereford breed and the introduction of the Red Dane bulls into the Red Poll Breed in order to improve the performance of that dual purpose breed.
- Differences between breeds are differences in inherited traits, and these differences may be large or small. The difference between a Jersey cow and a Hereford cow is a big difference which includes colour, size, conformation, milk production, butterfat production and beef characteristics. On the other hand, Red Friesians differ from Black Friesians by having one pair or one of a pair of genes differing. Black and White Friesians can carry a gene which is a recessive for the red colour. If two of these animals are mated, a red and white calf can be produced.
|BB||Br||Br||rr – red calf|
If the red calf is produced, it means that both of the parents must be carrying the red recessive gene. The calf cannot be registered with the Friesian Breed Society, although both parents are full pedigree and registered. To be logical, both parents should be removed from the herd book and lose their pedigree status, but this does not happen. The red and white Friesian has exactly the same characteristics of conformation and milk production as the black and white Frisian except for the red colour of the coat.
Breed types are laid down by the Breed Societies. Some of these characteristics are sensible because they relate to commercial needs. The useful life of a cow depends on sound legs, and the small teats and straight udder of the Ayrshire was bred for use with a milking machine. A fine shoulder and a long neck and head are associated with dairy cows, while beef cows have a wide shoulder and a short neck and head. Furthermore, breeding for milk and meat is basically incompatible. Beef breeds are successful and dairy breeds are successful, but dual purpose breeds do not succeed well.
Some characteristics laid down by Breed Societies are ‘show points’. They improve the look of the animal but they have no commercial advantages. Examples are a straight back, neat tail set and, of course, horns. Selecting for sound commercial characteristics, particularly with dairy cattle, is difficult enough. Selecting and breeding for show points as well is almost impossible.
One way in which crossing or outcrossing is used in practical breeding is in the grading up of cattle. By using pedigree, registered bulls a farmer can grade up his cattle from non-pedigree to full pedigree, registered status in about four generations. Breed Societies have their own rules about the number of generations required to obtain full pedigree status, and all non-pedigree animals are recorded in the grading up registers. After the required number of generation, the final calf is registered in the pedigree herd book. Beef breeds are judged on conformation, colour, horns, ‘type’ etc., but dairy breeds are judged on both conformation and milk production with standards for milk yields and butterfat being laid down for each stage of the grading-up process.
To give you an idea of grading up, the example below gives the regulations required by the British Friesian Society to produce a registered pedigree animal in four generations and starting with unrequested cows.
BRITISH FRIESIAN SOCIETY
SUPPLEMENTARY REGISTER – GRADING UP SCHEME
The Supplementary register is designed so that owners of typical but unregistered black and white cows and heifers shall be enabled to breed cattle into the British Friesian Herd Book. The supplementary register is divided into four classes, namely A, B, C and D as follows:
- Class A: Well marked typical and good Black and White cow, or heifer, which, officially recorded, has produced in a completed lactation of not more than 365 days. The minimum milk and butterfat yields as laid down by the Breed Council, and which is approved on examination by an Honorary Inspector acting for the Society.
- Class B: For well-marked and typical Black and White Heifer Calf by a British Friesian Bull registered in the herd book and out of a cow or heifer registered in Class A.
- Class C: Same as above, but out of a cow or heifer registered in Class B.
- Class D: Same as above, but out of a cow or heifer registered in Class C.
A heifer calf out of a registered Class D heifer or cow and by a registered British Friesian Bull will be eligible to have its entry registered in the Society’s herd book, provided the calf is of Friesian type both as regards quality and markings.
NOTE: Bull calves out of dams registered in any of the classes of the supplementary register are not eligible for entry to the Supplementary Register or the herd book.
SUPPLEMENTARY REGISTER A
5900kg at 3.7% fat, 1st calves
5900kg at 3.6% fat, Late Lactations Plus Inspection
9000kg at 3.5% fat, Any Lactation
|Herd Book |
|93 ¾ % Pedigree |
87 ½ %Pedigree
75 % Pedigree
50 % Pedigree
|4th cross Heifer |
3rd cross Heifer
2nd cross Heifer
1st cross Heifer
Using Registered Friesian bulls on Classes A – D
(a) Pure Breeding
(b) Cross Breeding