Livestock populations provide people with a variety of products and services, including meat, milk, eggs, fiber, and draught power, in a wide range of environments. This diversity of function is only possible because of the diversity of their genetic make-up.
For more information please visit Pritish Kumar Halder’s article.
The genetic variation within livestock populations also provides the raw material both for evolution through natural selection in response to changing conditions and for human-managed genetic improvement programs. It is vital both to efforts to increase production and to the adaption of livestock populations to challenges such as climate change, emerging diseases, and pressures on feed and water resources.
Animal genetics is one of the pillars of livestock development (alongside animal health, animal nutrition, and husbandry issues such as housing). It is a broad field, ranging from characterization to conservation to genetic improvement, and involves actions at local, national, regional, and global scales.
Characterization is the assessment of livestock populations and their production environments to establish their current status and identify strengths that can be enhanced and weaknesses that need to be overcome, for example via a genetic improvement program. It can also help to inform conservation strategies.
Conservation of animal genetic resources refers to measures taken to prevent the loss of genetic diversity in livestock populations, including to protect breeds from extinction. It can involve both the conservation of live populations and the curio conservation (preservation through freezing at extremely low temperatures) of material such as semen or embryos.
Genetic improvement (animal breeding) is based on the principle that the products (milk, meat, wool, etc.) and services (e.g. transport, draught power, or cultural services) provided by animals are a function of their genes and the environmental influences that they are exposed to.
Improvement can be achieved by selecting genetically superior animals to be the parents of the next generation. “Genetically superior” means superior in terms of a particular set of characteristics, which usually include productivity in the environmental conditions expected in the future, but should also consider traits such as fertility, disease resistance, or longevity that relate to costs of production.
Since the middle of the twentieth century, genetic improvement efforts have concentrated on a very small number of breeds worldwide, frequently without due consideration to how local production environments affect animals’ ability to survive, produce and reproduce.
The main objectives of animal breeding are :
(i) improved growth rate,
(ii) increased production of milk, meat, egg, wool, etc.,
(iii) superior quality of milk, meat, eggs, wool, etc.,
(iv) improved resistance to various diseases,
(v) increased productive life, and
(vi) increased or, at least, acceptable reproduction rate.
Two methods of animal breeding are inbreeding and outbreeding, based mainly on breeding work with cattle.
When breeding is between animals of the same breed for 4-6 generations, it is called inbreeding. Inbreeding may be explained by taking the example of cows and bulls. Superior cows and superior bulls of the same breed are identified and mated. The progeny obtained from such mating is evaluated and superior males and females are identified for further mating. A superior female, in the case of cattle, is the cow that produces more milk per lactation.
On the other hand, a superior male is that bull, which gives rise to superior progeny as compared to those of other males. As the homozygous pure lines developed by Mendel as described in Chapter 5, a similar strategy is used for developing pure lines in cattle as was used in the case of peas. Inbreeding, as a rule, increases homozygosis.
Expose of harmful genes
Thus inbreeding is necessary if we want to develop a pure line in any animal. Inbreeding exposes harmful recessive genes that are eliminated by selection. It also helps in the accumulation of superior genes and elimination of less desirable genes. But continued inbreeding reduces fertility and even productivity. For more information please visit Pritish Kumar Halder ‘s page.
This is called inbreeding depression. In this condition, the selected animals of the breeding population should be mated with superior animals of the same breed but unrelated to the breeding population. This often helps in restoring fertility and yield.
Outbreeding is the breeding between unrelated animals which may be between individuals of the same breed (but having no common ancestors) or between different breeds (cross-breeding) or different species (interspecific hybridization).
It is the mating of animals within the same breed but having no common ancestors on either side of their pedigree for up to 4-6 generations. The offspring of such a cross is called an outcross. Outcrossing is the best breeding method for animals that are below average in productivity in milk production, the growth rate in beef cattle, etc. Sometimes only one outcross helps to overcome inbreeding depression.
In cross-breeding superior males of one breed are mated with superior females of another breed. Many new animal breeds have been developed by this strategy. It gives better breeds. Cows of an inferior breed may be mated to bulls of a superior breed to get better progeny. Hazardable is a new breed of sheep developed in Punjab by crossing Bikaneri ewes and Marino rams.
(iii) Interspecific Hybridization:
In this approach, male and female animals of two different species are mated. The progeny obtained from such a mating is usually different from both the parental species.
But in some cases, the progeny may combine the desirable characteristics of both parents. A mule is produced from a cross between a female horse (mare) and a male donkey. Mules are harder than their parents and are well suited for hard work in mountainous regions.
Controlled Breeding Experiments:
These are carried out using artificial insemination and Multiple Ovulation Embryo Transfer Technology (МОЕТ).
(i) Artificial Insemination (AI):
The semen of the superior male is collected and injected into the reproductive tract of the selected female by the breeder. The semen can be used immediately or can be frozen for later use. When a bull inseminates a cow naturally approximately 5 to 10 billion sperms are deposited in the vagina. However, when semen is deposited artificially, considerably fewer sperm are required to achieve conception. Therefore, artificial insemination is very economical. The spread of certain diseases can be controlled by this method.
(ii) Multiple Ovulation Embryo Transfer Technology (МОЕТ):
In this method, hormones (with FSH-like activity) are given to the cow for inducing follicular maturation and superovulation instead of one egg, which they usually give per cycle, they produce 6-8 eggs. The cow is either mated with the best bull or artificially inseminated.
The embryos at the 8-32 cell stage are recovered and transferred to surrogate mothers. The genetic mother is available for another superovulation. МОЕТ has been done in cattle, sheep, rabbits, buffaloes, mares, etc. High milk-giving breeds of females and high-quality (lean meat with less lipid) meat-giving bulls have been bred successfully to obtain a better breed in a short time.