Animal breeding and husbandry

Summary

Animal husbandry is the production and care of animals. In universities, animal husbandry is usually called animal science since academic studies involve research and the application of scientific principles. Animal breeding is often considered part of husbandry and involves applying genetic principles to develop breeds and lines of animals for human purposes. Animal breeding principles are also used in captive breeding programs to propagate endangered wildlife species. The development of a leaner line of pigs and a strain of chickens that produces more eggs are examples of animal breeding.

Definition and Basic Principles

Animal husbandry is concerned with all aspects of farm animals' management, care, and breeding. Animal husbandry aims to provide the best conditions (given economic constraints) to maximize productivity in terms of body weight, wool, milk, or eggs. The animals must remain healthy to attain this productivity and reproduce. Animal husbandry involves the choice of proper feeds, housing, and suitable animals.

Animal breeding begins with measuring desirable traits (phenotype) related to improved animal production. The breeding value of an animal, however, is the degree to which its underlying genotype can be transmitted to its offspring. Modern methods of breeder selection combine traditional measurements of quantitative traits with the new technology of genome analysis, which aids in determining the breeder's genotype. The rate of genetic change (in animal populations) is directly related to the accuracy of selection, selection intensity, and genetic variation in the population and is inversely related to generation interval. There are two primary types of breeding programs: the development of breeds or lines that can be used as breeders (seedstock) and the development of crossbreeds for production. Crossbreeds demonstrate improved productivity because of hybrid vigor and complimentary traits exhibited by their parents.

Background and History

Animal husbandry began with the domestication of animals for human purposes from around 10,000 to 5000 Before the Common Era (BCE). Sheep were the first to be domesticated, followed by cattle, horses, pigs, goats, and finally, chickens and turkeys. A relatively small number of species have been domesticated because they must possess several suitable characteristics that allow them to adapt to interaction with humans. Their diet must be simple (the early domesticated animals depended on grazing and foraging for their food). They must be able to breed in captivity and must grow and reproduce over a relatively short time interval. They must have calm, predictable behavior and a cooperative social structure.

Animal breeding started in the Roman Empire or perhaps earlier. Early breeders recognized desirable traits in animals that they wanted to propagate, so they selected those animals for mating. The characteristics of domesticated animals began to vary greatly from those of their wild cousins, and they became totally dependent on their human captors. Systematic selective breeding methods began with the English sheep farmer Robert Bakewell in the late 1700s. Bakewell sought to increase the growth rate of sheep so that they could be slaughtered at an earlier age, to increase the proportion of muscle, and to improve feed efficiency. The application of genetics in animal breeding began in the twentieth century. Jay Lush, a professor at Iowa State University, is considered a pioneer in applying genetic techniques in animal breeding. His Animal Breeding Plans (1937) advocated breeding based on quantitative measures and genetics rather than just the animal's appearance.

How It Works

Husbandry. Farm animal production is an economic venture undertaken to produce food (meat, milk, and eggs) or other animal products, such as wool, hides, hair, and pelts. Through animal husbandry, growers seek to create conditions that maximize the production of animal products at the lowest cost. With advancing technology and improvements in breeds, animal production has evolved from extensive systems to increasingly intensive systems.

Intensive systems put more demands on good husbandry practices because the animals are often under more stress and depend more on humans for their well-being. Extensive systems involve keeping animals on pastures or in small pens with minimal housing. Intensive systems are most advanced in the case of poultry. Broilers (meat animals) are kept in total confinement indoors, while laying hens are kept completely in cages. Swine are also commonly kept in confinement, usually on slat or grid floors made of metal. Confinement operations require closer attention to ventilation, sanitation, and animal interaction requirements. Beef cattle are still grown on range or pasture, but it is more common to finish them in large feedlots, concentrating thousands of animals. Dairy cattle usually have pastures for grazing but are practically always milked by machines in parlors. Sheep are still largely grazed on range or pasture for most of their growing cycle.

Traits and Breeding Value. The selection of animals in the early days of animal breeding depended on physical or quantitative traits exhibited by the animal without understanding the underlying genetic principles. These observed or measured traits are known as the phenotype of the animal. The animal's phenotype results from the interaction of its genotype (genetic makeup) with the environment. Animal breeding aims to produce animals in a herd, flock, line, strain, or breed that possess superior phenotypes that can be passed on to future generations. The degree to which observed phenotypes can be transmitted to offspring is known as heritability and is a measure of the breeding value of the animal. The selection of desired traits depends on the species of animal and the intended purposes for raising them. The selection also depends on the management practices adopted by the farmer and the relationships between farm inputs and the value of the animals. Examples of traits include calving interval for beef cattle, milk yield for dairy cattle, litter size for swine, first-year egg numbers for hens, and breast weight for meat chickens. The performance of traits can depend on the environment. A high-producing Holstein cow may not produce as well in the tropics because it is not heat tolerant.

Rate of Genetic Change. Progress in a breeding program is related to the rate of genetic change in a population. Several factors affect this rate of change: accuracy of selection, selection intensity, genetic variation, and generation interval. The accuracy of selection relates true breeding values to their prediction for a trait under selection. Selection intensity refers to the proportion of individuals in a population that are selected. Populations selected more intensely will be genetically better than the average, leading to a faster rate of genetic change. Populations exhibiting greater genetic variation among individuals have the potential for more rapid genetic change. Finally, species with a short generation interval will have a faster rate of genetic change.

Multiple Trait Selection. Breeders seldom select just one trait for improvement since a combination of traits is essential for the economic success of the enterprise. Selection for one trait usually affects the response to traits not selected for because of the phenomenon of correlated response. The primary cause of correlated response is pleiotrophy, in which one gene influences more than one trait.

Breeders practice multiple trait selection through three primary means. Tandem selection involves selecting one trait and then another. Independent culling levels set minimum standards for traits undergoing selection, and animals that do not meet all the standards are rejected. Finally, the method of economic selection indexes assigns weighted values to the various traits.

Applications and Products

Seedstock. A term commonly applied to breeding stock is “seedstock.” The purpose of breeding stock is to provide genes to the next generation rather than to be producers of meat, milk, wool, or eggs. Traditionally, seedstock have been purebreds, but the number of nonpurebred stock is increasing. Seedstock animals are obtained by inbreeding programs. These programs increase homozygous or similar genotypes. As a result, seedstock have a greater tendency to pass on performance characteristics to their offspring, an ability known as prepotency. One risk of inbreeding is the expression of harmful genes resulting in reduced performance, known as inbreeding depression. Outcrossing or linebreeding is a milder form of inbreeding and involves mating animals from different lines or strains within the same breed. This process still maintains a degree of relationship to highly regarded ancestors but is less intense than breeding first-degree relatives. Outcrossing allows for the return of vigor that can be lost by inbreeding while still maintaining the genetic gains obtained by inbreeding.

Crossbred Animals. Mating animals from different species is known as crossbreeding. The resultant offspring are known as hybrids. In modern animal husbandry, even hybrids are commonly used in crossbreeding systems. Crossbred animals are used for production and are designed to take advantage of hybrid vigor and breed complementarity. Hybrid vigor is the increased performance of hybrid offspring over either purebred parent, especially in traits such as fertility and survivability. A classic example of complementarity is the crossing of specialized male and female lines of broiler chickens. Individuals from male lines are heavily muscled and fast-growing but not great egg producers, while individuals from the female line are outstanding egg producers.

Artificial insemination.Artificial Insemination is a reproductive technology that has been used for a long time. Semen is collected from males and is used to breed females. Because semen can be frozen, it can be used to eventually sire thousands of offspring. This expanded use of superior males can markedly increase the rate of genetic change. Estrus synchronization facilitates artificial insemination by ensuring that a group of females comes into estrus at the same time.

Embryo Transfer.Embryo transfer involves collecting embryos from donor females and transferring them to recipient females. Although the motive for embryo transfer is to propagate valuable genes from females, the number of progenies is much fewer, and the procedure is more difficult and costly than artificial insemination.

A variation of embryo transfer is in vitro fertilization. This technology involves collecting eggs from donor females, which are then matured, fertilized, and cultured in the laboratory. The embryos can then be transferred to recipient females or frozen for later use. The procedure is very expensive and time-consuming. However, it has the potential to aid genetic selection and crossbreeding programs. The genotype of the embryo could be determined before pregnancy. Knowing the genotype could be particularly important for dairy cattle, which frequently have fertility problems.

Cloning.Cloning is the production of genetically identical animals. Cloning allows the breeder to predict the characteristics of offspring, increase the unifority of breeding stock, and preserve and extend superior genetics. The preferred method of cloning, somatic cell nuclear transfer, involves removing the nuclei from multiple unfertilized eggs and transferring somatic cells from the animal to be cloned. If the process is successful, the resulting embryo is placed in the uterus of a surrogate mother for development.

Genetic Marker Technology. Genetic marker technology was made possible by developing reasonably inexpensive and efficient genomic analysis of farm animals. The term commonly used in the genetic marker field is quantitative trait loci (QTI). Animal breeders select traits of economic importance that are primarily quantitative traits. These traits are usually controlled by many genes, even thousands of genes. Each gene can contribute a small portion to the total genetic variation of the trait. Since the location (locus) and identity of these genes on the DNA molecule are frequently unknown, genetic markers have become important. Genetic markers are associated with quantitative genes and can be identified in the laboratory.

Single Nucleotide Polymorphisms. Another term associated with genetic markers is single nucleotide polymorphisms (SNPs). Nucleotides are the building blocks of DNA, and polymorphism means “many forms.” Nucleotides are made of one of four different bases. Genes are made of many nucleotides. The exchange of one base for another in a nucleotide is an SNP, which can change the expression of a gene. Instead of analyzing the entire genome of an animal, the dense SNP array test measures around 50,000 SNPs, which is then related to the genetic merit of the animal. With traditional breeding programs, each offspring is assumed to have inherited an average sample of genes from their sire (father) and dam (mother). Full siblings have equal parent average (PA) but are expected to share only half of their genes as copies of the same genes in their parents. Considerable improvements in breeding value have been demonstrated by the use of a genomic predicted transmitting ability (gPTA) calculation, which combines genomic data with the traditional parent average data.

Careers and Course Work

The field of animal husbandry is called animal science in colleges and universities to reflect scientific study and applications in the field. Students specifically interested in animal husbandry should concentrate on coursework and experience related to production and management. Animal science also encompasses agribusiness, government, and research and teaching. Job titles in animal husbandry can include livestock or dairy herdsperson, stable manager, veterinary technician, feed mill supervisor, or farm manager. Previously, on-farm experience was enough to work in animal husbandry, but it has become a more complex field. A two-year associate's degree should be considered minimal for the field, while a four-year bachelor's degree would benefit managerial positions. Coursework can include animal production, biology, chemistry, animal growth and development, physiology, animal nutrition, biotechnology, farm management, and economics.

A career in animal breeding and genetics requires a doctoral degree, whether employment is in academia or industry. The careers can include such specialties as quantitative or molecular genetics, bioinformatics, immunogenetics, and functional genomics. The prerequisites for graduate studies typically include undergraduate coursework in animal science. Graduate courses can include animal breeding, statistics, endocrinology, genome analysis, population and quantitative genetics, animal breeding strategies, statistical methods, and physiology and metabolism. Specific coursework varies depending on the school.

Social Context and Future Prospects

Some controversy has arisen in the animal sciences fields because state agricultural experiment stations (SAES) are increasingly entering into collaboration with and receiving funding from private firms. Because the agricultural experiment stations are public institutions, some people feel that they may be compromising their independence and objectivity. However, these stations are primarily involved in basic research, and the private firms conduct the necessary practical research leading to the commercialization of new products.

Modern factory farming methods, with poultry, swine, and other farm animals kept in confinement and crowded conditions, have been condemned by animal rights activists. They claim that because the animals are often unable to perform their natural and instinctive behaviors, they suffer. Animal science departments have been aware of these criticisms and have developed a new field of farm animal welfare. Animal welfare is being studied academically in a manner that is validated and measured objectively and, therefore, is reliable. The discipline considers the relationship of farm animal welfare to the animals' environment in three areas: how the animals feel, their fitness and health, and their natural behaviors.

One issue addressed by the Food and Agriculture Organization of the United Nations is animal diversity. As globalization of agriculture encourages breeding for high-input, high-output animals, some livestock breeds are becoming extinct. This leads to fewer breeds, which means less flexibility when confronted with an emerging disease or changed environmental conditions. Another problem is that genetic breeding occurs mostly in advanced countries under the conditions of intensive agriculture and the local environment. These conditions and farming methods are different in some lesser developed countries, and animals that produce well in one country may not do as well in another because of environmental factors.

Controversies also surround the genetic engineering of animals. Despite social concerns, however, the field has increased as new technology allows scientists to sequence the genomes of domestic animals. This information can be used to positively affect the health of animals and the environment. For example, a breed of pig called the Enviro-Pig has proven to emit up to 60 percent less phosphorus than traditional pigs, thus lessening the pig's impact on the environment. The potential benefits and profits from improvements in animal breeding and husbandry are great, and the field is likely to remain active.

Innovations continue to be made in animal breeding and husbandry in the twenty-first century to optimize animal health and profits. Genome editing has been advanced by CRISPR-Cas9 technology. Reproductive technologies, like artificial insemination, in vitro fertilization, and embryo transfer, have revolutionized the field. Nutritional, health, and animal welfare concerns are at the forefront of the industry as those in the field keep a keen eye on ethical and sustainable practices. Finally, artificial intelligence and data analytics help those in the field analyze existing data and allow it to inform future decisions. 

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