Animal Breeding. For: ADVS 1110 Introduction to Animal Science

Transcription

1 Animal Breeding For: ADVS 1110 Introduction to Animal Science

2 Genetics

3 Components of Animal Husbandry

4 Content Topics: Basics of Genetics Selection Applied Animal Breeding

5 Basics of Genetics

6 Basics of Genetics Genetics science of heredity Heredity the hereditary transmission of genetic or physical traits of parents to offspring.

7 Basics of Genetics Gregor Johann Mendel, An Austrian Monk and Botanist, founder of modern genetics. Mendel conducted basic breeding experiments with garden peas between 1857 and 1865, during the time of the U.S. Civil War.

8 Basics of Genetics Mendel in his monastery at Brunn (now Brno, in Czechoslovakia), applied a powerful curiosity and a clear mind to reveal some of the basic principles of hereditary transmission. In 1866, he published in the proceedings of a local scientific society a report covering eight years of his studies, but for 34 years his findings went unheralded and ignored.

9 Basics of Genetics Finally, in 1900, sixteen (16) years after Mendel s death, three (3) European biologists independently duplicated his findings, and this led to the dusting off of the original paper published by the Monk thirty-four (34) years earlier.

10 Basics of Genetics Mendelian inherited according to Mendel s laws. Mendelism the theory of heredity as formulated by Gregor Mendel.

11 Basics of Genetics

12 Basics of Genetics Mendel s basic phenotypes with his pea studies:

13 Basics of Genetics

14 Basics of Genetics

15 Basics of Genetics Mendel s Laws The four (4) principles of hereditary phenomena discovered and formulated by Gregor Mendel:

16 Basics of Genetics 1. The Law of Independent Unit Characters 2. The Law of Segregation 3. The Law of Dominance 4. The Law of Independent Assortment

17 Basics of Genetics 1. THE LAW OF INDEPENDENT UNIT CHARACTERS; which states that characters, as height, color, etc., are inherited separately as units.

18 Basics of Genetics 2. THE LAW OF SEGREGATION; which states that body cells and primordial germ cells contain pairs of such unit characters and that when gametes are produced, each gamete receives only one member of such pair.

19 Basics of Genetics 3. THE LAW OF DOMINANCE; which states that in every individual there is a pair of determining factors (genes) for each unit character, one from each parent, if these factors are different (heterozygous), one character (the dominant) appears in the organism, the other (recessive) being latent; the recessive character can appear in the organism only when the dominant is absent.

20 Basics of Genetics 4. THE LAW OF INDEPENDENT ASSORTMENT; which states that any one pair of characters is inherited independently, notwithstanding the simultaneous transmission of other traits. (This principle has been modified by the discovery of linkage and pleiotrophy.)

21 Basics of Genetics Robert Bakewell of Dishley, Leicestershire, England, , found and Father of Animal Breeding.

22 Basics of Genetics Bakewell was an English farmer of remarkable sagacity and hard, common sense. He was the fist great improver of cattle in England.

23 Basics of Genetics Bakewell s objective was to breed cattle that would yield the greatest quantity of good beef rather than to obtain great size.

24 Basics of Genetics Bakewell s efforts with cattle were directed toward the perfection of the English Longhorn, a class of cattle common to the Tees River area.

25 Basics of Genetics Bakewell also contributed significantly to the improvement of the Leiceister breed of sheep, and the Shire horse.

26 Basics of Genetics Bakewell established the Dishley Society to ensure the purity of livestock breeds. The Dishley Society was organized in the Leicester Secular Hall.

27 Basics of Genetics Careful analysis of Bakewell s methods reveals that three (3) factors were paramount to sound animal breeding: 1. Establish A Definite Goal 2. Breeding the Best to the Best 3. Use Proven Sires

28 Basics of Genetics Genes: Genes normally occur in pairs in each body (non-germ) cell of individual organisms, such as our farm animals. Genes are located on or in distinct rod shaped bodies called Chromosomes.

29 Basics of Genetics Chromosomes: Chromosomes are carried in the center or nucleus of each cell. Chromosomes occur in sets of two. Each set is usually identical. The number of chromosomes in each body cell of all members of a specie is normally the same. All of our farm animals are normally Diploid (sets of 2), aka 2 n.

30 Basics of Genetics Germ Cells, Sex Cells or Gametes: Gametes: Female = Ova or ovum Male = Spermatozoa or sperm cell Contain only 1 member of the hereditary factors, and referred to as Haploid, aka 1 n. Haploid refers to one half of the Diploid number of Chromosomes for a given specie, as found in the germ cells or gametes.

31 Basics of Genetics Alleles: Genes are paired in each animal, and each kind of gene has a particular chromosome location (loci or locus) and is called an Allele. Alleles are two (2) genes that occupy the same (locus) on Homologous Chromosomes and affect the same trait.

32 Basics of Genetics Alleles (continued): Homozygotes are Homozygous i.e. TT or tt Heterozygotes are Heterozygous i.e. Tt TT, Tt, and tt are genetic allelic pairs that express the tall (T) or short (t) traits in pea plants; also called Genotypes found in cells. Tall or Short are Phenotypes or outward appearances of pea plant traits.

33 Basics of Genetics Gametogenesis and/or Gamete Formation: Meiosis, aka Reduction Division - Takes place in the Gametogenic process. Zygote New individual genetic organism. Chance When more than one kind of Gamete is produced by reproducing parents, Chance determines according to the Laws of Probability, which Gametes will unite to produce the new individual organism(s) (Zygotes).

34 Basics of Genetics Mitosis: The process of division of body cells. This regular process keeps the number of Chromosomes constant in body cells throughout the organism. When body cells divide this is the process by which tissue growth and renewal is accomplished. Each Chromosome is normally duplicated exactly along it s entire length.

35 Basics of Genetics Meiosis: Meiosis means to make smaller. Meiosis is a form of cell division that reduces Chromosome numbers from Diploid (2 n ) to Haploid (1 n ). Meiosis occurs in the formation of Sex Cells (Gametes) or Spermatozoa or Ova (Ovum).

36 Basics of Genetics Sex Determination: Mammals Avian Species

37 Basics of Genetics Sex of an Ova (egg) and Sperm united are referred to as a Zygote, and is determined by the kind of Sperm it unites with at Fertilization.

38 Basics of Genetics Sex Determination: Sex Chromosomes Mammals: Female Mammals = XX Male Mammals = XY The Sex Chromosomes are carried by the Gametes (Sex Cells; sperm & ova).

39 Basics of Genetics Sex Determination: Sex Chromosomes Mammals: Female Mammals produce only one kind of egg, those containing an X Chromosome. Male Mammals produce two (2) kinds of Sperm, those containing an X Chromosome and those containing a Y Chromosome.

40 Basics of Genetics Sex Determination: Sex Chromosomes Mammals: In Mammals then: Females are Homogametic. Males are Heterogametic.

41 Basics of Genetics Sex Determination: Sex Chromosomes Avian Species (birds): Female Avians = ZW Male Avians = ZZ The Sex Chromosomes are carried by the Gametes (Sex Cells; sperm & ova).

42 Basics of Genetics Sex Determination: Sex Chromosomes Avian Species (birds): To avoid confusion, the Sex Chromosomes in birds are usually referred to as Z and W Chromosomes. In Avians then: Females are Heterogametic. Males are Homogametic.

43 Basics of Genetics Genetic Composition of Various Animal Species: Animal Specie Diploid (2 n ) Haploid (1 n ) Cattle Swine Sheep Goat Horse 64 32

44 Basics of Genetics Genetic Composition of Various Animal Species: Animal Specie Diploid (2 n ) Haploid (1 n ) Mouse Rat Human Cat Dog 78 39

45 Basics of Genetics Genetic Composition of Various Animal Species: Animal Specie Diploid (2 n ) Haploid (1 n ) Duck Turkey Chicken Rabbit Donkey Bison 60 30

46 Basics of Genetics Modes of Inheritance: Dominant Recessive Incomplete Dominance, aka Blending Sex-Linked Epistasis, aka Overdominance Semi-lethal Factors Lethal Factors

47 Basics of Genetics Examples of Dominant and Recessive Traits in Farm Animals Specie Dominant Trait Recessive Trait Cattle Black Hair Coat Red Hair Coat Cloven Hooves Mulefoot Normal Muscling Double Muscling Normal Size Polled Dwarfism Horns

48 Basics of Genetics Examples of Dominant and Recessive Traits in Farm Animals Specie Dominant Trait Recessive Trait Horses Bay Non-Bay (Black) Black Hair Coat Normal Hair Chestnut or Sorrel Curley Hair Poultry Barred Plumage Non-Barred Plumage Broodiness Non-Broodiness

49 Basics of Genetics Examples of Dominant and Recessive Traits in Farm Animals Specie Dominant Trait Recessive Trait Poultry Feathered Shanks White Skin Clean Shanks Yellow Skin Sheep Wooly Fleece Hairy Fleece White Wool (except for Karakul and Black Welsh) Black Wool

50 Basics of Genetics Examples of Dominant and Recessive Traits in Farm Animals Specie Dominant Trait Recessive Trait Swine Black Hair (Hampshire) Cloven Hoof Erect Ears Red Hair Mulefoot Droopy Ears

51 Basics of Genetics Single Gene Effects: Horned vs Polled P = Dominant Polled p = Recessive Horned

52 Basic Genetics Single Gene Effects: Angus Coat Color B = Dominant Black Angus b = Recessive Red Angus

53 Basic Genetics Single Gene Effects: Poultry Skin Color W = Dominant White w = Recessive Yellow

54 Basics of Genetics Single Gene Effects: Dwarfism in Cattle, Snorter Calves or Snorter Dwarfs N = Dominant Normal n = Recessive Dwarf Normal (N) Dwarf (n)

55 Basics of Genetics Single Gene Effects: Partial Dominance; Lack of Complete Dominance or Co- Dominance RR = Red Coat, Shorthorn Cattle rr = White Coat, Shorthorn Cattle Rr = Roan or Spotted Coat, Shorthorn Cattle

56 Basics of Genetics Biochemical Basis of Heredity: A living organism is a complex chemical factory. Many chemical reactions are going on all the time in living organisms. Scientists, James D. Watson and Francis Crick discovered DNA in 1953.

57 Basics of Genetics Biochemical Basis of Heredity: DNA = Deoxyribonucleic Acid DNA is the basic material of heredity. A genetic protein-like nucleic acid on plant and animal genes and chromosomes that controls inheritance. Each DNA molecule consists of two strands in the shape of a Double Helix.

58 Basics of Genetics

59 Basic Genetics Biochemical Basis of Heredity: Most inheritance characteristics can be predicted but some cannot, because some genes jump (are promiscuous). Such genes can result in resistance to pesticides, drugs, etc. DNA can reproduce itself, molecule by molecule, by joining various chemical building blocks.

60 Basics of Genetics Biochemical Basis of Heredity: DNA is the material of which genes are made. The DNA molecule has been described by a few in the scientific community as the backbone of the chromosome, analogous to the vertebral column, which is the backbone of the vertebrate animal body.

61 Basic Genetics Biochemical Basis of Heredity: DNA molecule resembles a long, twisted ladder in which two strands (sides) are joined together by runs. Each strand is called a Polymer (poly = meaning many) and (mer = meaning parts), because it is composed of many repeating units of nucleotides.

62 Basics of Genetics Biochemical Basis of Heredity: Nucleotides are composed of a nitrogenous base (either a purine or a pyrimidine) linked to a sugar, which is in turn linked to a phosphoric acid molecule.

63 Basics of Genetics Heredity The Make Up of an Individual Animal Sire Sperm Dam - Ovum Environment Ration Disease Climate Housing Water Manager Injury Zygote Birth Weaning Sexual Maturity Death Conception

64 Selection

65 Selection Natural Selection: Refers to the influence of the environment on the probability that a particular phenotype survives and reproduces. Not all phenotypes are equally Fit to compete in a particular environment. Fitness is the capability of a phenotype and the corresponding genotype to survive and reproduce in a given environment.

66 Artificial Selection: Selection Refers to a set of rules designed by humans to govern the probability that an individual survives and reproduces. Individuals capable of surviving may not be allowed to survive under an Artificial Selection program because their appearance or performance does not meet some standard.

67 Selection Population Genetics: Animal breeders are interested in characteristics of whole groups or large Populations. Animal breeders are concerned with individuals and particular matings only because they make a Population what it is.

68 Selection Population Genetics: Basic to Population Genetics is the concept of Gene Frequency (q). Gene Frequency is a value characteristic of a particular gene in a particular Population.

69 Selection Population Genetics: Gene Frequency is a fraction of the number of genes of that kind in the total number of genes in that Allelic Series within that Population. Gene Frequency will have a numerical value between 0.0 & 1.0. Permanent genetic changes in Populations are produced by changes in Gene Frequencies.

70 Selection Factors Which Change Gene Frequencies: 1. Mixture of Populations 2. Mutations 3. Selection (Artificial Selection) 4. Genetic Drift

71 Selection Factors Which Change Gene Frequencies: 1. Mixture of Populations: Example: Assume a herd of 20 white registered Shorthorn cows (rr). The Gene Frequency of white gene (r) in this group of Shorthorn cows = 1.0 The Gene Frequency of red gene in this group of Shorthorn cows = 0.0 If a red bull (RR) was mated to these white cows (rr) and produce 20 calves, all of these calves would be roan (Rr), and the Gene Frequency of the white gene (r) in the calves would be 0.5 as compared to 1.0 in their dams.

72 Selection Factors Which Change Gene Frequencies: 2. Mutations: A change in a gene which causes a sudden change in the phenotypic expression of that gene. Genes and alleles mutate at different rates. Basically most, if not all mutations are due to a change in the code sent to ribosomes by the gene by means of RNA m to form a particular protein.

73 Selection Factors Which Change Gene Frequencies: 2. Mutations: (continued) If the wrong code is sent, different proteins are formed. The missing protein or the new protein may cause a defect in a new genetic trait to appear. Differences we can see or measure between individuals are due to an accumulation of different mutations (old or new) within populations. These mutations are responsible for differences in coat color, size, shape, behavior, and other traits in various animal species.

74 Selection Factors Which Change Gene Frequencies: 3. Selection (Artificial Selection): Can be a potent force in changing the frequency of a gene or genes in a population. Main genetic effect of artificial selection, if selection is effective, is to change Gene Frequencies. Artificial Selection is the most valuable tool for an animal breeder.

75 Selection Factors Which Change Gene Frequencies: 4. Genetic Drift: Frequency of a gene may be quite different than in a larger population from which the smaller population was derived. Example: If in a large population the frequency of genes (A) is 0.5, and if a small group of individuals leave this larger population (migrate) and settle in another area where they become isolated, they interbreed.

76 Selection Factors Which Change Gene Frequencies: 4. Genetic Drift: (Example continued) The frequency of the Alleles (A) and (a) in this small group could be between 0.7 and 0.3, or any frequency depending upon the change. Thus, the frequency of genes (A) and (a) in the new isolated population could be quite different than in the original population from which they came. Another way the Gene Frequency could be altered in the other direction for this small isolated group would be if a male from another population becomes introduced to the group, thus a form of Genetic Drift.

77 Selection

78 Selection Selection Response: Five (5) factors that determine a herd and/or an animal s response to Artificial Selection: 1. Heritability 2. Accuracy 3. Selection Differential 4. Generation Interval 5. Number of Traits Selected

79 Selection Heritability: Heritability is the portion of Total Phenotypic Variation that is due to Hereditary differences between individuals. Heritability is also a technical term used by animal breeders to describe what fraction (portion) of differences in a Trait are due to differences in Genetic Variation rather than Environmental Variation.

80 Selection Heritability: (continued) Heredity Variation is due to additive gene action. Phenotypic expression of genes is that which can be measured by our senses. Phenotype is what can be seen visually as a Trait or outward appearance of the animal in question. Heritability Estimate = h 2.

81 Selection Heritability: (continued) Heritability Estimate = h 2. h 2 = Genetic Variation h 2 = Additive Genetic Variation Total Variation* *(Total Variation = Genetic Variation + Environmental Variation)

82 Selection Heritability: (continued) Genetic Improvement of farm animals depends upon the existence of Genetic Variation! To improve a productive characteristic of an animal, requires animals within the population (i.e. herd, flock, breed, etc.) must differ in the genes they possess that govern the ability to manifest a specific productive trait.

83 Selection Heritability: (continued) Two (2) kinds of Genetic Variation: 1. Additive 2. Non-additive

84 Selection Heritability: (continued) Additive Genetic Variation: Refers to differences from animal to animal in those genes in which the changes in genetic ability due to replacement of one gene with another which are additive; That is, there is a linear change in breeding value with the addition of successive allelic substitutions.

85 Selection Heritability: (continued) Non-additive Genetic Variation: 1. Dominance Type 2. Epistatic Type

86 Selection Heritability: (continued) Dominance Variation arises when the change in genetic ability is nonlinear after the substitution of one allele for another at the same genetic locus. Epistatic Variation arises when nonlinear changes in genetic ability occur at a locus due to a gene substitution at a different locus.

87 Selection Heritability: (continued) Heritability Estimates (h 2 ): Heritability Estimates reflect the degree to which additive gene effects exist in controlling variation in performance. Heritability Estimates fall between 0.0 and 1.0; apart from errors of estimation. A Heritability Estimate value near 1.0 suggests that environmental variation plays little part in variation in performance from animal to animal.

88 Selection Heritability: (continued) Patterns are expected in heritability estimates for various aspects of animal performance. Traits associated with reproduction are expected to have very low heritabilities. Traits associated with rate of gain, growth, carcass traits, body dimensions, and wool traits will have higher heritabilities.

89 Selection Heritability: (continued) Heritability Estimates reflect the degree to which additive gene effects exist in controlling variation in performance. When substantial Additive Variation exists, selection on phenotypic differences among animals will change the genetic ability of the animal population (herd, flock, etc.). When Non-additive Variation is substantial, breeding methods such as crossbreeding or crossing of different genetic lines will be effective in improving performance.

90 Selection General Trait Values for Heritabilty Estimates (h 2 ) of Livestock: Trait(s) (h 2 ) Range Level of Heritability Reproductive Low Growth & Milk Medium Carcass High Wool/Fleece Very High

91 Selection Types of Traits: 1. Qualitative Traits 2. Quantitative Traits

92 Selection Qualitative Traits: Traits associated with quality of the animal; Non-measurable; Great value to purebred breeders; Little value to commercial breeders; Examples, i.e. coat color, horns vs polled, ear carriage, conformation, etc.

93 Selection Quantitative Traits: Traits associated with quantity of product(s) produced by the animal; Measurable via scale, ruler, number count, etc. Great value to commercial breeders; Less value to many purebred breeders; Examples, i.e. weaning weight, average daily gain, fleece weight, number of piglets weaned, eggs laid, feed efficiency, yield grade, etc.

94 Selection Statistics: Statistics is the quantitative study of Populations. Population Statistics includes all of the individuals of that Population (that is identified). Sample is a smaller part of the larger defined Population. The Bell Curve, the basic statistical Binomial Distribution or Normal Distribution is shown on the next page.

95 Statistics: Selection

96 Selection Arithmetic Mean, aka Average: Mean provides no information about variability.

97 Selection Variance: Variance is the best measure of dispersion. Standard Deviation is a measure of dispersion off of the Mean. Properties of Standard Deviation: Easy to calculate Use of all values Uses Mean as a base Same units as original observation.

98 Selection Genetic Correlations: Genetic Correlations may be determined statistically, or they may be determined by selection for only one trait. If one is successful in improving one (1) trait, changes may also happen in another unobserved and unselected trait. A Genetic Correlation means that two (2) or more traits are affected by the same Genes. Genetic Correlations between two (2) traits may influence the amount of progress (+) or retrogression (-) made in selection for one of them.

99 Selection Genetic Correlations: Positive Genetic Correlation (example): Positive + Weaning Weight Milk Production

100 Selection Genetic Correlations: Negative Genetic Correlation (example): Grade of Wool Negative - Clean Wool Yield

101 Selection Selection Differential (SD): The Selection Differential is the difference between the Mean of the selected parent(s), and the average of the population for their Generation.

102 Selection Selection Differential (SD): or The average superiority of those selected for parents over the average of the population from which they were selected. SD = ½ X ( SD + SD)

103 Selection Selection Differential (SD): The magnitude (size) of the Selection Differential is dependent upon the number of individuals that can be Culled. (Culling is the opposite of Keeping)

104 Selection Selection Differential (SD): The more traits selected for, the smaller the Selection Differential. This fact is because it is more difficult to find an individual that is excellent for two (2) or more traits; than it is to find an individual that is excellent for only one (1) trait.

105 Selection Generation Interval (GI): The Generation Interval (GI) is the average age of the parents when the offspring are born. Early breeding of both males and females in a herd or flock can reduce Generation Interval.

106 Selection Generation Interval (GI): Generalizing the following are samples of typical Generation Intervals for the identified animal species: Swine 2 to 3 years Cattle 4 to 6 years Sheep 3 to 4 years Horses 9 to 13 years Humans 30 to 35 years

107 Selection Artificial Selection Response: Annual Genetic Progress = SD x h 2 GI + Average for Trait Selected SD h 2 GI = Selection Differential = Heritability Estimate value for a specific trait = Generation Interval

108 Selection Genetic Progress for one (1) Year: Annual Genetic Progress = SD x h 2 GI + Average for Trait Selected Example: A herd of cattle had an average weaning weight, with calves adjusted to a standard. The average adjusted 205 day weight for the herd was 400 lbs. The bull calves selected for breeding weighed an average of 500 lbs. The heifer calves selected for breeding weighed an average of 450 lbs.

109 Selection Genetic Progress for one (1) Year: Thus: SD for bull calves = = 100 lbs. SD for heifer calves = = 50 lbs. SD for both selected parents = = 75 lbs. If h 2 for adjusted 205 day weaning weight of beef cattle is 0.25 (25%) Then the expected improvement in weaning weight should be lbs. (0.25 x 75 lbs.).

110 Selection Genetic Progress for one (1) Year: Thus: (continued) If the average age of the cows at the time of calving was 4 years, and the average age of the bulls used to produce these calves at the calving of these calves was 3 years, then calculate the Annual Genetic Progress expected from these potential selected breeding parents. GI = = 7 2 = 3.5 Years Generation Interval The expected AGP; = 5.36 lbs. increase/year in adjusted 205 day weaning weights.

111 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding 2. Linebreeding 3. Outbreeding 4. Crossbreeding

112 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding: The production of progeny by parents that are more closely related than the average of the population from which they came. Common ancestors within the last 3 or 4 generations. Major genetic effect of INBREEDING is to increase the number of pairs of genes that are HOMOZYGOUS in an INBRED POPULATION.

113 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding: INBREEDING does not create RECESSIVE GENES! Increased HOMOZYGOSITY due to INBREEDING increases BREED PURITY.

114 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding: Increased HOMOZYGOSITY due to INBREEDING also tends to fix RECESSIVE GENES in a small INBRED POPULATION.

115 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding: Increased HOMOZYGOSITY due to INBREEDING when combined with SELECTION, can be used to eliminate detrimental RECESSIVE GENES from an INBRED POPULATION.

116 Applied Animal Breeding Mating Systems for Livestock Improvement: 1. Inbreeding: The main reason for INBREEDING is to produce INBRED LINES that may be used in crossing purposes to take advantage of HYBRID VIGOR.

117 Applied Animal Breeding Mating Systems for Livestock Improvement: 2. Linebreeding: This is a form of INBREEDING in which an attempt is made to concentrate the inheritance of one (1) or more outstanding ANCESTORS IN A PEDIGREE. HOMOZYGOSITY does not usually increase as rapidly as it does in INBREEDING.

118 Applied Animal Breeding Mating Systems for Livestock Improvement: 3 Outbreeding: This is the mating of UNRELATED FAMILIES WITHIN THE SAME BREED.

119 Applied Animal Breeding Mating Systems for Livestock Improvement: 4. Crossbreeding: This is the mating of individuals from DIFFERENT BREEDS. A CROSSBRED is more intense in its GENOTYPIC and PHENOTYPIC effects than in OUTBREEDNIG, but the effects are similar.

120 Applied Animal Breeding Mating Systems for Livestock Improvement: 4. Crossbreeding: Main genetic effect of CROSSBREEDING is the INCREASED HETEROZYGOSITY or DECREASED HOMOZYGOSITY within the population that it is practiced. Main PHEONOTYPIC effects of CROSSBREEDING is an increase in PHYSICAL FITNESS or traits related to PHYSICAL FITNESS.

121 Applied Animal Breeding Mating Systems for Livestock Improvement: 4. Crossbreeding: Crossbreeding = Hybrid Vigor = Heterosis HYBRID VIGOR is the superiority of the CROSSBRED offspring over the average of the PUREBREDS used to make the cross. HYBRID VIGOR includes more than just HARDINESS it includes greater VARIABILITY, faster growth rate, greater milk producing ability, and fertility.

122 Systems of Selection 1. Tandem Selection 2. Independent Culling Level 3. Selection Index

123 Systems of Selection 1. Tandem Selection: One trait is selected for at a time until maximum progress has been achieved for the trait being selected. Then another trait becomes the focal target for improvement. This system of selection is slow.

124 Systems of Selection 2. Independent Culling Level: Minimum standards are identified or set for several traits within the herd or flock. The failure by any animal to meet any one trait, results in it being sold or slaughtered.

125 Systems of Selection 3. Selection Index: A SELECTION INDEX is a mathematical scoring system, giving weighted values for any number of ECONOMICALLY IMPORTANT TRAITS. Animals then are ranked based on an INDEX COMPUTED. The SELECTION INDEX system is widely accepted in the United States.

126 Systems of Selection 3. Selection Index: Below average animals are CULLED. The SELECTION INDEX system can be tailored to fit a scoring and weighted system to one s farm and/or ranch needs.

127 Systems of Selection 3. Selection Index: Computers and even programmable calculators can be utilized. Some livestock breed associations have CUSTOMIZED SELECTION INDEX selection systems for use by their breeders.

128 QUESTIONS?

129 More Math Anyone?