Masters Theses

Date of Award

12-1962

Degree Type

Thesis

Degree Name

Master of Science

Major

Animal Husbandry

Major Professor

R. J. Cooper

Committee Members

C. S. Hobbs, C. C. Thigpen

Abstract

Spectacular changes have occurred in the conformation of beef cattle since the time of Robert Bakewell, the so-called father of Animal Breeding. Anyone who compares a picture of a beef animal that was existent 200 years ago at Bakewell’s time with the present-day beef animal, will be amazed at the striking improvements in the physical form of the beef animal that have taken place during these two centuries. Selection has been mainly responsible for this remarkable success. In olden days the selection criteria were merely empirical. Visual standards were solely relied upon as the estimation of the genotype. Now conditions are changing. Selection in many herds is being based mainly on the records of performance, supplemented by the visual standards only wherever absolutely necessary. Already performance testing has yielded very good results. Records such as birth weight, growth rate, weaning weight, type score, condition grade, and feed efficiency are used for culling cows, selecting bull-prospects and replacement heifers, as a source of information for progeny testing and for merchandising cattle. Environmental factors such as sex of calf, age of dam, year of birth and season of calving affect these performance traits of calves very much. Information on the effects of these environmental factors is of immense value in comparing animals for selection. Appropriate correction factors are necessary for this purpose. These correction factors enable the breeder to more accurately identify the genetic differences between the individual calves and make the selection procedure more efficient and scientific.

Even selection based on the performance of the individual has its own limitations. When heritability is low, phenotype is a poor indicator of the genotype. For this reason, selection based on the individual performance will be successful only when heritability is high. A reliable estimate of heritability is desirable to decide which kind of breeding plan is most appropriate for the herd. Also knowledge of heritability is necessary in determining the relative emphasis to place on various traits in selection programs.

Heritability is a measure of the proportion of the observed phenotypic variance which is additively genetic. In other words, the heritability estimate tells us how much is attributable to heredity and how much is attributable to the environmental factors which cannot be accounted for by routine adjustment of data. These heritability estimates differ from population to population and from time to time for the same herd.

Repeatability is a concept closely allied to the heritability. In a statistical language, it is the Intraclass correlation between the records of the same cow. It tells the breeder the degree or extent to which a particular trait is repeatable from one calf to another calf from the same cow. It is also useful for estimation of the increased accuracy in evaluating Inherent qualities from averaging repeated records of the same animal, and for estimating the gain from correcting records for known environmental influences. Logically, repeatability represents the upper limit of heritability, since it includes all type of genetic, as well as some environmental influences, which contribute to real differences among animals (Dickerson, 1959).

The breeder will also be interested in the interrelationship of various performance traits. A knowledge of the phenotypic, genetic and environmental correlations will be required to formulate efficient selection procedures (such as using a selection index) and to predict response from selection (Dickerson, 1959), The phenotypic correlation gives a measure of the relationship between the two variables, without making a distinction between genetic and environmental influences. But the genetic correlations provide a measure of the association between two variables taking into consideration only the genetic component involved. The genetic correlation is of practical importance to the breeder since the selection on the basis of one trait automatically improves the other trait provided these two traits have high positive genetic correlation.

Considerable work has been done in the past on the influence of various environmental factors on the performance traits like average daily gain from birth to weaning and type score at weaning; but such information is limited for the period ‘birth to pre-weaning’ and ‘pre-weaning to weaning’ separately. (The term pre-weaning is used here to denote the period from birth to July. Similarly weaning performance refers to the period from birth to weaning. This is a deviation from the common usage in the literature where pre-weaning performance refers to the period from birth to weaning.) Information is inadequate on the heritability and repeatability estimates for the performances in these periods. Although the literature concerning estimates of phenotypic correlations among traits of beef cattle is abundant, there is considerably less information on genetic correlations.

The objectives of this study are:

1. To evaluate the influence of sex of calf, age of dam, month of birth and year of birth on the following traits: birth weight, average daily gain from birth to July (pre-weaning), average daily gain from July to October (pre-weaning to weaning), average daily gain from birth to October (weaning), type score and condition grade in July, and type score and condition grade in October.

2. To develop appropriate correction factors for these traits for use in adjusting the data for genetic analysis and selection procedures.

3. To calculate the heritability and repeatability estimates for the above traits by using the adjusted data.

4. To estimate the phenotypic, genetic and environmental correlations among the various traits.

5. To evaluate the usefulness of taking weights and grades at about 120 days from birth (July) in measuring the performance of calves from birth to weaning and the mothering ability of beef cows.

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