John W. Massey and Dale W. Vogt

Department of Animal Sciences, University of Missouri-Columbia

How much advantage for a particular trait do superior animals transmit to their offspring?

Heritability estimates help us answer this important question. This publication explains the

meaning of heritability estimates, how they are calculated and their influence in changing

livestock performance.

Heritability is the single most important consideration in determining appropriate animal

evaluation methods, selection methods and mating systems. Heritability measures the

relative importance of hereditary and environmental influences on the development of a

specific quantitative trait. More specifically, it measures that part of the total variability of

the trait caused by genetic differences among the animals on which the measurements were

The numerical value of the heritability estimate is given as a percentage or a decimal and

should, of course, lie between 0 and 1. In some instances, the value falls outside this range.

This is a chance occurrence caused by statistical manipulation.

A heritability estimate is a partial description of one trait in one group of animals at some

particular time. It may vary (for each trait) during one time period from herd to herd, or it

may vary in the same herd from time to time. This is natural because herds differ in genetic

makeup and because there are many different environmental circumstances from herd to

herd or within a herd from year to year. These genetic and environmental differences

The numerical value of a heritability estimate can be increased or decreased by changes in

either of its component parts. An increase results from a reduction in the environmental

variance or from an increase in genetic variance. Conversely, a decrease results from an

increase in environmental variance or from a reduction in genetic variance.

A number of factors affect genetic variance. Introduction of new and unrelated animals

into the herd may increase the genetic variance. Effective selection within a group of

animals over a number of generations decreases the genetic variance. The use of

inbreeding as a system of mating also reduces the genetic variance.

Any management practice that ensures uniform treatment of animals reduces

environmental variance. For example, if you give each animal the same amount and

quality of feed, you reduce environmental variance. When you make adjustments for any

environmental differences, your objective is to remove performance differences that result

because animals are "treated" differently.

Weaning weight provides an example. To get accurate figures, you make adjustments for

the dam's age or the calf's season of birth to remove their effects on weaning weight.

Effective adjustment also reduces total variance of weaning weight within individual

herds. This reduction makes recognizing genetic differences among animals in the herd

easier. When you adjust records and treat animals uniformly, you are attempting to reduce

environmental variance so you can more easily recognize genetic differences.

The heritability estimates in Tables 1, 2 and 3 are average values based on many studies

conducted at USDA research centers and college of agriculture experiment stations in

many states.

Calving interval 3 0.08

Fertility — 0.10

Birth weight 75 0.45

Gain, birth to weaning 62 0.30

Weaning weight 83 0.24

Feedlot gain 43 0.34

Pasture gain 14 0.30

Final feedlot weight 36 0.46

Final yearling pasture weight 19 0.44

Weaning score 52 0.38

Final feedlot score 16 0.36

Yearling pasture score 12 0.30

Feed efficiency 20 0.45

Weaning, shoulder, creep fed 6 0.82

Weaning, shoulder, non-creep

fed

6 0.88

Weaning, hip, creep fed 6 0.82

Weaning, hip, non-creep fed 6 0.95

Grade 0.50

Dressing percent 0.45

Ribeye area/cwt 0.70

Fat thickness/cwt 0.45

Tenderness 0.60

Retail product, percent 0.30

Retail product, pounds 0.60

conformation characters of young beef cattle. Departmental Tech. Rpt., No. 103, 2nd

Ed., Texas Ag. Exp. Station, Texas A&M University, College Station, Texas.

Gestation length 0.45

Multiple birth 0.15

Birth weight 0.30

Rate of gain 0.30

Weaning weight (at 60 days) 0.10

Weaning weight (100+ days) 0.30

Mature body weight 0.40

Weaning type score 0.10

Yearling type score 0.40

Face cover 0.56

Neck folds (at weaning) 0.39

Skin folds 0.40

Fleece weight, grease 0.38

Fleece weight, clean 0.40

Staple length (at weaning) 0.39

Staple length (at yearling) 0.47

Grade 0.35

Loineye area 0.53

Fat thickness over loineye 0.23

Weight/day of age 0.22

Length 0.31

Fat weight 0.57

Bone weight 0.30

Lean weight 0.39

Grade 0.12

Retail cut weight 0.50

Dressing percent 0.10

Number farrowed 0.10

Number weaned 0.10

Birth weight 0.10

Weaning weight 0.15

Growth rate 0.30

Feed efficiency 0.35

Meat tenderness 0.30

Meat color 0.30

Marbling (in loin) 0.30

Meat firmness 0.30

Backfat thickness 0.50

Loineye area 0.50

Length 0.60

Percent ham, chilled carcass wt. 0.60

Percent fat cuts, chilled carcass wt. 0.60

Percent lean cuts, chilled carcass wt. 0.50

In general, each estimate of heritability is based on the degree of resemblance among

related individuals vs. non-related individuals in some animal population. Family units

most often used to evaluate degree of resemblance include parent and offspring; parents

and offspring; full sibs (i.e., full brothers and/or sisters); and paternal half sibs (i.e., half

brothers and/or sisters).

The statistical technique used in calculating heritability is chiefly dependent upon available

records. One practical consideration is the kind of family units represented in the data. This

may dictate the technique used. If you have a choice of techniques (for example, if more

than one kind of family unit is represented in the data set), then you must consider which

technique gives the least amount of bias from a variety of sources.

Complications in any technique arise most often when you try to account for the effects of

environmental factors. You must equalize environmental factors as much as possible and

adjust for other non-genetic factors that influence animal performance. For example, if you

are evaluating resemblance between weaning hip heights of cows and their calves, you

might make adjustments for differences in age at the time of measurement, age of dam, sex

of calf, season of birth, and so forth.

If you fail to adequately account for environmental contributions, you will reduce the

estimate of heritability. From a practical standpoint, this means you will be less able to

recognize genetic differences among animals you are considering for breeding purposes.

One of the most common estimation techniques, and the one described here, is the paternal

half-sib analysis of variance. With this method, the total variance is divided into two parts.

In one, variation is attributed to differences among progeny of different sires. In the other,

the variance is attributed to differences among offspring of the same sire. The analysis of

variance is generally put into tabular form as follows:

Total N - 1

observations minus number of constraints. For example, N - 1 is the total number of

measurements (observations) of a specific trait, such as hip height, minus 1.

and "within sires" sources of variance.

the genetic variance for the trait concerned.

variance.

You can use heritability estimates to estimate progress and setbacks in different traits that

you can expect from different matings. For example, a particular mating may bring

improvement in rate of gain if the parents are genetically superior. If they are inferior,

however, they may cause a decline in rate of gain in their offspring.

To illustrate how to figure expected progress from particular matings, assume you have a

herd with an average daily gain in the feedlot of 2.40 pounds per day. From that herd, you

kept bulls that gained 3.20 pounds and heifers that gained 2.80 pounds per day for

breeding purposes.

How much gain in genetic improvement could you expect in the progeny of these selected

parents?

To answer this question, first calculate just how superior these parents were to the average

in the herd.

Calculate the superiority of the breeding animals as follows:

The next question is, "How much of this 0.60 pound advantage is transmitted to the

offspring?" To answer, you must know the heritability of feedlot average daily gain. The

average estimate for this trait is 0.34 (See Table 1).

Expected genetic gain, then, is equal to the average superiority of the parents multiplied by

the heritability (i.e., 0.60 x 0.34 or 0.20 pounds/day).

The herd average was 2.40 pounds feedlot gain per day. With all other things equal, you

would expect the offspring of the selected parents to gain an average of 2.40 + 0.20 = 2.60

pounds per day. This is the average of the herd plus the genetic advantage transmitted by

the parents.

The calculations above illustrate two important points. First, if the selected parents had not

been superior in rate of gain over the average of the herd, there would have been no

genetic improvement in rate of gain of their offspring, regardless of the degree of

heritability of the trait.

Second, the amount of genetic progress is also dependent on how highly heritable a trait is.

Though the parents had an advantage over the herd average of 0.60 pounds per day in gain,

they would not have transmitted any of this advantage to their offspring if the trait had a

herd heritability.

The general conclusion, then, is that the greater the superiority of the individuals selected

for breeding purposes and the higher the heritability of the trait, the more progress will be

made in selection.

A knowledge of the size of the heritability estimate is also important in deciding which

animal evaluation method should be used. When heritability of the trait is medium to high

(above about 0.30), selection based upon the individual's own level of performance allows

a relatively rapid rate of improvement. When the trait has a low heritability, you should

use other methods to identify genetically superior individuals. The "other" methods

involve various schemes for including the level of performance of related individuals such

as siblings or progeny.

You can make more improvement in low heritability traits by using mating systems that

take advantage of heterosis (hybrid vigor). As a general rule, the lower the heritability of a

trait, the greater the heterotic response from various outbreeding mating systems.

Low-heritable traits, such as those associated with reproductive efficiency, show the

greatest benefit from using outbreeding mating systems. Highly heritable traits, such as

carcass quality traits, show very little if any heterotic response from outbreeding mating

systems.

An important difference lies between these two avenues of trait improvement.

Improvement from selection schemes is cumulative over the generations. But improvement

that comes from exploitation of heterosis is maximized in one generation and must be

recreated each generation thereafter.

Copyright 1999 University of Missouri. Published by University Extension, University of Missouri-Columbia. Issued in furtherance of

Cooperative Extension Work Acts of May 8 and June 30, 1914, in cooperation with the United States Department of Agriculture. Ronald J.

Turner, Director, Cooperative Extension Service, University of Missouri and Lincoln University, Columbia, Missouri 65211. • University

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