By
David R. Hawkins
Michigan State University

• Cattle have 30 pair of chromosomes in the nucleus of each cell.
• 29 pairs are similar and the remaining pair are the sex chromosomes which can be either XX (female) or XY (male).
• Regions on each chromosome called “loci” contain the genetic information “genes” which control the expression of a trait.

• Since the chromosomes and loci occur in pairs, so do the genes.
• Genes on homologous chromosomes are called alleles.
• If the alleles are similar the animal is
“homozygous”, but if they are different, it is
“heterozygous”.
• “Genotype” refers to the genetic makeup of an animal while “phenotype” refers to what we see or measure.

• Qualitative traits are those that are controlled by 1 or 2 gene pairs and tend to be distinct in their phenotype. Examples of qualitative traits are coat color, polled vs. horned, dwarfism, etc.
• Quantitative traits tend to be influenced by several gene pairs. Examples include production traits like weaning weight, milk production, carcass traits, etc.

• Five factors influence the rate with which populations can be changed by selection.
– Heritability
– Selection Differential
– Genetic Correlation
– Generation Interval
– No. of traits for which one selects

• Heritability is the proportion of the differences that we observe between animals that can be transmitted to their progeny.
• Heritability (h 2 ) is usually expressed as a % between 0% and 100%.
• In general, fertility traits have low heritability, growth traits are medium to high and carcass traits are highly heritable.

• Selection differential (SD) is the difference between the selected animals and the average of the population from which they were selected.
• If a selected bull had a weaning weight of
500 lb. and his herd average was 450 lb., the selection differential would be 50 lb.
(500 – 450 = 50)

• This can be calculated by multiplying h 2 x
SD.
• If the heritability were 30% and the selection differential were 50 lb. then the improvement per generation would be (.3 x
50 lb. = 15 lb.)

• Generation Interval (GI) is the average age of the parents when their progeny are born.
• In beef cattle herds the average generation interval is 5 to 6 years.
• Theoretically, it could be as short as 2 years, but one would have to mate yearling bulls to yearling heifers and replace the parents each year, which is not practical.

• This refers to the situation where different traits are influenced by the same genes.
• For example, weaning weight and yearling weight are genetically correlated.
• Genetic correlations can be either positive or negative.
• If traits are correlated, selection in one trait will result in change in the other trait.

• Selection for more than one trait, reduces the amount of progress one achieves in one trait alone.
• Change occurs at a rate of 1/square root of n, where n = number of traits being selected.

• Tandem is selection for one trait at a time.
It is usually not very effective.
• Independent culling level establishes minimum or maximum values that are acceptable for each trait.
• Selection index provides a means of including several traits and different economic weighting factors for each trait.

• A pedigree is a written record of ancestry that is usually maintained by the respective breed association.
• Performance pedigrees contain a large amount of information including name, birth date, identification, owner, parentage, performance, EPDs, etc.

• Abbreviated as EPD
• Can be calculated for any trait that can be measured quantitatively.
• Expressed in same units as measured (lb., cm., sqin., etc.)
• Are computed by the respective breed associations.

• Calculations are possible because of A. I. which permits sires to be used in more than one herd.
• High speed computers can quickly solve the mathematical equations.
• Information from several sources (pedigree, individual performance, progeny data, etc.) are combined to arrive at a single value for each trait.
• EPDs change over time as more data are included in the analyses.

• Accuracy is a measure of how much an
EPD might change or reliability.
• Accuracies range from “0” to “1.00”, and they increase as an animal ages.
• Pedigree information results in an accuracy of .05 to .10. Individual performance will increase accuracy to about .35 and progeny data will increase accuracy to .99.

• EPDs can be used to compare animals across a breed in the same analysis.
• Breed average for each trait is not “0”.
• Many breeders are using breed percentiles to determine where an animal ranks in the breed.
• EPDs are an important tool for selection but should not be the sole criteria for evaluating a potential breeding animal.

1. Obtain a sire summary
2. Establish selection goals based on your cow herd, breeding program and resources
3. Prioritize traits
4. Find the breed’s average EPD values and percentiles
5. Determine an acceptable range of EPDs for your herd

6. Establish some trade-offs that you are willing to make
7. Keep records of how bull’s perform in your herd and how their EPDs correlate with actual progeny data
8. Reevaluate your goals and decide which sires to use for the next mating season

1. Decide which traits are economically important.
2. Focus on traits with moderate to high heritability.
3. Be aware of genetic antagonisms.
4. Be objective in your evaluation.
5. Set realistic and obtainable goals.

6. Consider a selection index for multiple trait selection.
7. Use proven sires with high accuracy.
8. Use all emerging technologies.
9. Breed cattle that are functionally sound for your environment.

10. Develop a long range plan and follow it.
11. Listen to your customers.
12. Continually reevaluate your plan and make adjustments as needed.