International genetic evaluations:
don't let the numbers confuse you!
 
Dr. Martin Sieber,* Dr. Rex Powell, and Ms. Suzanne Hubbard
*National Association of Animal Breeders, P.O. Box 1033, Columbia, Missouri 65205, USA
Animal Improvement Programs Laboratory, Agricultural Research Service, United States Department of Agriculture, Beltsville, Maryland 20705-2350, USA

    As more and more genetic material is traded around the world, the need for accurate comparisons of the genetic merit of bulls from different countries has become increasingly important. The mission of the International Bull Evaluation Service (Interbull), which is located in Sweden, is to promote the development and standardization of international genetic evaluations for cattle. Formed in 1983, Interbull now has 34 member countries. Twenty-two countries provided national data for the February 1998 Interbull evaluations.

Yield evaluations
    Interbull evaluations for milk, fat, and protein yields currently are released twice a year and will soon become available four times a year (February, May, August, and November). The evaluations are calculated with a statistical procedure called Multiple Across-Country Evaluation (MACE), which allows bulls from many countries to be evaluated simultaneously. Only bull evaluations are needed for MACE, not daughter records. The end result is an international genetic evaluation for each bull for each country.
    MACE takes into account that bulls do not rank exactly the same in any two countries because of interactions between genetics and environment; that is, genetic correlations between countries are less than 1.0. Therefore, the true ranking of bulls can differ by country because of the evaluation system used or the environment. As a result, bull rankings for yield traits are "custom made" for each country to provide evaluations on that country's scale with the appropriate measurement units.
    Although many people think that Interbull results are provided in one combined international list of all bulls from all countries, this is not the case. Interbull does not rank animals in any way. The member countries of Interbull are responsible for ranking bulls based on their own breeding objectives and for releasing those rankings. There is no single international ranking of bulls for yield traits.

Type evaluations
    At present, Interbull does not calculate international evaluations for type (conformation). Joint research projects on international type evaluations currently are underway with Canada (the Canadian Dairy Network and the Centre for Genetic Improvement of Livestock, University of Guelph) and with the United States (Holstein Association USA). In August 1997, Holstein Association USA began to release comparable type evaluations for bulls from Canada, France, Germany, Italy, the Netherlands and the United States. Those evaluations are calculated with a MACE procedure that combines domestic and international type data and are expressed on a U.S. basis.

    Because type traits are not defined the same worldwide, MACE evaluations take into account the different genetic correlations and heritabilities of type traits from different countries. Just as for yield traits, the international ranking on a U.S. scale could differ from the rankings in other countries. How international bulls rank on overall type (PTAT) or individual traits in the United States depends on how much the country of origin has emphasized selection for type and on how genetically related (correlated) type traits in the country of origin are are with the corresponding traits in the United States.

Which international ranking should I use?
    Most international suppliers of genetics participate in international evaluation programs for yield and type. Ask them for the combined ranking of evaluations for domestic and international bulls on their country's scale. For yield traits, this ranking often combines the country's domestic evaluations with Interbull evaluations. Use only this combined ranking to select your bulls so that you will be able to select among domestic and international bulls based on comparable evaluations. For type traits, insist on access to the combined MACE evaluations.

Misleading advertisements
    As the dairy genetics industry has become global, the values for genetic merit that are reported in advertising have become more difficult to interpret and often are confusing. The explanation of genetic terminology that follows will help you understand the value of the product that is being offered.

Expressions of genetic merit
    An estimated breeding value (EBV), which also commonly is shortened to breeding value (BV), is a measure of the expected performance of an animal relative to population average. By definition, a predicted transmitting ability (PTA) is half of BV and, therefore, is an estimate of the genetic merit transmitted to offspring by a parent. The measure of an evaluation's accuracy generally is called its reliability (REL) and is expressed as a percentage.
    For yield traits, many countries use EBV's or BV's expressed in kilograms; PTA's are expressed in pounds in the United States and in kilograms in Ireland, Israel, and the United Kingdom. Because 1 kg is equivalent to 2.2046 lb (often rounded to 2.2 lb), a BV in kilograms and a PTA in pounds are almost the same number. The following examples show the similarity between BV in kilograms and PTA in pounds:

  • Bull A has a PTA for milk of 2000 lb. His BV in kilograms would be calculated as:
     Conversion from pounds to kilograms:   PTA of 2000 lb 2.2 lb/kg = PTA of 909 kg
     Conversion from PTA to BV:                    PTA of  909 kg 2 = BV of 1818 kg
  • Bull B has a BV for milk of 2000 kg. His PTA in pounds would be calculated as:
     Conversion from kilograms to pounds:   BV of 2000 kg 2.2 lb/kg = BV of 4400 lb
     Conversion from BV to PTA:                    BV of 4400 lb 2 = PTA of 2200 lb

    Conversion of units and expression of evaluations to one scale, however, is not sufficient for making valid comparisons of genetic evaluations from different countries. To directly compare evaluations converted to the same units and expression of merit, the following assumptions must be true:

  • The genetic base populations for the two countries are the same.
  • The methodology used to calculate the evaluations is the same.

    These assumptions are almost never true. Therefore, you should not compare a BV in kilograms from one country with a PTA in pounds from another country just by converting the units and expression of genetic merit.
    What if an advertisement doesn't specify the units and scale with the numbers? The best solution is to ask the advertiser exactly what the values are. Do not assume that numbers that seem to indicate a U.S. PTA in pounds are really U.S. PTA's. Make sure that you compare evaluations only on one country's scale and not just similar values on scales from different countries.

How do I interpret the numbers?
    To find out what advertised numbers represent, use the following checklist:

  • What country's scale is being used? (United States? Canada? Spain? Australia?)
  • What is the evaluation date? (July 1997? January 1998? February 1998? May 1998?)
  • How is the evaluation expressed? (PTA? BV?)
  • What units are used for the evaluation? (pounds? kilograms? points?)
  • How is evaluation accuracy reported? (original REL? converted REL?)

    Genetic evaluations published in trade journals and advertisements should be expressed only in the original units of the official evaluations released by each country. Only Interbull evaluations or evaluations converted with the most current official conversion equations should be published for bulls from other countries. The converted evaluation should be used only if an Interbull evaluation does not exist. If a converted evaluation is published, it should be labeled with a "c" suffix and include a converted reliability. All evaluations should be the most current information available at the time of publication. Most importantly, all evaluations should be labeled as to source, date, expression of genetic merit, units, and trait. If an evaluation is not labeled, make sure you find out what the numbers mean before you make breeding decisions.
    If you make sure that you have all the information in the checklist, sorting through the available bull lists, brochures, catalogs, and advertisements will be much easier. Always make sure to request a complete list of bulls on one country's scale to see exactly which numbers are valid comparisons. Do this before you spend your money for dairy genetics that seem to be too good to be true, and you will be rewarded with better returns from your investment.

SIDEBAR

    The official U.S. evaluations for yield traits can be found at the Animal Improvement Programs Laboratory's (AIPL's) internet web site (http://aipl.arsusda.gov). Interbull evaluations on a U.S. scale for any bull evaluated by the Interbull Centre are available at this site through "animal queries." To use those queries, you will need to know the bull's registration number. Links to genetic evaluations of other countries and to the Interbull web site also are provided at the AIPL web site.
    Holstein Association USA, Inc., makes available a list of the top 100 international Holstein bulls based on its Type-Production Index at its web site (http://www.holsteinusa.com). This ranking also shows the bulls' MACE type evaluations.
      In the "databases" at the National Association of Animal Breeders' (NAAB's) web site (http://www.naab-css.org), you can find identification information for any dairy bull ever enrolled in the NAAB cross-reference program and U.S. and international evaluations for yield, type, and calving ease. Searches by NAAB code number, registration number, or name (short or registration) are all available.