Understanding 2005 genetic base updates
Dr. Rex Powell,* Dr. Martin Sieber,** and Ms. Suzanne
During 2005, the United States and many other countries will be updating their genetic bases for dairy cattle evaluations. Will those changes affect your breeding decisions in selecting the bulls and cows to be parents of the next generation of animals in your herd? Having evaluations expressed on the same genetic base is important so that your breeding decisions will be based on the most accurate information available.
A genetic evaluation can be expressed as either a predicted transmitting ability (PTA) or an estimated breeding value (EBV). Both are measures of performance relative to a base population. A PTA indicates the difference in performance that can be expected from an animal's daughters relative to that base; an EBV is the genetic merit of the animal itself relative to the base and, therefore, is twice PTA. Regardless of whether evaluations are expressed as PTA's or EBV's, the genetic evaluation of a bull should be used to predict only:
When looking at either PTA's or EBV's, the difference between bulls is what is important, and that difference is not dependent on which genetic base is chosen (how an evaluation of zero is defined).
What is a genetic base?
A genetic base is a benchmark or reference point from which genetic merit is expressed. Typically, a group of animals (bulls or cows) at a given date is selected, and the average of their evaluations is set to zero. For example, the genetic base in the United States for the past 5 years has been defined as the average PTA of cows that were born in 1995.
Genetic bases often are updated periodically to reflect genetic progress. Production trends for U.S. Holstein cows born between 1995 and 2000 indicate that PTA's for Holstein cows were increasing by about 120 pounds (54 kilograms) of milk each year. Genetic trends usually are expressed in EBV's (double the PTA); therefore, genetic improvement in the U.S. Holstein population is 240 pounds (109 kilograms) per year.
What types of genetic bases are used?
There are three different types of genetic bases: fixed, rolling, and stepwise. With a fixed genetic base, the year and group of animals chosen as the base population remain the same forever. All evaluations can be compared regardless of the evaluation date, but evaluations for current animals become larger and larger because of genetic progress. Because breeders may become complacent and believe that all bulls have high genetic merit, a fixed base is rarely used today.
With a rolling genetic base (for example, in Canada and France), the base group of animals is updated each year. Although this approach allows sire selection standards to remain constant from year to year, comparison of evaluations over time is more difficult. Also, because the zero point is based on past and not current sire selections, breeders can mistakenly believe that a positive evaluation means that the animal is above average today, which is incorrect. Some Scandinavian countries use an average index value of 100 for their rolling base, and the evaluations are scaled around that value.
A stepwise genetic base (which is used in the United States and many other countries) is a combination of fixed and rolling bases in which the base population is updated at regular intervals but less frequently than yearly (for example, every 5 years). A stepwise base features a fixed base for a specified number of years, and the periodic updates clearly reflect the amount of genetic improvement that has occurred since the last base change.
Genetic bases around the world
A stepwise base with updates every 5 years (1995, 2000, 2005, etc.) is the international standard that is recommended by the International Bull Evaluation Service (Interbull) for all traits. In most countries, the base is established in the first national evaluation calculated during those years by setting the average genetic merit of cows born 5 years before to zero. However, every country has the freedom to choose whether a cow or bull base is used and which year is appropriate for the evaluation system in that country. Each country also decides whether to use a fixed, rolling, or stepwise genetic base.
Comparison of evaluations from different genetic bases
If genetic bases differ between countries, how can you compare evaluations? Even though genetic bases may be defined in the same way (for example, cows born in a specific year), the bases will not be comparable. Many other factors need to be considered to compare evaluations from different countries accurately. In fact, such a process is so complex that Interbull was formed to provide a forum for discussing methods of comparison, which in turn led to the establishment of the Interbull Centre in Uppsala, Sweden. That Centre uses a multitrait across-country evaluation system to combine national evaluations from many countries and express the resulting combined evaluations on each national scale. Because the Interbull evaluations are on the scale of any participating country of interest, the various genetic bases used by other participating countries are irrelevant.
One of the objectives of Interbull is to increase the ease of understanding genetic evaluation systems in different countries. Whether you live in one of the 26 countries that currently submit national evaluations to Interbull or in another dairy country (for example, Argentina, Brazil, China, Korea, or Mexico), you still can use the results from Interbull on the scale with which you are most familiar. For example, you can go to a U.S. Department of Agriculture (USDA) Internet web site (Animal Improvement Programs Laboratory) and find official evaluations on the current U.S. scale for almost 235,000 bulls. Although that database includes about 149,000 U.S. and 86,000 Interbull evaluations, you would need to be familiar with only the U.S. scale and genetic base. However, because only bull rankings and differences are important, knowledge of even that one base is of marginal interest. For example, if PTA milk of two bulls differed by 300 pounds (136 kilograms) under a 2000 base, those bulls would still differ by 300 pounds (136 kilograms) if their PTA's were expressed under a 2005 base.
Interbull began to provide evaluations for Holstein conformation (type) traits in August of 1999 and now calculates type evaluations for Ayrshire, Brown Swiss, Guernsey, Holstein and Jersey bulls from 21 countries. In the United States, Holstein Association USA calculates PTA for Holstein type traits, and USDA calculates PTA for type traits of other breeds; regardless of breed, the same 5-year stepwise genetic base is used as for U.S. yield traits. Interbull evaluations are expressed on the scale of each participating country so that you do not have to be concerned about different bases for type traits. Just use the current Interbull evaluations for type on the scale with which you are most familiar.
Interbull evaluations for udder health traits were first released in May 2001 for the major dairy breeds. Two separate Interbull evaluations are computed: one for somatic cell score (SCS) and one for clinical mastitis. For countries with no information available on clinical mastitis, SCS evaluations are included as an indicator trait in the Interbull evaluation for clinical mastitis.
The United States evaluates only SCS and uses the same genetic base as for yield and type traits. In the past, a breed average for SCS was included in U.S. SCS, but a base constant of 3.0 will replace the breed average in February 2005. This practice was introduced in Canada in 1996 and also is used in Belgium. Although Canadian and U.S. evaluations will still not be exactly comparable because of slight genetic differences in base cows, the two scales will be much more similar than in previous years, and the base constant of 3.0 will be much easier to remember than breed averages. A proposal to reverse the scale for U.S. SCS evaluations so that higher numbers would be favorable as in many European countries is under consideration for future evaluations.
The first Interbull evaluations for direct longevity of Holsteins were released in November 2004; release of evaluations for other breeds is planned for February 2005. The genetic base for productive life, the longevity measure used in the United States, is the same as for yield and type traits.
Calving ease and fertility
Routine Interbull evaluation for calving traits for Holsteins is planned for February 2005. Those evaluations will include direct and maternal calving ease as well as direct and maternal stillbirth.
In the United States, genetic evaluations for daughter calving ease measure the ability of daughters to calve easily and are expressed as the percentage of difficult births in first-lactation heifers; evaluations for service-sire calving ease measure the tendency of calves from a particular service sire to be born more or less easily. The genetic base for daughter calving ease is the average evaluation of bull daughters born in 1990; the base for service-sire calving ease is the average evaluation of the mates of bulls born in 1995. The National Association of Animal Breeders began to sponsor calving-ease evaluations for Holstein service sires in 1980. Although USDA now calculates calving-ease evaluations, those evaluations still are distributed to the U.S. dairy industry by the National Association of Animal Breeders. Release of USDA calving-ease evaluations for Brown Swiss are planned for February 2005.
The United States also began to evaluate daughter pregnancy rate as a fertility measure of cows in 2003. Daughter pregnancy rate is defined as the percentage of nonpregnant cows that become pregnant during each 21-day period and is a direct function of days open. Although the genetic base for implementation of evaluations was the average evaluation of all bulls born in 1995, the base population will change in February 2005 to be cows born in 2000 to be consistent with most other traits.
Impact of evaluation frequency
Interbull evaluations are calculated four times a year (February, May, August, and November); frequency of national evaluations varies by country and performance trait. Only information from the most recent evaluation on one national scale should be used, especially if a base change has occurred. Using some evaluations from 2004 and others that reflect base updates in 2005 will lead to improper genetic decisions. Evaluations must be expressed on the same national genetic base, ideally from the most recent evaluation, if you want your breeding selections to be based on the most accurate information available. Interbull guidelines state that the minimum requirements for advertising that includes information on genetic evaluations are evaluation source, date, base, expression, units, and reliability.
How much will the U.S. bases change?
If only the genetic base were changed in February 2005, U.S. evaluations would decrease only by the amount of PTA progress for cows born in 2000 relative to those born in 1995:
However, because of other changes that are being made to the U.S. genetic evaluation system at the same time, those decreases will not be uniform for all animals.
In February 2005, the United States also plans to implement adjustments for inbreeding depression and for differing standard deviations across lactations, expression of yield evaluations to a 36-month age base instead of a mature equivalent, and a slightly higher heritability for SCS. Those improvements could affect individual bulls and cows to varying degrees. The new procedures will cause little reranking among current U.S. bulls and are not expected to change Interbull rankings. Cow evaluations will change somewhat more than bull evaluations because a cow's own inbreeding directly affects her performance. Greater detail on the impact of the U.S. base changes can be found at a USDA Internet web site.
How do you make the best breeding decisions?
Regardless of whether you are interested in yield or type traits or whether the genetic base has been updated, the key to wise bull selections is to use current evaluations expressed on the same national scale. Interbull evaluation procedures account for different genetic bases from different countries. Therefore, breeders in any country do not need to adjust evaluations or question if they are comparable as long as comparisons among bulls are made using the most recent Interbull evaluations on a single country's scale.