Interbull's procedure for weighting data from different countries and estimating reliability (REL) has been based on numbers of daughters without consideration of the many factors that can affect how much is known about each daughter. Differences in lactation length, numbers of lactations, and numbers of herdmates are obvious differences among daughters. Having a given number of daughters in more herds gives more information. The simple number of daughters can be uniformly reported by all countries, and does a reasonable job of combining data, but is certainly not an optimal measure of the amount of information. If only the count of daughters is used to estimate REL, one would expect that young bulls (high percentage of short records) would have REL overestimated relative to older bulls (many daughters with multiple records).

The September 2000 test run included a new weighting procedure (effective daughter contribution). This was studied relative to the impact on REL estimates. The particular interest was in how well the new Interbull REL compared to the national REL. Evaluations for US bulls with only US data were from three sources: US national and Interbull for August and the test run. National REL were compared to the two Interbull REL and differences were examined.

Evaluation records were matched for 15,193 Holstein bulls. Interbull REL are often slightly lower than national evaluations because the latter include information from the dam, which is not counted in REL by Interbull. On the other hand, that can be offset by other relatives in other countries such that the Interbull REL might be higher. Both Interbull REL were lower than USDA, but only by an average of about 1%. The test REL were closer to the USDA REL than were the August Interbull REL. The improvement in correlation with USDA REL was dramatic, .919 for August but .996 for the test.

The new weighting accounts for incompleteness of lactation records, multiple records, and other factors. Impact on PTA for these bulls was minimal. Correlations were above .9998 and mean absolute differences between USDA and either Interbull result were 10, .2, and .2 pounds for PTA milk, fat, and protein, respectively. The new weighting improved the REL of single-country bulls and additional improvement from more precise weighting for multi-country bulls is likely but was not examined in this report.