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Changes in Predicted Difference Milk with Increased Repeatability

Animal Improvement Programs Laboratory, Animal Science Institute, Agricultural Research Science and Education Administration, US Department of Agriculture, Beltsville, MD 20705

ABSTRACT

All sire evaluations for 3,733 Holstein bulls initially sampled through artificial insemination were studied for changes in Predicted Difference milk as evaluations increased in Repeatability. Individual bulls contributed from 2 to 15 pairs of earlier and later evaluations. Average change in Predicted Difference milk was 5 kg. Change in Predicted Difference was correlated positively with change in records per daughter. Predicted Difference increased by 5 kg of milk for each .1 increase in numbers of records per daughter. Thus, Predicted Difference tended to increase with later records of daughters but to decrease when added records were for new daughters. Predicted Differences tended to increase as Repeatability increased to 70 to 79% and then to decrease as Repeatability continued to increase. Minor misevaluation may be from genetic bases for first and later lactations not being comparable. Average absolute changes in Predicted Difference were larger for larger changes in Repeatability. Variability of pairs of sire proofs generally agreed with changes expected from a part-whole formula.