Impact of Culling on Modified Contemporary Comparison Sire Evaluations

Sire evaluations by Modified Contemporary Comparison routinely are calculated separately for first and later lactations. The published proofs, based on all lactations, are a function of information from these two evaluations. Four data sets from daughters of 200 widely used Holstein sires were used to examine the impact of selection on Modified Contemporary Comparisons. Set 1 contained over 670,000 daughters with first and second records in the herd where the first was initiated. Sets 2 to 4 contained these same first records but only second records from the best 90, 80, and 70% of daughters based on Modified Contemporary Deviations for milk in first lactation. Means for Predicted Difference milk based on second records increased from 367 to 476, 573, and 670 kg as elimination of second records increased from none to 10, 20, and 30% with corresponding standard deviations of 274, 250, 238, and 228 kg. Correlations between Predicted Difference milk from first and second lactations declined as culling intensity increased (.85, .81, .77, and .73) for no, 10, 20, and 30% culling of seconds. Regression of second lactation milk proof on first declined with increased culling (.87, .75, .68, and .62). Adjustment for selection of second lactation proofs