Relations Between Sire Summaries of First and Second Lactations

Sire summaries of one or more progeny in first lactations in each of 20 or more herds were compared to progeny in second lactations of the same 133 Holstein bulls. Progeny in second lactations were also in 20 or more herds, but these herds were completely different from those having first lactation progeny. Since it is unlikely that any of the progeny were common to both "the first and second lactation summaries, associations between summaries should estimate genetic associations. Progeny were compared to either contemporaries in first or second lactation or herdmates of all ages. Correlations between summaries for first and second lactations for deviations of milk yield ranged from .56 to .62 compared to an expected correlation for deviations from herdmates of .66. Correlations were highest when progeny were deviated from herdrnates of all ages. A progeny test on first lactation milk is about 85% as accurate in predicting second lactation performance as a progeny test on second lactations. Progeny tests for milk yield from only progeny in first lactation are sufficient for most objectives of selection.