A lactation measure of somatic cell count.

Adjustments and weights to combine individual sample day SCC into a lactation measure were developed from 182,446 Holstein lactation records completed in 1984 from herds primarily in the western United States. Data included 1,660,782 sample day SCC expressed as log base 2 scores. Stage of lactation effects were investigated within average lactation score. Curves for lactations with averages less than 3.5 were essentially identical, which indicated that effect of stage of lactation did not increase with average somatic cell score. Lactations with averages above 4.5 did not display the typical curve of an early minimum followed by a gradual increase and, therefore, were excluded from estimating effects of stage of lactation and season. Curve for lactations with an average somatic cell score of 4.0 was intermediate. Small season effects were detected; scores were lowest in February and March and highest in August, September, and October. Parties after first had similar curves for effect of stage of lactation, but first parity showed relatively little increase in late lactation. Weightings were developed to combine adjusted sample day scores into a lactation measure. Weights were squares of correlations (computed from sample days grouped by month) between adjusted score for a given sample day and average of adjusted scores from other sample days. Weights were lowest at start of lactation, highest in midlactation, and intermediate at end of lactation.