Age and seasonal effects on Holstein yield for four regions of the United States over time.

Multiplicative factors to adjust Holstein yield for age and season of calving were calculated and analyzed from 20 data files for region and time. Regions were 1) California, 2) Minnesota and Wisconsin, 3) New York and Pennsylvania, and 4) six southern states (Texas, Louisiana, Mississippi, Alabama, Georgia, and Florida). Time periods were calving years of 1964 to 1968, 1971 to 1975, 1976 to 1980, 1981 to 1985, and 1986 to 1990. The same statistical model was used that had been used to derive current USDA factors from 1964 to 1968 data. The number of lactation records for each data file ranged from 81,394 to 2,238,201 and increased with time. Effect of calving season on milk yield generally decreased with time; the largest decrease was for the southern region. Effect of calving age also generally decreased for recent data, and the largest decreases were for California. Updated factors to adjust yield for calving age and season are indicated for some regions.