Relation of Days Open and Days Dry to Lactation Milk and Fat Yields

The relationships between days open, days d~T, and 90-day and 305-day production were studied, using 4,385 lactation records from nine North Carolina Institutional Holstein herds from June, 1950, through December, 1958.Relationships were studied separately for first, second or later, and all lactations to determine if adjustment of 305-day lactation records for days open snd days dry are warranted in compiling sire summaries.The intra-herd-year-season phenotypic correlations between 90-day production and days open were not significant and ranged from 0.05 to 0.08, suggesting that level of production had very little influence on this meeasure of fertility.The heritability estimates for days open were also very low, ranging from 0.01 in first lactations to 0.09 for all records.Length of the previous dry period had very little influence on production, accounting for only 0.6 and 0.3% of the variation in 90-day and 305-day fat yield and less than 0,1% of the variation in the two measures of milk production.Number of days open during the lactation significantly influenced production and accounted for 6.5, 4.3, and 4.2% of the variation in 305-day milk yield for first, second or later, and all lactations, respectively.Factors for adjusting 305-day production for differences in days open are presented.