Relationship of yield during early lactation and days open during current lactation with 305-day yield

To measure and to partition the effect of pregnancy on yield, the relationships among milk, fat, and protein yields during early lactation, current days open, and 305-d yields were investigated using sample day records of 247,310 Holstein cows. The model included fixed effects of calving herd-year-season, calving age, and days open; the continuous variable of early cumulative yield to 80, 100, 120, or 140 d; and a random residual effect. As days open during first lactation increased from 30 to 100 d, 305-d milk yield increased by 876 kg; as days open increased from 100 to 200 d, milk yield increased by only 172 kg. The impact of current days open was greater on second lactation than on first; the difference in 305-d milk yield between cows open 40 and 290 d was 1199 kg for first lactation and 1613 kg for second lactation. If early yield to 120 d was included in the model, the corresponding difference was reduced to 860 kg for first lactation and 1001 kg for second lactation. Inclusion of early yield in the model reduced regression coefficients for days open during first lactation by 22% for 80-d yield, 24% for 100-d yield, 27% for 120-d yield, and 30% for 140-d yield and by 31, 35, 38, and 41%, respectively, for second lactation. Statistical models to derive adjustment factors should account for early lactation yield so that those factors can remove effects of pregnancy but not correlations between yield and fertility caused by early yield.