Effects of previous days open, previous days dry, and present days open on lactation yield.

Influences of days open present lactation, days open previous lactation, and days dry previous lactation were fit simultaneously to determine their effects on measures of yield, which were fat-corrected milk, milk, and milk fat, all adjusted to a 305-d mature equivalent basis. Best linear unbiased estimates were obtained. Multiparity analyses were conducted using a model in which later parity records could be compared with more unselected first parity records. Additional parity information made little difference in the influence of previous days dry and previous and present days open on yield. As present days open increased from 20 to 300 d, lactation yields for FCM, milk, and milk fat increased approximately 1250, 1350, and 45 kg. As previous days open increased from 20 to 300 d, lactation yields for FCM, milk, and milk fat increased approximately 625, 650, and 25 kg. Cows dry 60 to 69 d gave the most milk the following lactation. Cows dry less than 40 d produced much less milk the next lactation. Heritability estimates for previous days dry were less than 7%. Multiplicative adjustment factors were developed to adjust lactation yield records for the largely environmental effects of days open and days dry.