Sources of variation of dry period length in Italian Holstein cows

The magnitude and the effects of several sources of variation of dry period length (DPL) were investigated using field data recorded in 101 herds located in northern Italy throughout 4 years of study. Dry period length (69 ± 33 d), available for over 11000 lactations of Italian Holstein cows, was analyzed according to a model accounting for the effects of herd, year and season of dry-off and data of previous lactation such as calving season, parity, lactation length, cumulated milk yield at 100 DIM, occurrence of mastits and lameness, daily milk yield near dry-off and several interactions. R2 of the model was 21% and most effects were highly significant (P<0.01). Herd effect alone explained over 11% of dry days variation, and LS means for dry days in different herds ranged bewteen 48 and 118 d. DPL increased at increasing parity number and was highest for previous lactation shorter than 280 d. Milk yield potential after 100 DIM of lactation slightly affected dry-days, but DPL significantly decreased at increasing daily milk yield near dry-off. The occurrence of mastitis or lameness during previous lactation did not affect DPL. Similarly, DPL was not affected by BCS at previous calving, but tended to increase at increasing condition score near dry-off.