Characterization of days dry for United States Holsteins.

This research characterized days dry (DD) for modern US Holsteins. This included investigation of factors influencing DD, summarizing variation in herd means as well as within-herd variation, and estimation of parameters related to DD. Data consisted of records initiated before 2003 from cows first calving after 1996. A total of 459,075 records from 295,067 cows in 3527 herds were included. The overall mean DD was 60.5 d, and standard deviation among herd means was 5.8 d. Approximately 91% of all herds had mean DD between 50 and 70 d. There were 41 herds that had mean DD of < or =45 d and 2 herds with mean DD of < or =30 d. Some herds (125) had unusually long mean DD (exceeding 71 d). Variation across regions of the United States was minimal, although southwestern herds did show somewhat longer DD than other regions. Days open (DO) accounted for the most variation in DD with longer DO leading to longer dry periods. Heritability of DD was 7% and repeatability 12%. Error correlations indicated that longer dry periods benefit both milk yield and somatic cell score in the subsequent lactation. Genetic and permanent environmental correlations were large, which indicated that 1) bias would result if genetic evaluations for milk yield included an adjustment for DD, and 2) correction for cow effects is warranted in estimation of DD effects on performance. The relationship between DD and DO is due primarily to permanent cow effects. Improved fertility will enhance the feasibility of shortened dry periods.