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item Kuhn, Melvin
item Hutchison, Jana
item Norman, H

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/9/2004
Publication Date: 3/1/2005
Citation: Kuhn, M.T., Hutchison, J.L., Norman, H.D. 2005. Characterization of days dry for United States Holsteins. Journal of Dairy Science. 88(3):1147-1155.

Interpretive Summary: Factors that affect length of dry period (the period between lactations when a cow is not being milked) were examined. Herd and regional averages and genetic parameters related to days dry were calculated with data from a national database. Herds averaged 61 d dry, with minimal variation among herd averages. Heritability of days dry was estimated at 7% and repeatability at 12%. This research can be useful in determining appropriate models for field data or factors to control in designed studies. Dairy producers can also benefit by knowing the magnitude and extent that various factors play in decisions for dry off.

Technical Abstract: This research characterized days dry (DD) for modern day 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. Herds were required to be on test during the entire period. Strict edits were applied to ensure quality of data. A total of 459,075 records from 295,067 cows in 3527 herds were included. 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. Although most herds were maintaining the traditional 60 DD, some were utilizing shorter dry periods. There were 41 herds that had mean DD of <=45 d and 2 herds with mean DD of <=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 SCS 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.