Submitted to: Journal of Dairy Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: August 8, 2005
Publication Date: May 1, 2006
Citation: Kuhn, M.T., Hutchison, J.L., Norman, H.D. 2006. Effects of length of dry period on yields of milk fat and protein, fertility and milk somatic cell score in the subsequent lactation of dairy cows. Journal of Dairy Research. 73(2):154-162. Interpretive Summary: Determining the effect of days dry (the number of days between lactations when a cow is not being milked) on subsequent lactation records was the objective in this study. A dry period length of 60 to 65 d maximizes subsequent lactational fat and protein yield; however, a short dry period is favorable to fat and protein %. Fertility and udder health benefit from a 60-d dry period compared to short dry periods. This research is useful not only on a farm level, but also throughout the industry because there is very little research on the effects of days dry on other economically important traits.
Technical Abstract: The objective of this research was to utilize data from US dairy cattle to determine the effect of DD on fat and protein yield, fat and protein %, DO, and SCS in the subsequent lactation. Field data collected through DHI from January, 1997 to December, 2003 and extracted from the Animal Improvement Programs Laboratory national database were used for analysis. Actual lactation records calculated from test-day yields using the test-interval method were used in this study. The model for analyses included herd-year of calving, year-state-month of calving, previous lactation record, age at calving, and days dry as a categorical variable. Fat and protein yield is maximized in the subsequent lactation with a 60-d dry period. Furthermore, dry periods of 20 d or less result in substantial losses in fat and protein yield in the subsequent lactation. On the other hand, a short dry period is beneficial for fat and protein %. Short dry periods also result in fewer days open in the subsequent lactation; however, this is entirely due to the lower milk yield associated with shortened dry period. When adjusted for milk yield, short dry periods actually result in poorer fertility in the subsequent lactation. Long days dry benefit SCS in the subsequent lactation. Herds with mastitis problems should be cautious in considering shortened days dry because short dry periods leads to higher cell scores in the subsequent lactation, compared to 60 d dry.