Dry period length in US Jerseys: characterization and effects on performance.

The objectives of this research were to characterize dry period lengths for US Jerseys, determine the effects of days dry (DD) on subsequent lactation actual milk, fat, and protein yields, fat and protein percentages, somatic cell score (SCS), and days open (DO), and to determine the dry period length that maximizes yield across lactations. Field data, collected through the Dairy Herd Improvement Association, on US Jersey cows first calving between January 1997 and November 2004 were used. Characterization of DD included a frequency distribution of dry period lengths as well as factors affecting US Jersey DD. Of the factors considered in this research, the primary ones affecting dry period length were DO, milk yield, and SCS. Cows with longer DO, lower milk yield, and higher SCS received longer dry periods. The model for analyses included herd-year of calving, year-state-month of calving, parity of calving, previous lactation record, age at calving, and DD as a categorical variable; records were preadjusted for cow effects. A total of 123,032 records from 73,797 cows in 808 herds were used for estimation of DD effects on subsequent lactation actual milk yield. Jersey milk, fat, and protein yields in the subsequent lactation were maximized with 61 to 65 DD. Dry periods of 30 d or fewer resulted in large reductions in subsequent lactation production. A short dry period was beneficial for fat and protein percentages in the subsequent lactation. Short dry periods also resulted in fewer DO in the subsequent lactation; however, this was entirely due to the lower milk yield associated with shortened dry periods. The biggest difference between Jerseys and Holsteins was a much larger detrimental effect on SCS in Jerseys for dry periods of 30 d or less. Jersey SCS increased 10%, relative to the overall mean, for dry periods of 20 d or less and 4.6% for DD between 21 and 30 d. Dry periods of 45 to 70 d maximized yields across adjacent lactations. A dry period length, after first lactation, of 45 to 70 d also maximized actual milk yield across lactations 1, 2, and 3. The final recommendation to Jersey producers is to avoid dry periods of <45 d. Long dry periods (>70 d) should also be avoided because these are even more costly to total yield than dry periods <30 d.