Do biological and management reasons for a short or long dry period induce the same effects on dairy cattle productivity?

The dry period is a well-established factor that determines lactation success. A retrospective observational study used 32,182 lactations from 16 farms to determine whether management versus biological reasons for deviations from the targeted 60-d dry period have the same associations with subsequent lactation performance. Herd inclusion criteria were Holstein cows, herd she >= 900 cows, breeding by artificial insemination, and (minimally) bimonthly milk testing. Dry period length (DPL) and gestation length (GL) were each categorized as short [>1 standard deviation (SD) below mean within herd; means 45 d DPL, 269 d GL] or long (>1 SD above mean within herd; means 73 d DPL, 284 d GL) and combined to generate the following 7 study groups: short DPL, short GL (S(D)A(G), n = 2,123); short DPL, average CL (S(D)A(G), n = 1,418); average DPL, short GL (A(D)A(G), n = 1,759); average DPL, average GL (A(D)A(G) , n - 19,265); average DPL, long GL (A(D)A(G), n = 3,325); long DPL, average GL (L(D)A(G), n = 2,573); and long DPL, long CL (LDLG, n = 1,719). Responses evaluated included milk and component yields at first test and over the whole lactation, days to first service, first service conception risk, days open, and herd retention through 60 and 365 d postpartum. Continuous data were analyzed by mixed models and time to event data by Cox proportional hazard models, both accounting for clustering at the herd level. First test and whole-lactation milk arid component yields were lowest for SDSG. Within cows that experienced calving difficulty, rates of receiving first service were 13 and 20% less for SDSG and A(D)S(G) compared with A(D)A(G). Hazard of leaving the herd by 60 d in milk (DIM) was 34% greater for A(D)S(G) than A(D)A(G). Similar effects between SDSG and ADSG but not S(D)A(G) indicated that short GL was a greater contributor to poor performance than A(D)A(G) and S(D)A(G); however, somatic cell linear score at first test was greater for S(D)A(G), and milk yield at first test was lesser for S(D)A(G) cows with greater milk at last test before dry-off. Although short DPL might be a successful strategy for some herds or cows, cows with high milk yield at dry-off should not be subjected to a short dry period. Long DPL or GL did not influence early-lactation or whole-lactation milk yield. Cows with a long DPL due to early dry-off (L(D)A(G)) likely experienced issues related to excessive lipid mobilization, as milk fat concentration and fat:protein ratio at first test were greater and hazard of leaving the herd was 30 and 24% greater compared with A(D)A(G) by 60 and 365 DIM, respectively. We conclude that deviations in DPL length caused by biology (short GL) were associated with greater effects than management causes of short DPL, whereas management reasons for long DPL were associated with more negative outcomes tlian long CL.