Production, Management, and the Environment 2

摘要

The objective of this study was to quantify the effect of dry period length(days dry, DD) in the current lactation on the risk of culling in the subsequentlactation. For this retrospective study, we obtained DHI test recordsof 1,287,659 Holstein cows from 5,155 herds with the last dry date in2014 or 2015 from DRMS, NC, USA. Three groups of adjacent lactationswere constructed: 1 and 2 (par12; n = 605,082), 2 and 3 (par23, n =362,169), and 3 to 6 (par36, n = 320,408). Included cows had between 21to 120 DD which were classified into 9 categories (shortest 21–30 DD tolongest 101–120 DD). Survival analysis was conducted with proc GLIMMIXin SAS with a 30-d piecewise exponential model with 9 DD categories,2 time-dependent variables (4 seasons (winter, spring, summer,fall) and 4 reproductive statuses (open, pregnant, bred ≤ 30 d, bred > 30d), and 5 covariates including total milk yield in the current lactation,milk yield on d 14 before dry-off, and DIM at dry-off. Next calvings,lactations in progress or cows sold for dairy purposes or reason-not-reportedwere censored. The model was run separately for each group for0 to 60 and 61 to 450 DIM after calving to quantify the effects of dryperiod length on culling in early and later lactation. Hazard ratios (HR)for each DD category were calculated with the HR of the 51–60 DD categoryset to 1. The HR for the 0–60 DIM ranged from 0.81 (21–30 DD)to 1.73 (101–120 DD) for par12, 0.77 (21–30 DD) to 1.64 (101–120 DD)for par23, and 0.74 (21–30 DD) to 1.52 (101–120 DD) for par36. TheHR for the 60–450 DIM were also generally greatest for long DD andsmallest for short DD, but were closer to 1, implying that differences inDD had greatest effects early in lactation. Models without time-dependentvariables and covariates showed greater HR for short DD (closer to 1)and greater HR for long DD (>2) compared with models that includedthese variables. In conclusion, a longer dry period length in the currentlactation increased risk of culling in the subsequent lactation and needsto be considered when optimizing dry period lengths for individual cows.