Associations between dry period length and time to culling and pregnancy in the subsequent lactation

摘要

The association between dry period length (DPL) andtime to culling and pregnancy in the subsequent lactationmay be important for the economically optimallength of the dry period. Therefore, this study aimed to(1) quantify the association between DPL and hazardof culling and pregnancy in the subsequent lactation;(2) develop continuous functions of DPL for the hazardratios of culling and pregnancy; and (3) investigate theeffect of a cause-specific hazards model and a subdistributionmodel to analyze competing events. The dataused in this observational cohort study were from dairyherd improvement milk test lactation records from 40states in the United States. After edits, there remained1,108,515 records from 6,730 herds with the last daysdry in 2014 or 2015. The records from 2 adjacent lactations(current, subsequent) were concatenated with theDPL of interest, 21 to 100 d, in between both lactations.We defined 8 DPL categories of 10 d each. Kaplan-Meier survival curves were used to show associationsbetween DPL and time to culling or pregnancy for 3lactation groups: lactation 1 and 2, lactation 2 and 3,and lactation 3 and greater. To control for confoundingfactors in Cox proportional models, we included6 current lactation covariates and 3 time-dependentvariables in the survival models. Hazard ratios of cullingwere estimated for 4 days in milk (DIM) categoriesfrom 1 to 450 DIM. Hazard ratios of pregnancy wereestimated for 3 DIM categories from 61 to 300 DIM.Competing risk analysis of 8 disposal codes (i.e., farmerreported reasons) for culling and the culling event forpregnancy were conducted by a cause-specific hazardsmodel and a subdistribution model. Hazard ratios werealso estimated as quadratic polynomials of DPL. Comparedwith the reference DPL category of 51 to 60 d,hazard ratios of culling and pregnancy of the other 7DPL categories ranged between 0.70 and 1.49, and 0.93and 1.15, respectively. Short DPL were associated withlower risk of culling in the early lactation but not overthe entire lactation. Short DPL were associated withgreater hazard of pregnancy. Trends in hazard ratiosover the ranges of the 8 DPL categories were not alwaysconsistent. Competing risk analysis with both modelsprovided little differences in hazard ratios of cullingand pregnancy. In conclusion, variations in DPL wereassociated with meaningful differences in the hazardratios for culling and pregnancy and minor differencesin the relative frequency of disposal codes. Subdistributionhazards models produced hazard ratios similar tocause-specific hazard models. The quadratic polynomialsmay be useful for decision support on customizationof DPL for individual cows.