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Effect of dry period length on risk of culling in the subsequent lactation.

A. De Vries

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06-23-2020

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Abstract:

T71
Effect of dry period length on risk of culling in the subsequent lactation.
P. Pattamanont1, M. I. Marcondes2, A. De Vries*1. 1University of Florida Gainesville, FL, 2Federal University of Vicosa Vicosa, MG, Brazil.

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 subsequent lactation. For this retrospective study, we obtained DHI test records of 1,287,659 Holstein cows from 5,155 herds with the last dry date in 2014 or 2015 from DRMS, NC, USA. Three groups of adjacent lactations were 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 21 to 120 DD which were classified into 9 categories (shortest 21—30 DD to longest 101—120 DD). Survival analysis was conducted with proc GLIMMIX in 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 > 30 d), 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-reported were censored. The model was run separately for each group for 0 to 60 and 61 to 450 DIM after calving to quantify the effects of dry period 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 category set 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. The HR for the 60—450 DIM were also generally greatest for long DD and smallest for short DD, but were closer to 1, implying that differences in DD had greatest effects early in lactation. Models without time-dependent variables and covariates showed greater HR for short DD (closer to 1) and greater HR for long DD (>2) compared with models that included these variables. In conclusion, a longer dry period length in the current lactation increased risk of culling in the subsequent lactation and needs to be considered when optimizing dry period lengths for individual cows.

Keywords: dry period, culling, survival.