RELATIONSHIP BETWEEN PRODUCTION AND DAYS OPEN AT DIFFERENT LEVELS OF HERD PRODUCTION

Five production variables and days open were analyzed using 611,680 re cords from 348,243 cows in 5694 herds enrolled in the Wisconsin DHI pr ogram. Production variables included 305-d milk production and several production measures adjusted for combinations of mature equivalent, f at and protein content, and effects of days open. Herds were divided i nto four groups by herd production. Heritability estimates for product ion variables ranged from .27 for mature equivalent milk that was corr ected for fat and protein content and adjusted for days open to .34 fo r mature equivalent milk and for mature equivalent milk that was adjus ted for days open. Adjustment of production records for days open had little impact on heritability estimates of production traits. Heritabi lity for days open was approximately .05. Heritability estimates were larger for all measures of production and for days open for the herds with higher mean production. The effects of parity and season were imp ortant for both adjusted and unadjusted measures of production. After production variables were adjusted for mature equivalent factors, larg e differences remained between REML estimates of fixed effects of pari ty and season. For days open, estimates were larger for later parity c ows and were highest for cows calving during spring. For measures of p roduction, estimates were largest between first and second parity cows and between winter and summer calvings. Within herd, days open were l onger for cows with higher production. Regressions of days open on pro duction records that were not adjusted for days open were significantl y larger for the herds at lower production.