Relationship between production and days open at different levels of herd production.

Five production variables and days open were analyzed using 611,680 records from 348,243 cows in 5694 herds enrolled in the Wisconsin DHI program. Production variables included 305-d milk production and several production measures adjusted for combinations of mature equivalent, fat and protein content, and effects of days open. Herds were divided into four groups by herd production. Heritability estimates for production variables ranged from .27 for mature equivalent milk that was corrected for fat and protein content and adjusted for days open to .34 for mature equivalent milk and for mature equivalent milk that was adjusted for days open. Adjustment of production records for days open had little impact on heritability estimates of production traits. Heritability 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 important for both adjusted and unadjusted measures of production. After production variables were adjusted for mature equivalent factors, large differences remained between REML estimates of fixed effects of parity and season. For days open, estimates were larger for later parity cows and were highest for cows calving during spring. For measures of production, estimates were largest between first and second parity cows and between winter and summer calvings. Within herd, days open were longer for cows with higher production. Regressions of days open on production records that were not adjusted for days open were significantly larger for the herds at lower production.