Evaluation of production per day of life calculated on cull cows as a comprehensive monitor of dairy herd performance

The primary objective of this study was to determine the contribution of several management variables to average production per day of life (PPDL). A secondary objective was to compare PPDL between two geographic regions in North America and between dairy herds within the two regions. PPDL was determined for animals culled from 4 'large' dairy farms in Florida, USA (FL), and 8 'small' dairy farms in Prince Edward Island, Canada (PEI). Average milk PPDL was calculated by summing the kilograms of milk produced per cow and dividing by the total number of days from birth until culling. PPDL for the 4 FL herds (9.50 kg/day) was significantly higher than PPDL for the 8 PEI herds (8.64 kg/day). PPDL increased in a quadratic fashion with number of lactations. PPDL did not differ between the 8 PEI herds. Significant differences in PPDL were identified between the 4 FL dairy farms. The regression of PPDL on average lactational milk yield (AVGFCM), number of lactations (LACT), age at first calving and average days to conception explained 94.5% and 92.4% of the variation of PPDL for multiparous cows in the PEI and FL herds, respectively. One variable (AVGFCM) accounted for 68% and 55% of the variance in PPDL for PEI and FL, respectively. The additions of LACT to the model explained 87% and 81% of the variance in PPDL for PEI and FL, respectively..