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Abstract
The objective of this study was to investigate environmental effects of conception rate and the trend over time in Holsteins using individual artificial insemination records. Data for cows born from 1988 to 2004 were obtained from Dairy Records Management Systems in Raleigh, NC. Records without calving or birth date, parity >1, days to service after calving <21 or >250, and without next calving date were eliminated. Conception rate was significantly affected by DIM, season, and milk production, and it varied by region. Conception rates increased with DIM, but there was a lag for high milk producing cows. Also, conception rates were negatively affected by heat stress in southeast USA. Conception rate can be improved by increasing the voluntary waiting period, especially for high producing cows. High producing cows in hot climates should be inseminated in spring. Conception rates decreased over the years while milk production increased.