Dairy Herd Improvement testing records of 201 dairy herds of sizes from under 100 to over 1000 cows and herd average milk yields from under 5,000 to over 10,000 kg were studied. Average days to first postpartum breeding tended to be less in herds of over 500 cows. Herds of 300 to 600 cows had highest production per cow. Herds with higher average yields averaged shorter intervals to first postpartum breeding and fewer days open. Days open included farrow cows arbitrarily assigned 305 days open. Number of breedings changed little as herd yield increased, but days open for highest producing herds averaged one estrous period shorter than for low producing herds, suggesting better detection of estrus. For the individual cow, high yield or associated factors have a small but real antagonistic association with reproductive efficiency. Days to first breeding, to last breeding, and days open increased .27, .80, and .61, and number of breedings increased .014 for each 100 kg increase in 180-day yield of fat-corrected milk. The record averages of high-producing herds indicate this antagonism may be overshawdowed by good management; effective estrus detection probably is a major factor.