The National Animal Health Monitoring Systems (NAHMS) Dairy +96 Study of dairy herds in 20 states examined herd management practices that varied between high-producing (top 25%) and low-producing (bottom 25%) U.S. dairy herds. Only 100% Holstein herds were included in the study. Within four size categories (30-99 cows, 100-199 cows, 200-499 cows, 500 or more cows) herds were ranked by milk production per cow. High-producing herds were more likely to use Dairy Herd Improvement Association records or an on-farm computer system than low-producing herds. High-producing herds were also more likely to feed a total mixed ration, use forage test results in balancing feed rations, use bovine somatotropin (bST) and/or systematic prostaglandins, and vaccinate against a larger number of diseases than were low-producing herds. Additionally, high-producing herds more frequently culled healthy cows using a break-even milk production level than low-producing herds and, on a per-cow basis, more frequently used the services of veterinarians than did low producers. High producers were more likely to separate calves at birth before nursing, feed four or more quarts of colostrum than low producers, and supplement colostrum delivery with hand-feeding. Producers who used pastures were more likely to be low-producing than high-producing. Specific practices varied with herd size. Vaccination and the use of bST and systematic prostaglandins increased with herd size; the use of producer-grown grains for feeding cows and the use of veterinary services on a per-cow basis decreased with herd size. Average milk production per cow increased with herd size. Contact for this paper: Stephen Ott