Technical Abstract: A genotype x zinc supplementation interaction study was conducted involving 85 primiparous heifers. Heifers were from three genotypes [xbred composite (XCP), Hereford (H) or Charolais (Ch) sired] bred AI to three sire breeds [H, Limousin (L) or Piedmontese (P)]. The dry lot diet was 85% corn silage, 10% alfalfa hay and 5% barley with (ZN) or without (NZN) 170 mg daily supplemental zinc fed the last trimester of gestation. Precalving data were obtained 10 ñ 4 d prior to the predicted calving date. Dystocia was recorded 0=no assistance to 4=major difficulty. Dams and calves were weighed and blood sampled within 48 hr postpartum; a calf blood sample was obtained in mid May when calves were approximately 6 wk old. Dam precalving body weight (P<.05) and pelvic area (P<.05) averages were: 469 and 260; 454 and 247; 582 kg and 308 sq. cm. for XCP, H and Ch dams, respectively. Effects of ZN on dystocia were not significant. Birth weight was affected by ZN x sire breed (P<.01; H, 38.8 vs 35.4; L, 35.6 vs 36.4; P, 38.8 vs 38.8 kg, NZN vs ZN, respectively) and ZN x dam breed (P<.01; XCP, 35.5 vs 35.6; H, 36.8 vs 36.8; Ch, 41.2 vs 38.1 kg, NZN vs ZN, respectively). Plasma zinc of calves at birth was affected by dam breed x ZN treatment interaction (P<.05): .79 vs .93, .75 vs .87 and .86 vs .80 ppm for XCP, H and Ch dams, NZN vs ZN, respectively. At 6 wks of age, plasma zinc concentrations of calves suckling dams receiving ZN during gestation (1.23 ppm) exceeded those of calves suckling control dams (1.12 ppm; P<.05). We conclude zinc requirements may be breed dependent and zinc supplementation during gestation affects plasma zinc of the calf directly in utero plus transfer in the milk postnatally.