Infertility is one of the manifestations of clinical copper deficiency in dairy cattle that results from high dietary molybdenum intakes. This trial investigated the effects of slow release glass copper, cobalt and selenium boluses (Cosecure) on the number of inseminations required for confirmed pregnancy, calving interval and trace element status in dairy cattle. Sixty Holstein Friesian dairy cattle were allocated to one of two treatment groups, 30 cattle were treated with two soluble glass trace element boluses (Cosecure®,Telsol Ltd.) on day 0 and 169 while the remaining 30 cattle were left untreated (Controls). The cattle were grazed at pasture (day 0 to 165) which has been shown to contain high levels of molybdenum (range from 2.3 to 3.1 mg/kg DM). Blood samples were taken between days 0 and 280 to measure copper and selenium status and the number of inseminations and calving intervals were recorded. The control cattle required significantly more inseminations to confirmed pregnancy compared with the Cosecure treated cattle (2.5 v 1.7 SEmean 0.16, p<0.01) and had a significantly longer calving interval (397 v 372 days, SEmean 9.2, p<0.05). Plasma copper concentrations were significantly higher in the Cosecure treated cattle on days 30, 78 and 135 (p<0.01) compared with the controls, although the controls were all in the normal range (> 12 μmol/l). Cosecure treated cattle had significantly higher glutathione peroxidase (GSHPx) activities on day 246 (P.0.05). However, on all sample dates, the GSHPx activities were in the normal range for both treatment groups (>40units/ml PCV). The dietary cobalt intakes of the cows were measured (1.5 to 2.1 mg/kg DM in grass and 2.1 to 2.7 mg/Kg DM in compound) and found to be well in excess of the requirement of 0.2 mg/Kg DM. It was concluded that treatment with slow release glass copper, cobalt and selenium boluses (Cosecure) reduced the number of inseminations required for conception in the dairy cows due to the slowly released copper.