A total of 187 animals (mainly Friesian dairy heifers) on nine farms in Wharfedale, Nidderdale, Calderdale and the Vale of York were weighed, condition scored and blood sampled on three occasions during the grazing season 1977. On each site, half of the animals each received two injections of a Cu preparation (200 mg Cu total), one in May and one in July. Herbage samples were taken in May, July and September.

Despite the fact that many animals started the experiment with low blood total Cu concentrations, growth rates during the experimental period were all satisfactory (lowest mean value 0·70 kg/day) with a significant increase due to treatment being observed on only one site. The Cu treatment, however, had a significant effect on blood and plasma total Cu values with the treatment either increasing the level to normal in animals which had started with low values, or maintaining the value in animals starting with near normal concentrations. On certain sites, animals in the untreated group ended the experiment with very low blood and plasma total Cu values (0·02–0·03 mg/100 ml). To prevent the possibility of a Co deficiency on some sites interfering with the experiment, all animals received a Co bullet initially. Despite these, concentrations of vitamin B12 in serum samples taken during and at the end of the experiment were low by present interpretation. This suggests either that Co bullets were an ineffective treatment or that the present interpretation of serum vitamin B12 concentration in bovines is incorrect.

The calculated concentrations of available Cu estimated from herbage analysis indicated that on most sites the value was below the optimum for the growing cattle under study. The results of the experiment suggest that even when blood Cu values and herbage available Cu values are low a growth response to Cu injections may not occur over the course of a grazing season.