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Four experiments described determined the effect of copper and cobalt therapy on the growth and productivity of cattle on the marine plains of central coastal Queensland. Copper was administered by subcutaneous injections of copper glycinate, and cobalt by dosing per os with heavy cobalt pellets. The growth of weaned cattle was significantly improved by copper, particularly from June to October when limited palatable feed on the high ground forced the animals to forage on the para grass swamps. During the same period, 2-year-old heifers also showed a growth response to copper. Their conception rate increased after 19 months of copper therapy but not after 10.5 months. The growth rate of their calves bas significantly increased by copper supplementation. Liver copper concentrations were always low in untreated cattle. Copper therapy maintained these reserves at higher levels, which varied according to the season and the rate of growth of the animals. Calves born to treated cows had higher initial liver copper reserves than those from untreated cows, but in the absence of copper therapy these reserves declined to low and comparable levels in all calves at weaning. Pasture analyses suggest that the copper deficiency revealed was due to interference with copper metabolism rather than to a low copper status in the diet; this interference did not appear to be due to molybdenum. Weaned cattle appeared to respond to cobalt during 1960 but not subsequently, while the cows and calves showed no response. The vitamin B12 status in liver and serum appeared adequate in both treated and untreated cattle.