Upland pasture improvement can induce sub-clinical cobalt deficiency in grazing animals, leading to lowered productivity. A trial was set up to investigate three methods of combating this problem, namely: monthly vitamin B12 injections, administration of a slow-release bolus at the start of the grazing season and treatment of the pasture with cobalt sulphate.

A recently improved permanent grassland of 5 ha was split in two and one half treated with cobalt sulphate at 2 kg/ha. During two summers, 15 Blackface ewes with twin lambs grazed each pasture. One lamb from each pair on the untreated pasture was treated by monthly 1000 µg vitamin B12 injections in the first year, or by the administration of slow-release bolus in the second. Monthly measurements were made of liveweight, serum vitamin B12 and herbage Co.

Cobalt application to the pasture effected an immediate increase in herbage Co and maintained higher values for the whole of the first grazing season. The high value found one month after application would be due to leaf adherence.