EFFECT OF COBALT APPLICATION ON THE COBALT STATUS OF PASTURES .1. PASTURES WITH HISTORY OF REGULAR COBALT APPLICATION

Pasture and soil cobalt (Co) concentrations were monitored for 8-9 years on six properties that had been developed in pasture for periods ranging from 15 to 24 years on yellow-brown pumice soils in the Rotorua-Taupo area, New Zealand. All sites had been topdressed annually with Co before the survey began in 1979, but from then the amount and frequency of Co applied was reduced considerably. Despite this reduction, pasture Co concentration remained adequate for grazing livestock and soil Co extracted with EDTA was maintained at satisfactory values throughout the survey period. Serum vitamin B12in lambs grazed on the sampled pastures confirmed that they all had adequate Co intake. In a field trial near Taupo, small plots of white clover, perennial ryegrass, and lucerne were established on a site previously in permanent pasture which had received 16 applications of Co. Ryegrass had the highest Co concentration, lucerne the lowest, and white clover intermediate but the outstanding result was that Co concentration remained high in ryegrass and white clover and there was no decrease for 12 years even where no further Co was applied. Conclusions are that pastures on yellow-brown pumice soils, where soil Co reserves have been increased by repeated annual Co application, can be maintained as Co-sufficient for livestock by much lower rates than the traditional 350 g/ha per year of cobalt sulphate. This work indicates that 175 g/ha applied every third year or 100 g/ha annually are satisfactory and that the accumulated soil Co reserves can last for 10 years or more. © 1990 Taylor & Francis Group, LLC.