Monitoring cobalt status - experiences in the central North Island

The requirement for ‘cobalt (Co) of ruminants grazing pastures on the pumice soils of the Central Plateau was established in the late 1930s and Co topdressing to increase the Co intake of animals became commonplace from the mid-1950s. An on-farm survey in 1978-79 indicated that Co inputs could be halved because of adequate reserves of soil Co accumulated since the 1950s. However, the economic downturn in agriculture of the 1980s led to marked reductions in Co use. Follow-up surveys between 1991-93 indicated a general decline in soil and pasture Co levels. This decline was much more pronounced in areas with a poor history of Co topdressing. The 1991-93 survey results indicated a relationship between soil Co and pasture Co, Using probability analysis it was possible to indicate the soil Co level required to maintain a particular pasture Co concentration. For instance, for an 80% probability of achieving 0.08 mg/kg DM in the pasture (the critical level for sheep) a soil Co level of 1.7 and 2.2 mg/kg DM is required for the yellow-brown pumice soils and ~.yellow-brown.loams.respectively..F!urther-surveysare required in other areas to indicate soil Co levels for other soil groups. The 1991-93 surveys have indicated a need to increase the Co level of other soils to prevent Co deficiency in grazing animals. The evidence indicates that a regular Co input is required to build up soil reserves. This input needs to be of the order of 350 g CoSO+.7 H,O/ha/year for 7-10 years on the most deficient areas. The surveys also highlighted the farm by farm variation in their Co status which can exist in a district and hence the need for individual farmers to monitor their soil, pasture and animal Co status. Other factors such as feed quality and grazing strategy can also be important.