Sodium deficiency in Canterbury and Central Otago sheep pastures

Abstract

Sheep diets are likely to be deficient in sodium (Na) if pasture contains less than 0.04% Na (lambs and maintenance) or 0.05% Na for ewe flushing and lactation. Herbage from four species was sampled in autumn 2002 from 12 sites on a transect south of the Rakaia River from the coast to 110 km inland. All species had >0.10 % Na near the coast but browntop and cocksfoot declined to < 0.10% Na at 10 km and 20 km inland respectively. White clover and ryegrass declined to about 0.10% Na at 40 km and 80 km respectively. In autumn 2003, soil and herbage samples were collected from 8 farms over a range of distances from the coast. Soil quick test (QT) Na from the 8 farms had a strong negative correlation with distance from the sea. Soil QT Na was <5 at 40 km inland and declined to 2 at Mt Grand in the Upper Clutha basin 150 km from the east coast. The herbs, plantain and chicory, had highest concentrations of Na at both high and low soil QT Na. Ryegrass and white clover had moderate Na content at high soil Na but ryegrass appeared to be less sensitive to low soil Na. Although cocksfoot is classified as a natrophile and it had high herbage Na% at high soil Na, it was sensitive to low soil Na. The very low soil QT Na of 2 at Mt Grand resulted in all species except plantain and chicory having <0.04% Na in their herbage. Merino ewes and lambs, which were present when coarse salt (NaCl) was spread at rates ranging from 50 to 300 kg/ha, were strongly attracted to +NaCl plots on south-facing Mt Grand pastures. In two days the sheep grazing and trampling increased bare ground from 25% before salt was spread to between 40 and 63%. It was concluded that natrophobic species grown on inland soils with soil QT Na <5 are likely to be deficient for sheep; all species except plantain and chicory will produce deficient herbage (<0.04%) if soil QT Na is 2 or less and lactating Merino ewes grazing Na deficient pasture will be strongly attracted to areas where coarse salt is spread. The increase in % bare ground resulting from sheep seeking salt may provide a tool for enhancing the establishment of pasture seeds broadcast on to hill country.