Predicting the likelihood of animal sodium deficiencies based on soil and herbage sodium concentrations : a dissertation submitted in partial fulfilment of the requirements for the degree of Bachelor of Agricultural Science with Honours at Lincoln University

Abstract

Pasture plants are broadly classified as either natrophiles that preferentially accumulate sodium (Na) in the leaves, and natrophobes, which store Na in the lower stems and roots. Na has not been proven essential for growth of most temperate pasture species, thus determining critical Na levels in plants is based on the requirements of grazing animals. Na requirement in animals is influenced greatly by lactation, with the minimum herbage Na concentration for ewes and cows recommended at 0.05 and 0.10% of dry matter respectively. Previous surveys have shown soil Na levels to be the highest bordering the coast, with the content decreasing further from the coast. Thus, inland high country soils are generally the lowest for soil Na. A field survey of common pasture species in Canterbury has demonstrated that plantain and chicory can accumulate up to 10 times the concentration of rye grass (1.0% Na vs. 0.10%) on high Na soils with a MAP Quick Test (QT) of 14. This effect was also shown on low Na soils (MAF QT 4), with the natrophile herbs accumulating approximately twice the Na concentration of rye grass and white clover (0.20% Na vs. 0.06 0.10%). Ryegrass did not accumulate very high Na concentrations on high Na soils (0.15%), but it provided adequate Na for lactating animals (>0.10%) when grown in to soils with Na QT values as low as 3. This is in contrast to white clover, which accumulated more Na (0.25%) than ryegrass on high Na soils. However, white clover is more sensitive to soil Na, and became deficient on soils with a QT of 6 or below. Lucerne is a natrophobe and was shown to be deficient (average less than 0.10%) for lactating animals irrespective of the soil Na status. Cocksfoot is a natrophile that will accumulate Na in the leaves if grown in high Na soils. However, this species is sensitive to soil Na, and is likely to be deficient if grown on soils of MAF QT 6 or less. A glasshouse pot experiment was carried out to assess the ability of ten pasture species to translocate Na to the leaves, when grown in either high (MAF QT 14) or low (MAF QT 3) Na soils. Values for Na were considerably higher than field values, with some results being uncharacteristic and difficult to explain. This study showed that the natrophiles ryegrass, plantain, chicory and lotus as well as the natrophobes tall fescue and browntop, all accumulated higher levels of Na in the leaves on high Na soils compared with low Na soils. White clover (natrophile) and lucerne (natrophobe) had about the same concentrations on both soils, whereas cocksfoot (natrophile) and rape (natrophobe) had higher concentrations of Na in the herbage when grown on low Na soils. This study has shown that the requirements for Na supplementation of grazing livestock maybe predicted by location in relation to distance from the sea, soil and herbage Na content, and the natrophilic or natrophobic characteristics of pasture species.