SODIUM FERTILIZER APPLICATION TO PASTURE .1. DIRECT AND RESIDUAL EFFECTS ON PASTURE PRODUCTION AND COMPOSITION

Results are given on the effect of sodium (Na) fertilizer applied as NaNO3 in five applications at 0(Nil), 32 (Low) or 64 (High) kg Na ha-1 yr-1 and the residual effect of sodium chloride (NaCl) fertilizer applied in the previous year on the growth rate and the chemical composition of herbage cut at fortnightly intervals over the grazing season. Na fertilizer at Low and High rates increased herbage Na contents by 20 and 40% respectively. Magnesium (Mg) content in herbage was increased at Low Na fertilizer application rate but there was no difference between the Nil and the High Na treatments. Calcium (Ca) content was increased in proportion to Na fertilizer level. Potassium (K) content of Nil Na herbage exceeded that of Low and High, but there was no effect of treatment on total harvested K yields. The contents of phosphorus (P), sulphur (S), true protein (TP) and modified acid detergent fibre (MADF) in herbage were not affected, but the dry matter digestibility (DMD) was increased with increasing Na fertilizer level. Applying Na fertilizer at the Low and High levels gave similar increases in water-soluble carbohydrates (WSC) and ash contents and decreased crude protein (CP) and non-protein nitrogen (NPN) contents. Net herbage accumulation (fresh and dry weight) was increased by 15 and 26% at the Low and High Na fertilizer application rates respectively. NaCl fertilizer residues from the previous year increased herbage Na, DM, DMD and WSC contents, reduced K contents and did not affect Ca, P, S, CP, TP and NPN contents. Residues from Na fertilizer decreased MADF and ash contents at the Low and High Na application rates. The main and residual effects of Na fertilizer were generally maximal in early season and diminished as the season progressed, with many of the effects eliminated by late season. Proportionally 0.70 of Na applied was recovered in the first two grazing seasons after application. The effects of Na fertilizer on mineral ratios are also described, together with the implications for animal health.