EFFECTS OF SODIUM AND POTASSIUM FERTILIZERS ON THE COMPOSITION OF HERBAGE AND ITS ACCEPTABILITY TO DAIRY-COWS

Fertilisation of herbage with Na can increase acceptability to cows, b ut the influence of fertiliser rate and fertilisation by K is unknown. In experiment 1, ten cows were grazed on pasture plots that had just been fertilised with 0-132 kg-Na ha(-1) (current Na) and had received 0-64 kg-Na ha(-1) in the previous grazing season (residual Na). Herbag e Na concentration increased in proportion to current Na from 2.7 to 4 .9 g-Na kg(-1) dry matter (DM) and also increased with increasing resi dual Na from 2.2 to 4.5 g-Na kg(-1) DM. Herbage K concentrations were low (10 g kg(-1) DM at 0 kg-Na ha(-1)) and were only slightly reduced by Na fertiliser. Herbage Mg and Ca concentrations and DM digestibilit y were maximum at 66-99 kg-current-Na ha(-1). Cows grazed current-Na-f ertilised plots to a lower height and spent more time grazing them. In experiment 2, pasture plots received no fertiliser, low and high isom olar and independent applications of Na and K or a combination of the two. The herbage was more mature than in experiment 1 and Na concentra tion of the herbage without Na fertilizer was high (5 g kg(-1) DM). Na fertiliser, therefore, only slightly increased Na concentration, more in clover than in grass, and had little effect on K concentration. K fertiliser increased K concentration from 16 to 20 g kg(-1) DM and red uced Na concentration to 3.5 g kg(-1) DM. Sodium fertiliser, therefore , only increased the acceptability of herbage to cattle when herbage N a concentrations were initially low (less than 5 g kg(-1) DM) and were increased substantially by the application of the fertiliser. (C) 199 8 SCI.