THE EFFECT OF TIMING AND RATE OF POTASH APPLICATION ON THE YIELD AND HERBAGE COMPOSITION OF GRASS GROWN FOR SILAGE

Replicated field trials were carried out at 23 sites in England and Wales between 1986 and 1988 examining the response of first-cut grass silage to different levels of early winter- or spring-applied potash on dry matter (DM) yield and herbage composition. Residual effects at the second cut were measured. In 1986/87 and 1987/88, the effect of additional potash at second cut was examined. Yield responses at first cut were measured at 13 sites (P < 0.05). Optimum potash rates (K-opt) ranged from 0 to over 160 kg ha-1, the highest rate tested. The mean rate of response up to K-opt at responsive sites was 10.4 kg DM (kg K2O)-1. Herbage K% increased with potash rate regardless of the requirement for maximum economic yield. Increasing potash rate reduced herbage Mg% by less than 0.020% which was less than the effect of soil Mg level. An increase of 50 mg l-1 in soil Mg (0-7.5 cm depth) increased herbage Mg by 0.034%. Early winter application of potash was generally as effective as spring application though it gave slightly lower herbage K and higher herbage Mg concentrations. Offtake of potash (K2O) at first cut was on average 12 kg ha-1 more from spring than from early winter application. Herbage analysis for N:K ratio was the best indicator of potash requirement. Regression analysis of the N:K ratio of herbage in control (nil potash) treatments against K-opt at first cut gave an r2 value of 51.4% (***). For herbage total K%, r2 was 39.7% (**) and for soil K (0-7.5 cm depth) it was 21.3% (*). Taking account of soil K below 7.5 cm depth was important at some sites, particularly those with a history of arable cropping and associated soil cultivation. Residual effects of a single early winter or spring application were measured, but additional potash applied for second cut was generally more effective. Optimum rates of potash for second cut were not determined.