NEGATIVE EFFECTS OF WINTER-APPLIED AND SPRING-APPLIED CATTLE SLURRY ON THE YIELD OF HERBAGE AT SIMULATED EARLY GRAZING AND FIRST-CUT SILAGE

Experiments were carried out on seven UK sites during 1985-1988 to eva luate the importance of negative effects (smoethering and scorch) of w inter- and spring-applied cattle slurry on grass yield, assessed at th e time of early grazing or first-cut silage. Slurry application has bo th positive and negative effects on herbage growth. Positive effects, usually associated with herbage yield increases, derive from the nutri ent value of the slurry. Negative effects, often resulting in herbage yield decreases, are caused by smothering and/or scorch. The net effec t on herbage depends on the balance between these positive and negativ e effects. These experiments were designed to isolate the negative eff ects of slurry whilst attempting to minimize the positive effects. Yie ld reductions of up to 20% for early grazing and 5% for first-cut sila ge were recorded, following applications of cattle slurry in February and March. Earlier applications smaller yield reductions. The threshol d for the occurrence of yield reduction was estimated at 3-4 t ha(-1) of slurry solids applied, at 6% solids content. In practice this would mean an application of 50-65 m(3) ha(-1) at 6% dry matter, which woul d supply 150-200 kg ha(-1) total nitrogen, based on typical slurry ana lyses. Within the limits of application rates tested in these experime nts, yield depression tended to increase with increasing rate of slurr y applied above this threshold. Although the results suggest that the effects are more likely to occur following spring applications of slur ry, earlier (autumn or winter) applications would increase the risk of losses of slurry nitrogen by leaching and denitrification. These resu lts, therefore, confirm current advice in the UK that, where possible, cattle slurry is best applied to grassland in early spring according to the fertilizer needs of the crop. High rates of application should be avoided after early March for cutting areas and, more especially, f or early grazing.