Negative effects of winter- and spring-applied cattle slurry on the yield of herbage at simulated early grazing and first-cut silage

Experiments were earned out on seven UK sites during 1985–1988 to evaluate the importance of negative effects (smoethering and scorch) of winter and spring-applied cattle slurry on grass yield, assessed at the time of early grazing or first-cut silage. Slurry application has both positive and negative effects on herbage growth. Positive effects, usually associated with herbage yield increases, derive from the nutrient value of the slurry. Negative effects, often resulting in herbage yield decreases, are caused by smothering and/or scorch. The net effect on herbage depends on the balance between these positive and negative effects. These experiments were designed to isolate the negative effects of slurry whilst attempting to minimize the positive effects. Yield reductions of up to 20% for early grazing and 5% for first-cut silage were recorded, following applications of cattle slurry in February and March. Earlier applications smaller yield reductions. The threshold 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 m³ ha^?1 at 6% dry matter, which would supply 150–200 kg ha^?1 total nitrogen, based on typical slurry analyses. Within the limits of application rates tested in these experiments, yield depression tended to increase with increasing rate of slurry applied above this threshold. Although the results suggest that the effects are more likely to occur following spring applications of slurry, earlier (autumn or winter) applications would increase the risk of losses of slurry nitrogen by leaching and denitrification. These results, 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, for early grazing.