A Review of Agronomic Factors Influencing Grass Silage Effluent Production in England and Wales

Silage effuent has a high biochemical oxygen demand (BOD) of about 60 000 mg/l. In 1991 it was estimated that the 29·7 Mt of bunker silage made per annum produced about 1·5 Gl of effluent. Silage effluent production is, however, extremely variable and may account for 25% of total agricultural water pollution incidents in wet years compared to 14% in dry harvest years. Incidents are highest in the grassland livestock areas of the west and least in eastern and south-eastern areas.A major factor in the increased pollution in recent years is the five-fold increase in the amount of silage made in the last 20 years. The average dry matter (d.m.) content of farm silages has also declined from about 270 to 250 g/kg in the past decade. Other reasons for increased pollution are inadequate and poorly maintained silos, crops being cut earlier and more late season aftermath cuts being taken in the autumn. There is evidence that increased use of nitrogen (N) fertilizer and N applied late produces grass with a low d.m. content. When this is ensiled with acid additives, effluent production is increased, but the effects of consolidation and mechanical pre-treatment are not clearly established. Large overfilled silos and poor silo design, structural deterioration and overflowing effluent tanks have also markedly contributed to silage effluent pollution.Effluent pollution could be reduced if crops were wilted to greater than 250g of d.m./kg. There are, however, difficulties in wilting in the wetter, western areas where rainfall and humidity are particularly high. The use of root and green crops together with higher d.m. crops for ensilage e.g. forage maize and whole crop cereals could potentially reduce effluent pollution; the present acreage is, however, very small in comparison with grass.