Optimising manure management for GHG outcomes

Abstract

This paper focuses on improvements to livestock manure management to reduce environmental pollution and emission of greenhouse gases (GHG). Livestock manures contain large amounts of plant nutrients and organic matter (OM). Structural changes to livestock production and ample supply of cheap chemical fertilisers have decreased the interest and possibilities of farmers in using manure for the fertilisation of crops and grasslands and maintenance of soil fertility. As a result, many livestock producers dispose of manure as cheaply as possible causing serious pollution of soil, water and atmosphere. In addition, livestock production systems contribute to climate change by emission of the GHG carbon dioxide (CO₂), methane (CH₄) and nitrous oxide (N₂O). Careful recycling of livestock manures to fertilise crops and grasslands and improve soil fertility is considered the most suitable and cost-effective option for environmentally friendly disposal. Manure management legislation in The Netherlands is described to explain the principles. These include complete collection of faeces and urine of confined livestock, adaptation of the period and rate of manure application to the N and P requirements of crops, and use of manure collection, storage and application techniques aiming at low ammonia (NH₃) losses. Effects of sustainable manure management on GHG emissions are described. Optimising the period, rate and technique of manure application to crops and grassland causes effective utilisation of manure N and reduces direct and indirect losses of N₂O. In addition, effective recycling of manure nutrients and OM allows a reduction in the use of chemical fertilisers and fossil energy and contributes to the maintenance or improvement of the carbon content of agricultural ecosystems. The relatively high costs of sustainable manure management stimulate farmers to optimise feed conversion and minimise manure production per unit of product by good livestock feeding and management practices. High feed conversion efficiency reduces CH₄ emission by enteric fermentation and may reduce feed imports and related GHG emissions. In addition, it is shown that livestock categories differ widely in feed conversion efficiency and N and P excretion per unit of product. Finally, anaerobic digestion of livestock slurries provides a valuable energy source and reduces CH₄ emission of stored slurry and, possibly, N₂O emission after field application of the slurry.