Managing Livestock Manures 4: Managing Manure on Organic Farms

Good manure management - minimising pollution and using nutrients

All farmers should follow a Farm Waste Management Plan to help reduce the risk of pollution of water. The plan helps identify areas of land where spreading of slurry or manure should not take place, due to factors such as proximity to water, soil type, slope and drainage. No additional guidelines exist in Organic Standards , but registered holdings are required to avoid pollution from both diffuse and point sources. Following Good Farm Practice is a requirement of the Organic Farming Scheme.

Complaints of smells emitted from manure during housing, storage, treatment, transport and spreading, has become an acute public relations problem. Emissions of odour are highest during manure spreading, but odours may persist for up to 12 hours following application. Organic farms that maintain aerobic conditions (e.g. by composting) should have fewer problems than those using slurry systems.

Using manures on grassland

Cutting grass for silage or hay can remove large amounts  of phosphate and potash. Manures are an effective way of replacing these removals apply solid manures in autumn/winter or slurries in spring.

Most of the nutrients from grass consumed by a grazing animal are excreted back on to the soil, so that the net removal from the field is small. In addition, using manures on grazed swards can lead to problems (disease risk, reduced intake, nutrient imbalances), and is best avoided.

Large applications of N will reduce the clover content of grass/clover swards.

Using manures on arable crops

Arable crops will benefit from manure applications. Solid manures could be applied before cultivating the soil either in the autumn before winter cereals, or in the spring before spring cereals or potatoes, sugar beet etc. Rapid incorporation after application decreases losses of nitrogen as ammonia.

Slurry and poultry manure applications will ensure best use of the nitrogen if they are made in spring before spring crops (again, incorporating rapidly). It is also possible to top-dress growing cereal crops with slurry in the spring.

Application to land

To make best use of the nutrients in manure, in particular N, it should be applied when potential crop uptake is at its maximum and risk of losses are at their lowest.

Ammonia losses, which are also associated with odour nuisance, can be greatly reduced by incorporating all manures as soon as possible into soil (see Table 3) whereas for slurry application to grass, the use of injectors, trailing shoe spreaders and band spreaders should be considered.

The dry matter (DM) content of surface-applied slurry affects ammonia losses 20% more N is lost from a slurry with a DM of 6% compared to a 2% DM slurry. The lower losses are the result of more rapid infiltration into the soil.

Livestock manures are frequently the greatest source of avoidable nitrate leaching. Where possible, avoid applications during the autumn or early winter period. Delaying applications (in particular those with a high proportion of readily available N) until late winter or spring will increase N utilisation by the crop. Figure 4 shows the potential nitrate leaching risk associated with manures applied at different times over the autumn/winter period.

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Reducing losses of nutrients from manures is a challenge, made more difficult by the interactions between loss pathways and, often, conflicts in best management practices to meet different environmental objectives. There is no simple, single answer. Best manure management to optimise nutrient supply to the soil will generally have the added benefit of reducing environmental impact.

In certain areas, there are restrictions regarding manure application timing and rates. In Nitrate Vulnerable Zones (NVZs), slurries and poultry manures must not be applied to sandy or shallow soils between 1 August 1 November (arable fields) or 1 September 1 November (grassland fields). All farmers affected by NVZ conditions will have been notified by the Environment Agency. Other schemes may have different restrictions. Check with Sector Bodies for any specific conditions.

Application rate

Standards laid down by the Sector Bodies will govern maximum application rates of manure. They will not be greater than the maximum annual N loading (averaged over all the agricultural land on a farm) of 170 kg/ha set by the EU Regulation. However, other schemes may have more stringent restrictions: for example, the Soil Association limits the application of manure N on unimproved meadows to 75 kg N/ha/yr.

It is important that slurry or manure is spread as accurately and as evenly as possible. See DEFRA Managing Livestock Manures: Booklet 3, (Appendix II) for further information on spreading systems for slurries and solid manures.

Solid manure composting

Composting is actively encouraged on organic farms because it:

  • reduces substrate mass
  • improves friability and handling characteristics
  • destroys weed seeds and potentially harmful pathogens by generating high (60-70 degC) temperatures during the process
  • provides phyto-sanitary effects on incorporation into the soil
  • incorporates inorganic N into the organic fraction, thus protecting from immediate loss after application
  • reduces odour and ammonia emissions during land spreading
  • concentrates plant nutrients, enabling application rates to be lower and the risk of crop smothering to be reduced.

Different degrees of composting are achieved on-farm during manure storage, ranging from actively managed frequently turned piles, through to simple stockpiles. The degree of composting will impact on nutrient and water losses during storage, and the composition of the end product.

For effective composting, the manure must remain aerobic throughout. This is normally achieved by regular turning or mechanical aeration. As a result of microbial activity, substantial rises in temperatures occur. Temperatures in excess of 60 degC destroy weed seeds, pathogens, chemical residues and antibiotics, and the composting process should aim to achieve this. Sector Bodies may have requirements for manure storage and composting.

The potential benefits of composting may be lost, and harmful environmental impacts caused by poor practice. Increased rates of ammonia volatilisation have been measured, with losses up to 70% of total N. Ammonia emission is dependent upon temperature, initial ammonium-N content, turning frequency, moisture content and the composition of the manure. Ammonia emissions can be reduced by composting manure with an initial carbon:nitrogen ratio >35:1 and by carefully balancing the frequency of turning to meet the conflicting requirements of sufficient aeration and minimum disturbance.

Typically, the final composted product has 85-95% of the total N in organic forms, 5-15% as ammonium-N and 15% as nitrate-N.

Slurry aeration

The aim of on-farm aerobic treatment of slurry is to produce a better product for spreading. Slurry aeration is frequently recommended for use in organic systems, but it may be difficult to achieve cost-effectively. Adequate aeration involves dissolving sufficient oxygen in the slurry to convert the anaerobic microbial activity to aerobic (much as composting does for solid manures). The expected benefits are:

  • a reduction in ammonium-N content (through nitrification), which results in more positive effects on soils and crops
  • removal of pathogens with temperatures between 50 and 60 degC
  • an improvement in slurry homogeneity
  • a reduction in offensive odours and methane emission during storage and land application.

However, incorrect operation of aeration can lead to increased losses of N (and odours). Some of the potential drawbacks are:

  • removal of the surface crust, thereby increasing the risk of ammonia emission
  • movement of ammonium-N to the surface, increasing the potential emission of ammonia
  • encouragement of nitrous oxide emission through intermittent aerobic and anaerobic conditions.

Although aeration may theoretically be the best approach to slurry management, it is expensive and the potential problems are high.

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