Improving Air Quality

All animals eat nitrogen through the protein in their feed. More than half of the protein animals take in through their food is lost as nitrogen in their muck. These losses can be lowered by matching (as closely as possible) the protein content of the animal’s diet to its needs. This simple change can lower ammonia emissions by around 10%.

Research¹ has found that dietary crude protein levels for lactating dairy cows can be lowered to 14% with little to no loss in milk yield or fertility. A project at the Institute of Biological, Environmental & Rural Sciences (IBERS) has also found no overall effect on growth from lowering crude protein, from about 14% to about 11.5%, in heifer diets. This has a big benefit in managing ammonia emissions on the farm because the losses at other stages (at housing, storage and spreading) becomes much less but it is important to speak to your nutritionist before making any big changes to cow diets.

When cattle are grazing they produce much less ammonia emissions because their urine is quickly absorbed in to the ground. By extending grazing in to the autumn and spring, less slurry is produced by the animals when they are housed over winter. With grass being the cheapest feed for producing milk, this approach can help cut costs. However, extending grazing in the autumn and spring needs to be carefully managed to avoid soil compaction and damage to the sward, which can lead to water pollution.

With more grass grown, rotational grazing can extend the grazing season but it may not be suitable for all farms. It needs careful planning and good grassland management. This needs to take into account the farm’s soil type, topography and infrastructure and needs to be flexible to adapt to changes in the weather. Multi-species leys can help because they have a longer growing season and offer other benefits like lowering the need to buy in fertiliser.

The sooner slurry reaches the store from livestock areas the less it is exposed to air, with 15% less ammonia lost at housing. Cubicle and feed passages should be designed to drain quickly and be scraped as frequently as possible. To get the slurry to the store as soon as possible, it needs to be scraped from the cow cubicle house, ideally at least once every two hours. More frequent scraping means slurry has less chance to build up and cause problems with lameness.

Although it is a relatively new concept in the UK, it is possible to design cubicle houses to lower ammonia emissions by keeping any contact between muck and urine to a minimum. This involves either a sloped floor with a gutter for urine or a grooved floor with specially designed ‘toothed’ automatic scrapers. Both allow urine to drain away quickly.

Concrete yards and passages contaminated with slurry emit ammonia. Collecting yards, livestock handling areas and parlours should be scraped and washed down as soon as possible after use. By washing these areas straight after milking has finished, the amount of ammonia lost can be lowered by around 70%. This may not be practical on every farm but it is a simple and relatively inexpensive way of lowering emissions. It needs be part of a dirty / clean water management system and could need extra storage for slurry or dirty water.

Putting a cover on a slurry store or lagoon can have a big impact by stopping ammonia escaping to the atmosphere. Watertight covers also have the benefit of stopping rain getting in to the store. Most covers include vents, which stop a build-up of methane, and also need to allow access for the slurry to be stirred.

There are three main types of covers. A tight lid, roof or a tent-type structure is very effective, reducing ammonia emissions at storage by about 80%. Floating sheets are also available and are probably the best option for smaller earth-banked lagoons. They are quite effective, reducing ammonia emissions at storage by around 50%. Slurry bags are a relatively new technology but have been widely used in the Netherlands for 30 years and are very effective, reducing ammonia emissions at storage by around 95%. If you are thinking of installing a bag, you can find out what the planning requirements are by contacting your local planning authority.

Up to 45% of the nitrogen can be lost from urea-based fertilisers. Switching from urea fertiliser to nitrate (or other types of nitrogen) can lower the amount of ammonia lost by 80% for solid fertiliser. Many manufacturers also now supply urea fertiliser with inhibitors (known as protected urea). These delay the urea being broken down and keep the nitrogen in the soil for longer. This stops some of the losses seen in straight urea fertiliser but is still not as effective as nitrate fertiliser is. Although it costs more per kg than straight urea, it can lead to costs savings because less nitrogen is lost as ammonia.

Whichever fertiliser you use, by using a nutrient management plan you can make sure you meet the needs of soil and crop and minimise nutrients lost to the air or water. Spreading bagged fertiliser when the weather and ground conditions are right and using a regularly maintained, calibrated spreader gets the best results. Lowering the need to buy in fertiliser by making use of legumes (like clover) will also improve air quality, save the farm money and lower your carbon footprint.

Trees and woodlands have the potential to recapture ammonia from animal housing, slurry stores and roaming areas, lowering emissions by 20%. Tree shelterbelts should be downwind (which is ordinarily to the northwest) from the livestock housing or slurry store and need to include a short, dense backstop. The shelterbelt needs to be as wide, or wider, than the building. Trees could also be planted upwind from a slurry store to lower the wind speed over its surface. It is important to get advice before planting to make sure the right trees are placed in right location to get the most benefits.