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Ammonia reduction strategies for the cattle sector

17 January 2022

Ammonia emissions from livestock present major challenges to the Northern Ireland agricultural industry. Funded by the Department of Agriculture, Environment and Rural Affairs, the Agri-Food and Biosciences Institute (AFBI) is currently conducting a major programme of research and has released this series of articles to help stakeholders understand the issue and adopt solutions to reduce emissions.

Ammonia is a gas emitted from natural and man-made sources. In Northern Ireland (and elsewhere), most of the ammonia in the air is released by agricultural practices, in particular from the management of animal manures and the application of nitrogen-containing mineral fertilisers.

Ammonia levels in Northern Ireland are high, which has wide-ranging negative environmental effects on sensitive habitats, human health and climate change. International targets have been set for the reduction of ammonia emissions, and the UK must achieve a reduction of 16% by 2030, in comparison to 2005 levels.

To address the ammonia issue in NI, a major programme of work, funded by DAERA, is currently underway at AFBI with a key focus on quantifying ammonia emissions and testing reduction strategies. As part of that research programme, AFBI, working in collaboration with Rothamsted Research, have been modelling ammonia emissions for typical Northern Ireland dairy and beef enterprises using the UK ammonia inventory model (NARSES). A number of proven and practical ammonia reduction measures have then been applied to these typical NI farms to determine the reduction in ammonia emissions on these farms as a result of the reduction strategies adopted.

This article is the 5th in a series of articles from AFBI looking at the ammonia issue. In a previous article the impact of ammonia abatement strategies adopted on a province-wide scale was shown to reduce ammonia emissions across NI by 25%. This article focuses on the impact of these strategies, when applied at farm scale, in order to demonstrate the reductions achievable on individual farms.

Dairy System Scenarios investigated

Two representative dairy systems were considered: (1) a grazing / housing system, and (2) a fully confined system, where grass is cut and carried during the summer. As outlined in Table 1, both scenarios were based on 100 cow herds (with 30 replacements) housed on solid floors with scrapers, outdoor slurry storage, splash plate spreading of slurry to grassland and typical calcium ammonium nitrate / urea applications. The only difference between the scenarios was the grazing period, set at 186 days for the grazing / housing system and milk yield, set at 7220 L for the grazing / housing system and 8500 L for the fully confined system, reflecting the higher milk yields typical of total confinement systems. These were considered the baseline scenarios.

Table 1. Parameters modelled for the baseline grazing / housing and fully confined dairy systems

Table 1. Parameters modelled for the baseline grazing / housing and fully confined dairy systems

Six ammonia reduction strategies where then applied across both scenarios to create ‘mitigation’ scenarios. The reduction strategies used were:

 

Results

Overall, a 43% reduction in ammonia emissions was achieved across both modelled dairy enterprises when the ammonia reduction measures were applied. Ammonia emissions were significantly higher in the fully confined system compared with cows grazing during the summer (Figure 1). Similar results were found when the size of the herd was increased.

Ammonia is formed when urine and faeces mix and since full-year housing means that all excreta is deposited in the house and additional manure management practices are required. This system has higher emissions compared with grazing / housing systems where a significant proportion of the excreta is directly deposited at pasture in small quantities.

Table 2. Ammonia emissions and milk yields derived from baseline typical practice scenarios (no mitigation applied)

Under the parameters modelled, for an identical herd size, fully confined systems produce approximately 57% more ammonia emissions per cow than grazing/housing systems. However, considering the higher expected milk yield for fully confined systems, these systems produced approximately 33% more ammonia per litre of milk than the grazing/housing equivalent (Table 2).

Whilst costs will be incurred to adopt a number of these reduction strategies, reducing ammonia losses throughout the manure management chain, thereby retaining more of the ‘Total Ammoniacal Nitrogen’ (TAN) content of slurry and, as a result the slurry has a greater fertiliser value.

Based on NARSES modelling it was estimated that the effective nitrogen fertiliser saving of this increased TAN was 461 kg per year for the grazing/housing system and 936 kg per year for the fully confined system. This equates to a cost saving in nitrogen fertiliser of £346 and £702 respectively based on a nitrogen fertiliser cost of £0.75 per kg (all savings per annum).

Beef System Scenarios

The beef sector in Northern Ireland is substantially more diverse than the dairy sector with a variety of different husbandry systems being used. However, the following two scenarios (outlined in Table 3) capture a large proportion of beef systems in NI:

  1. 24 month steer dairy-origin beef – 22 calves, 22 finishers
  2. Less Favoured Area (LFA) hill suckler cow spring calving – 18 beef cows, 18 calves

Table 3. Parameters modelled for the baseline beef systems

Ammonia Reduction Strategies

Whilst there are inherent differences between these systems, the same ammonia reduction strategies were applied for both:

Given the smaller-scale nature of the beef sector in Northern Ireland compared with the dairy sector, it was deemed inappropriate to model a move from under-slat stores to outdoor covered stores for the relatively small beef herds modelled. Likewise, a reduced crude protein diet was not modelled in these beef scenarios, although this is also a possible reduction strategy in the sector.

Results

The LFA suckler cow scenario (2) which is a slurry-only based system achieved an overall reduction in ammonia emissions of 42%, compared to the 34% reduction in the dairy origin beef scenario. This lower mitigation efficiency is due to a proportion of manure being managed as solid farm yard manure (FYM) in this scenario, from straw bedding. The mitigation options applied at land spreading (trailing shoe) are not applicable for FYM.

Based on NARSES modelling it was estimated that the effective nitrogen fertiliser saving was 75 and 70 kg for scenarios 1 and 2, respectively. This equates to a cost saving in nitrogen fertiliser of £56 and £53, respectively, based on a cost of £0.75 per kg (all savings per annum).

Overall, significant reductions in ammonia emissions are achievable across both dairy and beef systems, using existing and proven ammonia reduction strategies which are technically feasible and could be adopted within a 5-10 year timeframe.

In practical terms, longer grazing seasons and optimising dietary crude protein to requirements are both low cost strategies which could be adopted on most farms. Depending on existing infrastructure, increasing the housing scraping frequency may be an appropriate practice. Covering slurry stores and adopting Low Emission Slurry Spreading (LESS) technologies are solutions which require capital investment, but are important if significant reductions in ammonia emissions from the cattle sector are to be made. LESS in particular is fundamental to achieving significant on farm ammonia reductions as both an individual measure and an ‘end-of-line’ technique. Overall, it is promising that over 40% of ammonia emissions could be reduced from dairy and beef enterprises through the use of existing mitigation strategies.

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