Lowering Nitrogen Excretion from Pig Production

Dr Elizabeth Ball, a Senior Scientific Officer at AFBI, shared strategies for reducing the environmental impact of pig production by lowering nitrogen excretion and optimising feed ingredients, essential for sustainable farming practices, at the recent Teagasc Pig Farmers’ Conference.

One of the major challenges facing the agriculture industry is ensuring sustainable production while minimizing emissions of environmental pollutants. Although emissions from pig production are relatively small compared to total agricultural emissions, the intensive nature of pig farming and its concentration in specific geographic areas contribute to significant local nitrogen and ammonia emissions. Pollution from nitrogen and ammonia causes substantial environmental damage to waterways and surrounding areas. Leaching of nitrogen (nitrates) from soil due to excess slurry or fertilizer application can lead to eutrophication of freshwater and coastal ecosystems. Additionally, a portion of nitrogen excretion is converted to ammonia, which causes soil and water acidification and negatively affects biodiversity in sensitive areas, reducing their ability to capture carbon and thereby increasing total greenhouse gas emissions. Ammonia is also a precursor of nitrous oxide, a potent greenhouse gas, which further exacerbates emissions from livestock production. Reducing nitrogen excretion (and thus ammonia emissions) is a key objective in pig production, but how can this be achieved in practice?

Reducing Nitrogen Excretion Through Nutrition

Research at AFBI, as part of the Pig Research Consortium (AFBI, Devenish Nutrition Ltd., John Thompson and Sons Ltd., and Preferred Capital Management), has focused on strategies to reduce the environmental impact of pig production through targeted nutrition. Nitrogen, in the form of dietary crude protein (CP) and amino acids, is essential for growth. However, reducing nitrogen intake is not a simple solution for lowering nitrogen output. It is crucial to maintain an optimal supply to ensure peak production performance and sustainability, avoiding increased nitrogen and ammonia excretion due to poor growth efficiency. A series of trials has been conducted to maintain or improve production efficiency while reducing overall excretion. Results from the Consortium's work indicate that dietary CP levels can be safely reduced to 15% in finishing diets for pigs from 60 kg, provided that amino acids are balanced and adequately supplied. Based on this research, many producers in Northern Ireland have adopted 15% CP diets, reducing nitrogen excretion by 20% in finishing pigs, demonstrating the industry's commitment to addressing environmental challenges.

Phase Feeding

Phase feeding is a method used to tailor nutrition to the changing requirements of pigs as they mature. While it makes sense to adjust diets according to the animal’s needs, frequent diet changes can affect feed intake and efficiency. Although dietary nutrient requirements decrease as pigs age, most producers offer a single finishing diet from approximately 60 kg through to slaughter. With increasing slaughter liveweights (up to around 135 kg), questions have arisen about introducing an additional finisher diet at around 100 kg. The theoretical benefits include lower diet costs and reduced nitrogen excretion. Our study tested this by offering pigs (both boars and gilts) a 15% CP diet from ~60-100 kg, followed by a 13% CP diet from ~100-140 kg, comparing it with pigs on consistent 15% or 13% CP diets throughout. As shown in Table 2, phase feeding did not provide any benefits in terms of performance or nitrogen excretion. Thus, the practical recommendation is to offer a single diet from approximately 60 kg onwards.

Table 2. The effect of diet change on performance and N excretion from ~60-140kg

The results in Table 2 show that offering a 13% CP diet reduced performance, particularly on the commercial farm. Further analysis revealed that this was primarily due to reduced performance in boars, as gilts performed well on the lower CP diet. Splitting pigs by sex and feeding them accordingly can reduce nitrogen excretion. Boars require higher levels of amino acids due to their greater capacity for lean growth, while gilts can be offered a lower CP/lower lysine diet without compromising performance. This strategy resulted in an 18.5% reduction in nitrogen excretion from gilts. Hence, boars should be fed a 15% CP diet, while gilts can thrive on a 13% CP diet.

Going Lower in CP?

The next step in our research was to test if dietary CP could be reduced further by adjusting amino acid levels, particularly lysine. A trial compared diets with 13% CP containing either 0.9% or 1% lysine against a 15% CP control diet. The results (Table 3) showed that boars on the lower lysine diet had significantly lower growth rates and feed conversion efficiency. However, increasing lysine to 1% in the 13% CP diet allowed boars to maintain performance comparable to those on the 15% CP diet, reducing nitrogen excretion by 23%.

Table 3. The effect of gender and dietary lysine level of finishing pig performance and N excretion

Impact on Ammonia Emissions

Lowering dietary CP while maintaining production performance reduces nitrogen excretion and, consequently, ammonia emissions. A specialized trial at AFBI measured ammonia emissions, odour, water usage, and slurry output using pigs on diets with 18%, 15%, and 13% CP. As shown in Table 5, ammonia emissions decreased by 10% for every 1% reduction in dietary CP. Additionally, reducing CP lowered slurry output and improved slurry handling, with odour emissions and hydrogen sulphide levels also decreasing.

Table 5. The effect of reducing dietary CP on ammonia emissions, slurry output and odour.

*The reduction in ammonia emissions was significant at P<0.05 and the reductions in odour emissions and hydrogen sulphide were numerical but non-significant

AFBI's research has found that ammonia emissions from finishing pigs in Northern Ireland are 1.55 kg/pig place/year, 46% lower than the current UK standard. This updated figure reflects reductions in dietary CP and advancements in genetics, providing a more accurate baseline for implementing effective mitigation strategies.

Summary and Practical Recommendations

• House, manage, and feed boars and gilts separately, starting at weaning when pigs are easier to handle.
• Offer finishing boars a 15% CP diet and gilts a 13% CP diet from ~60 kg onwards, reducing nitrogen excretion in gilts by 18.5% while optimizing boar performance.
• Assess on-farm pig performance; if intake rates are high, consider using a 13% CP/1% lysine diet for all pigs from ~60 kg onwards.
• Avoid changing diets in the late finishing period (~100 kg) as this may reduce performance and economic returns without affecting nitrogen excretion.
• Reducing dietary CP by 1% can lower ammonia emissions by 10%, provided production performance is maintained.
• Lowering ammonia emissions also reduces odour emissions and slurry output.

Read the full Teagasc Pig Farmers’ Conference 2024 Proceedings