How to make the most of poultry manure and how it compares: part 1

In my first blog, we discussed the nutrient content of cattle slurry. This time, we will focus on poultry manure and its nitrogen profile, and next time we will focus on other nutrients.

Poultry manure is often referred to as "rocket fuel" for crops because it is rich in readily available nitrogen (RAN) and a valuable phosphate source. However, due to its high nitrogen and phosphate content, managing poultry manure can be challenging, and it poses environmental risks if not stored and applied efficiently.

The nitrogen content in poultry manure can be very high. A good proportion is in the uric acid and ammonium-N form; the remainder is organic nitrogen, which breaks down over time. Uric acid is the final product formed when poultry metabolise nitrogen within their bodies, and it, therefore, naturally occurs in their waste.

In fact, between 30% and 50% (40-70%, according to some studies) of the total nitrogen excreted by poultry is in the forms of ammonium-N and uric acid, both of which are readily available to crops. Typical percentages of organic and inorganic forms of nitrogen published in RB209 are given in Figure 1.

Both ammonium-N and uric acid pose a risk to the environment as these nitrogen forms can be lost as ammonia-N to the atmosphere, when applied to land and in animal housing. High ammonia levels in air reduce its overall quality and affect the health and wellbeing of the birds within poultry units. They also cause nuisance odours to the wider environment if steps are not taken to mitigate emissions from housing. Taking actions such as using litter additives, installing biofiltration or acid scrubbers, manipulating poultry diets, and adapting bedding materials are all strategies that farms adopt to reduce the impact of ammonia-N emissions.

Ammonia in the air poses not only an environmental problem by enriching sensitive habitats but also risks human health as it is associated with various respiratory issues. This is why, in the 25-Year Environment Plan published in 2018, the UK government committed to reducing ammonia emissions by 8% by 2020 (where a 9% reduction was met) and by 16% by 2030, using 2005 as the baseline measurement year.

Composting poultry manure reduces the risk of nitrogen loss

Composting has been shown to reduce uric acid content, as anaerobic bacteria consume uric acid as their nitrogen source. Poultry manures with a higher dry matter and, therefore, a lower moisture content also reduce the rate of uric acid decomposition during the process, leading to more nitrogen being maintained within the material. This benefits crops through a more consistent nitrogen supply when poultry manures are applied and reduces the need to supplement crops with nitrogen fertiliser.

NRM data analysis of poultry manure

NRM has examined all the poultry manure samples analysed over the last four years, and the results make interesting reading, particularly when compared to the standard figures published in RB209. The table below compares the average nitrogen content measured in the laboratory compared to the typical values in RB209. 

From a practical perspective, it is important to understand how rich in nitrogen the poultry manure you intend to spread is because it has an impact on the rate you are allowed to apply without contravening the 250kg N/ha field limit, for NVZ (Nitrate Vulnerable Zone) and CoGAP (Good Code of Agricultural Practice).

For instance, if poultry manure has a dry matter content of 60% and a typical nitrogen value of 28kg per tonne, you can only apply up to 8.9 tonnes per hectare to stay within the field limit of 250kg per hectare of total nitrogen. In the NRM dataset, the average measured nitrogen value was 7.6kg per tonne lower, meaning you could apply 3 tonnes per hectare more.

This is one of the advantages of routinely analysing livestock manures ahead of application. NRM has a quick and easy analysis service that enables you to have the up-to-date information required to help you apply the correct rates.

However, this situation can also work in reverse. If the poultry manure contains more nitrogen than the typical 60% dry matter content suggests, you risk oversupplying nitrogen if you apply the maximum rate of 8.9 tonnes per hectare based on typical values. You can see this illustrated in the total nitrogen distribution charts below.

These charts describe the variation of nitrogen concentrations measured in poultry manures with 20, 40, 60 and 80% dry matter contents (Figure 2). On average, the measured values are all less than the comparative typical values, but as we come to expect, there is a wide range of contents, which increases the greater the dry matter content.

When using poultry manure with a 60% dry matter content as an example (Figure 2C), the analysis shows that 25% of the sampled manure contained up to 15.0kg of nitrogen per tonne, while three-quarters contained 25.9kg of nitrogen per tonne. The median value is 23.6 kg per tonne, close to the 75th percentile (25.9kg per tonne), suggesting that the data is skewed towards the upper end of nitrogen values. Overall, the average value was 7.6kg per tonne less than the typical total value published in RB209 for poultry manure of 60% dry matter.

The analysis of manure with a 20% dry matter content is interesting because, for once, the data is “normally distributed”.  The average total nitrogen concentration of 7kg per tonne is very close to the median value of 6.9kg per tonne. This suggests that most samples consistently fall close to the average, but again, these data values are lower than those published as typical in RB209.

In addition to supplying nutrients, poultry manures are also useful sources of organic matter for soils. Navarro et al. (1993) reported a highly significant correlation between the loss of ignition (LOI) test and organic carbon (OC) contents in poultry manure.

Also, in a study by Nicholson et al. (1996), a good relationship between LOI and OC for poultry manures was also concluded, despite some poultry manures containing 2-5% calcium carbonate due to dietary additions of limestone dust acting as a bone and shell strengthener.

The study calculated typical amounts of organic carbon added when poultry manure inputs matched the application rate of 250 kg total N/ha would add:

o   1.1t ha - 30% DM poultry manure and % OC content of 28%

o   1.6 t ha – 60% DM poultry manure OC content of 35%

Based on a topsoil depth of 0-20cm and organic carbon content of 2%, typical of some arable land in England and Wales, these OC additions equal 2-3% of the OC already present in the topsoil. So, poultry manure is not only a great nitrogen supplier but over time can also condition the soil with organic matter and carbon. However, to understand the actual benefits of these materials, measurement plays a huge role in maximising the benefits and minimising the environmental risks associated with their use.

NRM: here to help

NRM provides a range of analytical services to help you get the most out of your livestock and organic manures. With accurate, timely data on manure nutrient content, you can make informed decisions on application rates, saving on fertilisers and boosting productivity.

To understand more about how analysis can help, contact your advisor or speak to us directly. You can also request sampling supplies on our website here.

References

Full article: Increased total nitrogen content of poultry manure by decreasing water content through composting processes

F. A. Nicholson, B. J. Chambers & K. A. Smith. Nutrient composition of poultry manures in England and Wales.  Bioresource Technology 58 (1996) 279-284. 

Navarro, A. F., Cegarra, J., Roig, A. & Garcia, D. Relationships between organic matter and carbon contents of organic wastes. Bioresource Technology 44 (1993) 203-207.

Ramesh Bahadur Bist, Sachin Subedi, Lilong Chai, Xiao Yang. Ammonia emissions, impacts, and mitigation strategies for poultry production: A critical review. Journal of Environmental Management Volume 328, 15 February 2023, 116919

Air quality: explaining air pollution – at a glance - GOV.UK