New science is enhancing the fertiliser value of slurry while reducing ammonia emissions

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In the vast majority of cases, it represents a home-produced resource that can be used to enhance grass and crop outputs.

Meanwhile, the effective use of slurry can help to secure the all-important sustainability targets that must be achieved across agriculture as a whole.

Slurry is a valuable source of plant nutrients, including nitrogen (N) phosphate (P) and potash (K). But unlike chemical fertiliser, it is a biological resource.

This means that the crop nutrients within slurry, particularly N,are both in a slow-release organic mineralised form and quick release plant available form, which is subject to volatile losses such as ammonia and leaching. Reducing these losses will substantially improve the fertiliser value of slurry.

Slurry research

The good news is that recent research work has identified how this can be achieved in an effective manner. The end result for the farmer is a resource that is considerably more valuable, both as a fertiliser and soil conditioner.

A case in point is Digest-it®, a microbial inoculant that has been identified and fully researched by Devenish Nutrition as a product that will enhance the fertiliser value of animal slurries under Irish farming conditions.

Digest-it® is available from United Feeds in Northern Ireland.

It is now widely recognised that three factors combine to maximise the effectiveness of soils to grow high quality crops. These are: soil structure, soil chemistry and soil biology.

United Feeds’ agronomist, Chloe Kyle, takes up that story:

“Digest-it® impacts positively on every aspect of soil health.”

“Driving all of this is the ability of treated slurry to enhance soil earthworm numbers.”

“This is at the very heart of achieving greater sustainability on farms.”

“Earthworms act to improve soil structure. Their burrowing activities are key to improving aggregate formation within a soil profile.”

Soil biology

Comprehensive trials, in tandem with its commercial on-farm use, have confirmed that the product will act to reduce ammonia emissions from slurry while it grows more, better quality grass. These enhanced production levels are achieved through an improvement in both fertiliser nutrient content and soil health.

Specifically, the Devenish trials have confirmed that treated slurry will produce an additional 2t of grass dry matter per hectare, relative to that achieved when untreated slurry only is used.

In addition, this work has consistently confirmed over 8 years a 19% increase in grass growth, a 31% decrease in ammonia emissions within slurry and up to a 50% reduction in agitation times prior to slurry being removed from a tank.

The man behind all these trials is Devenish consultant, Dr David Atherton.

He explained:

“Where ammonia emissions are concerned, two key trials with Digest-it were undertaken: one at Mountbellew College in Co Galway and the second at the former Devenish Research Estate, at Dowth in Co Meath.”

“Ninety per cent of ammonia emissions come from agriculture with a very clear focus on ruminant livestock.”

“The need to reduce ammonia emissions from agriculture is obvious. Digest-it represents a biological solution to this challenge.”

Other chemical solutions, including the treatment of slurry with acids has been looked at as a means of reducing ammonia emissions from livestock farms.

Dr David Atherton again:

“Acidification works on the basis of neutralising the alkaline nature of ammonia, which reduces emissions.However, acids are extremely hazardous substances.”

“And in addition, their use will act to depress the pH value of the soils on to which acid treated slurries are applied.”

“But Irish soils are already highly acidic. The average pH value of grassland soils in this country is already well below a value of 6.0. So, adding acid treated slurries to grassland areas will only act to further compromise soil fertility.”

“In contrast, Digest-it constitutes a source of microbes in the form of a liquid biological inoculant.”

Independent trial work

The Dowth ammonia trial was undertaken in 2021 under laboratory conditions: the work was directly coordinated by a post graduate student from Wageningen University in the Netherlands.

It confirmed an 80% reduction in ammonia emission levels from Digest-it treated slurries, relative to an untreated control over a ten-week period. The additive was included at the recommended commercial rate.

Also of significance was the fact that the ammonia emission levels from the untreated slurry varied considerably – rising and falling in synch with the ambient temperature.

However, emission levels from the Digest-it treated slurries remained consistently low throughout the ten-week trial period.

Dr David Atherton again:

“This was our first attempt at a replicated study. The slurry was contained in 85L barrels with the ammonia concentrations gathering directly above the surface measured using a Draeger tube.”

“Ammonia concentrations were measured on a weekly basis. The slurry was sourced from a neighbouring dairy farm.”

“Both treated and untreated slurry samples were made available to us. Initially, 60L of slurry was added to the barrels with additional slurry added over a four-week period. This allowed us to mimic the defecating activity of cattle.”

Reducing ammonia

The ammonia concentrations measured for the untreated slurries were in the region of 15-40 parts per million (ppm).

According to Dr David Atherton, these are the levels that one would expect to find in the air space above the slurry stored in slatted tank within a commercial dairy shed.

In contrast the Digest-it treated slurries never got above 7ppm throughout the ten-week trial.

The Devenish consultant points to the results generated by the untreated barrels vindicating the commercial relevance of the results obtained from the Digest-it treated slurries.

The laboratory research work at Dowth was the precursor to the full blown, commercial trial carried out at Mountbellew College.

This work centred on the comparative impact of Digest-it on the ammonia produced over a twelve week period by commercial beef cattle, housed in a slatted shed.

Dr David Atherton explained:

“The shed comprised two, 50,000 gall slurry tanks on either side of a feeding passage. One of the tanks was treated: the other wasn’t.”

“Cattle were allocated on both sides of the sheds on a balanced perspective in terms of their liveweights and genetics.”

He continued:

“Ammonia gas concentrations were measured twice daily at 5cm above the slurry surface.”

The Mountbellew trial was undertaken by a post graduate student at the college. Over the course of a twelve week period, the work clearly confirmed an ammonia concentration of 11.8ppm above the untreated slurry: the equivalent figure for the Digest-it treated tank was 8.2ppm.

“This works out at a 31% reduction, where the Digest-it treated slurry is concerned and is a highly significant result,” Dr David Atherton stressed.

Independent research work has also confirmed the benefits of Digest-it®. Trials carried out by the Kingshay Farming Trust, which is associated with the University of Bristol, evaluated several slurry additives. Untreated dairy slurry was compared with a range of inoculated substrates.

“These were laboratory trials: three replicates per treatment,” Dr David Atherton commented.

“The results confirmed a 17% increase in Nitrogen within the Digest-it® treated slurries, from 0.25% to 0.29%.”

“And, again, this figure was significant at a 95% probability level.”

He added:

“Half the N within slurry is tied up within the biological fraction the other half is in the form of aqueous ammonium ions, which is the direct source of ammonia gas.”

Improving soil health is one of the key attributes associated with Digest-it® treated slurries.

Specifically, the product is working to maximise the N value of slurries that are applied to grassland and crops.

Slurry mixing

It also acts to reduce the levels of agitation required to mix slurries. Depending on the tank design, the time required to mix slurry below slats thoroughly can be reduced by up to 50%.

However, it is important that Digest-it® is applied when a tank is low, usually at the start of winter and is subsequently allowed to inoculate the slurry for up to 12 weeks.

Evidence of this mixing activity is evident from the bubbles that percolate through the slurry during this period.

“Digest-it® is very good at breaking down sludge that accumulates at the bottom of tanks,”said Dr Atherton.

“This in turn is putting additional P and K into solution. These nutrients are not volatile and, as a result, will be plant available once the treated slurry is spread on to grassland and or crops.”

He continued:

“The enhanced fertiliser N value of Digest-it® treated slurries has been consistently demonstrated courtesy of field trials carried out at the Devenish Research Estate at Dowth over the past eight years.”

“This work has shown that the treated slurry will produce a 19% increase in grass silage yields. Both the control and trial plots received CAN plus sulphur at equivalent rates. The slurries were applied using a dribble bar.”

“The average increase in grass dry matters yield for the Digest-it® treated plots works out at 1.1t/ha on first cuts alone. This is a highly significant figure.”

In 2023 the Devenish team carried out a Digest-it® trial on both first and second cut silages. On the occasion the relative increase in dry matter yields, across both cuts, totalled over two tonnes of grass dry matter per hectare.

The financial benefits delivered in this context are significant, given that grass dry matter is valued at approximately £170/t, which equates to a return on investment of 12.5:1.

Dr David Atherton concluded:

“The microbes in the treated slurry are impacting beneficially on soil conditions. Improved worm counts in grassland soils to which Digest-it® treated slurry are indicative of this.”

“Additional research carried out at Dowth has shown conclusively that earthworm numbers increase significantly when Digest-it® treated slurry is applied to grassland.”

“These microbes multiply when mixed with slurry. In addition to improving slurry nutrient value,it is their subsequent activity in the soil that improves the potential to enhance both grass and crop growth.”

“Work on grassland soils at Dowth has confirmed that Digest-it® treated slurry will enhance earthworm numbers by 36% relative to the impact of untreated slurry. This trend is highly significant from an enhanced soil biology perspective.”

United Feeds’ Agronomist Chloe continued:

“As farmers face-up to the challenge of lowering the carbon footprint of their enterprises and becoming more sustainable, they must seek to identify ways of maximising the resources generated within their businesses.”

“Making best use of animal slurries is critically important in this context. It’s an approach that will allow farmers to grow more grass or produce the same amount of with a reduced dependency on chemical fertilisers.”

She concluded:

“Digest-it® will allow farmers achieve one or other of these objectives in a very meaningful manner.”

Digest-it® comes in 20L drums: one drum will treat 100,000 gallons of slurry. It takes 12 weeks for the slurry additive to generate its full effect.