Advice on assessing the nutritional value of manure on crops

Used properly on crops, muck is a valuable by-product that offers the opportunity to reduce artificial fertiliser inputs and improve soil health.

However, measuring its nutrient value and the associated costs of spreading it are essential to understand its full worth.

“The quality and value of manure varies massively, and it’s important not to treat them all the same,” says Zach Reilly, agronomist at Scottish Agronomy.

See also: How manure can increase soil oxygen and nitrogen use efficiency

Quality and nutritional value

“You’d treat a hen pen very differently to a cattle manure, and this impacts both nutrient status and how the muck can be used in a crop rotation.

“Some of the key differences include P and K [phosphorus and potassium] values – but most importantly, it’s the availability of nitrogen [N] that drives the difference in the value of that muck.”

It is not just the type of manure, but also how it has been handled and stored, he continues.

“If you leave cattle muck in a midden [dung heap] for a year, you’ll actually have a reduction in the availability of nitrogen over time, so it’s important to consider that too.

“Ask yourself: are you using manure straight out of a court, or has it been sat in a field for a while? This makes a difference to its nutrient value and availability.”

Method of storage, livestock diet and the type and quantity of straw used to bed animals also impact nutrient status, adds Zach.

“For example, we’ve found that with oat straw, because oats take up a lot more potash, that translates through to the dung, and it tends to contain more potash.”

Ammonia volatilisation can be minimised by covering heaps with a waterproof sheet.

Sampling and analysis

Testing each heap of muck before spreading is vital.

“This means not using a historic analysis – or standard values – because we find that even within the same species, dung heaps can vary in their value,” he advises.

The first step is identifying any potential differences between types of muck and analysing these separately, he says.

“For example, if you have some dry suckler cows housed and fed on a maintenance diet of low-quality silage or ammonia-treated straw, in one pen, and in another, cows fed an intensive cereals-based diet, you can pretty much guarantee they’ll have a very different nutrient status.”

Zach recommends taking multiple samples for analysis.

“Particularly with cattle muck, it can be hard to get a representative analysis, so taking samples from several parts of the heap and mixing it thoroughly before sending it to the lab is important.

“If it’s a big heap, it may even be worth carrying out two or three analyses and working out the average from those to  get the most representative picture of nutrient status.”

Financial value

Once the analysis has been done, it is possible to start attributing some financial worth to nutrient levels.

“Aside from nutrient content, there are obviously other benefits such as introducing organic matter and feeding the soils from a biological point of view.

“However, that is very hard to value financially, so when we think about manure value, it’s really just that nutrient content we’re talking about – N, P and K.”

Zach says the first step is working out typical fertiliser costs for a tonne N, P and K.

“This is done by looking at your straights costs. For example, take ammonium nitrate at 34.5% N – we’re getting quoted in Scotland about £330/t, which works out at £0.96/kg of nitrogen.”

To compare this with manure values, he says farmers should use these costs as a baseline to put a price on the available N – not total N – in the muck.

“That’s very important, as this is what the crop is likely to use. With the total nitrogen, there’s no guarantee we’re going to be able to utilise that and therefore we shouldn’t really place a value on that.

“However, we do tend to value all the P and K. Although this won’t all be available in the first year – it tends to be about 80% – it’s not overly mobile, so it’ll still be available for the following crops.

“It’s then just a case of multiplying these figures up and seeing how it [the muck] stacks up against bought in-fertiliser.”

Application

When it comes to applying muck, results from the analysis should be used in tandem with individual field requirements to tailor rates accordingly.

Zach says this should include:

• Soil nutrient status – taking into account any low indices and whether or not these can be corrected by applying manure
• Ratio of soil organic carbon to carbon – remembering that lighter soils have a more rapid turnover of organic matter because of their soil air content
• The timing of application to ensure maximum nutrient availability.

It is also important to consider the cost of applying muck, says Zach.

“This is probably more important for arable farms taking muck from livestock farms and being clear about the cost of the entire operation,” he explains.

“But regardless, applying muck still has a cost, even if it’s staying on farm.

“The key is not just to base the cost on contractor rates, but what it costs the individual farm.

“We’ve done a lot of machinery costings lately for clients and we’re finding huge variations.

“Consider how much the tractor is costing, the muck spreader, work rate and even having the telehandler on the side of the field, loading.

“Going into detailed machinery costings is really important when looking at the total value and costings involved.”