How to make the best use of manures this season #1

Author: Alli Grundy

1st November 2023

NRM

Agriculture

Livestock manures and other organic materials are a rich source of nutrients and organic matter. However, managing them within a sustainable system can be easier said than done.

The nutrients contained within livestock and organic manures aren’t homogenous in nature and that introduces the risk of under or over supply, particularly if we always rely on standard figures. Despite some uncertainty, organic inputs help to close the nutrient gap that arises when crops are grown and removed from the field—particularly when nutrient levels are determined through laboratory analysis. This informed use helps to maintain resources within the farming cycle.

Sustainable use of organic materials

The recycling of livestock and other organic materials is central to a more sustainable food system and land management approach. Their better use leads to a smaller carbon footprint through a reduced need for inorganic fertiliser, and their return to the soil has a myriad of soil health benefits.

Over time, farms have become more specialist and enterprise specific, moving away from the mixed farm blueprint, a system that relied on livestock recycling nutrients across the arable rotation. This shift away from the mixed farm approach has created a geographical split, where the eastern intense crop growing regions need all the benefits manures bring and the livestock intense western regions are having too much of a good thing with nature receiving the excesses, whether it likes it or not!

The challenge of its physical distribution—getting it to land which needs it—comes with a financial cost that generally has been tricky to square. In some cases, this might now stack up due to the increased cost of inorganic fertilisers.

Thankfully, mixed farming is once again receiving the attention it deserves but under a new guise: regenerative agriculture.

The regenerative approach to growing crops whilst also raising livestock is gaining mainstream momentum, and the use of organic materials is central to its success and ambition. We are beginning to see grass leys and animals return to parts of the country where they have been absent for decades, and measuring the impact of a system change is important to understand. Analytical measurement plays an important part in the sustainability of agriculture, and this blog is all about encouraging the analysis of livestock and other manures to ensure their sustainable use. The data that follows explains why measuring these materials is a good investment and the basis of sound nutrient and farm management.

NRM data analysis

It’s an interesting exercise to review how well RB209 standard values from cattle slurry correlate to analytical results being measured in the laboratory. On the whole, the average values published match reasonably well with the average content measured. There are, however, key differences between the average measured data and the standard values. Those underlined in the chart below show which measured values are significantly different to RB209 standard values.

On average, measured cattle slurry contains slightly higher total nitrogen and magnesium values, but the same or similar phosphate levels. The measured potash values do, however, vary significantly from the standard levels.

The data also shows that, as the dry matter content of the slurry increases, the nutrient content also increases. This trend is clear in the measured and the standard values.

Practically, it’s important to recognise how the difference in the measured and standard values impact the supply of nutrients. Relying on standard values might mean that you are over or undersupplying nutrients, and the only way to be sure is through analysis.

The table below highlights the supply of nutrients based on the standard and measured values described in the table above. At the application rate of 50m3/ha (4500g/ac), you would be supplying around 15% more total nitrogen, the same amount of phosphate, and 39% more potash and 3% more magnesium compared with standard values.

Mean nutrient composition of cattle slurry compared to standard figures (kg/50m³)

In this example, by relying on standard values, you risk inadvertently breaking NVZ rules by contravening the 250kg N/ha rolling field limit. Based on the standard value that a 6% dry matter (DM) slurry contains, the maximum application volume that can be applied to ensure you do not break the field limit rule is 96m³. On the other hand, based on the measured value that a 6% dry matter slurry contains, the maximum volume reduces to 84m³. This is important to recognise in terms of protecting the water and wider environment.

This comparison was based on comparing a measured average against standard nutrient values and, whilst it provides some insight into the differences, we all recognise that averages mask variation. Measuring enables us to reduce the uncertainty of nutrient supply, helps to better match nutrient supply to crop demand, and allows us to keep hold of resources that have an important fertiliser replacement value.

I’ll be continuing this discussion on standard values versus laboratory analysis and the importance of measuring nutrient levels in slurries in part 2 of my blog, which will be published very soon.