Turning organic nutrients into measurable value ahead of third-cut silage

Author: Alli Arden

14th July 2026

NRM

Agriculture

With first and second-cut silage now complete across the UK, attention quickly turns to building strong regrowth for third-cut silage. There is a critical window – as soon as the grass has been cut – where the effective use of slurry and livestock manures can significantly influence both yield and forage quality.

Rather than being viewed as a disposal exercise, organic manures should be treated as valuable, measurable nutrient resources. At a time of sustained fertiliser price volatility, making the most of nutrients already on farm is one of the best ways to protect margins.

Maximising the third-cut silage opportunity

Third-cut silage is typically taken 4-6 weeks after the second cut (depending on the weather and grass growth), meaning the nutrient demand from grass is both immediate and responsive.

Applying slurry or manure as soon as possible after the preceding cut allows nutrients removed in the harvested crop to be replaced, encourages rapid (and even) regrowth, and maintains the yield and quality potential of the next cut. Delays in application reduce the effective regrowth period and limit the crop’s ability to respond, ultimately impacting both output and forage quality.

From waste to resource: The importance of analysis

Livestock slurries and manures vary widely in nutrient content and depend on:

1. The type of livestock and their diet
2. Whether the slurry is diluted with rain or yard water
3. How it has been stored
4. How and when it is applied.

This variability presents both a risk and an opportunity:

• Risk – incorrect application rates, wasted nutrients, poor regrowth, and environmental impact
• Opportunity – significant fertiliser replacement value when properly quantified.

Laboratory analysis provides clarity on total and plant-available nitrogen (N), phosphate (P) and potash (K), as well as secondary nutrients and trace elements.

With accurate data, slurry becomes a predictable and manageable nutrient input, enabling:

• Accurate application rates
• Better field targeting
• Precise fertiliser top-ups for second cut.

Matching nutrient supply to third-cut silage demand

Third-cut silage still has significant requirements for N, P and K, depending on soil indices and the grass’s capacity to grow. On average, 6% dry matter cattle slurry contains approximately:

• 2.6 kg N/m³
• 1.2 kg P/m³
• 2.5 kg K/m³
• 0.80kg MgO/m³

The table below compares the standard nutrient values with those routinely measured by NRM. There are differences in the average measured content, which is why it is advisable to measure organic inputs before applications are made throughout the spring and summer.

These total nutrients can supply a substantial proportion of third-cut requirements, but only when nutrient content is known, applications are better targeted, and when shortfalls are topped up with fertiliser.

A more analytical approach allows you to develop a more effective nutrient management plan that focuses on the benefits of organic inputs and puts them to best effect. It does this by:

• Calculating the nutrients supplied via slurry application,
• Highlighting the comparison between third-cut demands and the nutrients that slurry or manure can supply,
• Encouraging strategic fertiliser applications rather than applying at the same rate across the entire cutting ground, and
• Informing which fertiliser analysis (ratio of N, P, K, S) should be applied.

Managing P & K in silage systems

The third cut is another opportunity to address nutrient imbalances that carry over through the season. Silage systems remove significantly more potassium than phosphorus:

• Typical grass silage (25% DM) removal:
  • 0 kg/t (K₂O)
  • 7 kg/t (P₂O₅)

This can create a common imbalance, particularly if you are not applying the correct fertiliser type to address it. It can result in:

• Declining soil K indices, limiting yield and quality
• Rising soil P indices, increasing environmental risk

The non-targeting of slurry applications can worsen this issue because it supplies more phosphorus (P) than potassium (K) relative to removal. A targeted approach is crucial. Always prioritise applying slurry to fields that need both P and K, avoid excessive P on high-index soils, and add potash where slurry cannot satisfy the demand.

Maximising nitrogen efficiency in summer conditions

Nitrogen from slurry is very valuable, but it’s also prone to loss. For silage, the efficiency of uptake depends largely on the timing and method of application. Applications in late spring and early summer align with high grass growth rates and strong nitrogen uptake and utilisation. Grass growth rates begin to decline as the season progresses, but using low-emission systems, such as a trailing shoe, hose, or shallow injection, can significantly improve nitrogen availability and uptake by reducing ammonia-N losses.

Matching application timing to soil type can help to improve uptake and minimise environmental losses.

• Light soils: higher leaching risk → apply little and often and at lower volumes
• Heavy soils: slower nutrient release → avoid applications in wet conditions
• Medium soils: generally easier to manage and provide a consistent response

Delivering measurable return from third-cut silage applications

Organic manures represent a major financial asset at the farm level. Across the UK, around 90 million tonnes of livestock manure are applied annually. Previous estimates of the fertiliser replacement value were circa £150 million, but at today’s prices could be more like £400–£500 million per year.

This is a good reason to better manage slurry applications ahead of grass cutting to enable:

• A reduction in fertiliser requirements
• Improved nutrient use efficiency
• Improved yield and grass quality.

However, this value can only be fully realised when nutrients are regularly analysed, applications match crop and soil demand, and fertiliser is used to fine-tune rather than add to and exceed nutrient supply unnecessarily.

A smarter approach

Cutting mid- and late summer is not just about supporting regrowth and quality; it’s also an opportunity to reset and rebalance nutrient management mid-season.

A data-driven approach should include:

• Regular slurry/manure analysis ahead of application
• Targeted fertiliser use based on crop and soil needs
• Ongoing soil testing (e.g. 20–25% annually) to check the increase or decrease of soil indices.

This ensures nutrients are used efficiently, supporting both grass performance and cost of production.

NRM’s expert insight: unlocking the full value of manures

While nitrogen often gets the most attention, phosphate and potash typically account for 40-60% of the total fertiliser value of livestock manures, depending on manure type, nutrient content and application efficiency. Yet this is frequently overlooked, probably because nutrient content varies and there are concerns about nutrient availability. The lack of regular analysis makes accurate accounting of the nutrient supply difficult.

However, analysing slurry third cut applications helps to:

• Reduce unnecessary fertiliser applications
• Provide quantifiable nutrient analysis data
• Reduce unnecessary fertiliser spend
• Enable better-informed nutrient decisions.

How NRM can help

Slurry and manure are not just by-products – they are valuable, measurable inputs that can directly influence grass management and animal performance. By analysing organic materials before application, you can:

• Unlock fertiliser value already on farm
• Optimise silage yields and quality
• Improve environmental performance
• Protect margins in a challenging cost environment.

Ultimately, in today’s economic climate, making better use of organic nutrients is essential.