Time is ticking to get everything in place for first-cut silage. When it comes to additives, compacting in the clamp, sheeting and weighting- down we have got you covered. But, what else can we do at this stage to set up our swards to get the most out of them? We get some top tips from Dr George Fisher.

At this stage of the year, we have to accept what we can and cannot fit in before first cut time is upon us. As we go through the thought process, we can park some issues to come back to as we highlight some ‘think ahead’ things, as well focus on some immediate actions.


Whilst we want compaction when it comes to getting the first cut into the clamp, compacted soils will reduce grass growth. This has been proven for the first cut by research at SRUC, Crichton Royal Farm. This showed that compaction caused by wheeling and trampling can reduce yields by 25% and 14.5% respectively (see chart on right).

Soil compaction reduces first-cut grass yields (SRUC, Crichton Royal Farm research (funded - AHDB Dairy)

Wheeled compaction is deeper down in the topsoil profile and it is too late do anything about that now, before first cut. Compaction from trampling or poaching affects the first few inches of the profile so the impact of this can be alleviated with spike aeration. The way to assess the situation, which might have been made worse by the heavy rainfall that we’ve had this winter, is to get your spade out and see how compacted the top of the soil profile is. If it’s difficult to separate the soil in this top layer, then spike aeration may help.

The trick is to make sure that the spikes get below the level of surface compaction so water can flow through the restricting layer and air can get in to feed the soil microbes. This will enhance the value of your slurries and purchased fertilisers. This can be done at the same time as we would traditionally roll a silage field to reduce soil contamination in the crop.

There is limited evidence that spike aeration gives yield results straight away, but alleviating that compaction will mean that the whole soil system works more effectively and should improve the health of your silage field soils if they are capped.

THINKING AHEAD – If you are worried about wheeled compaction, which is typically at least 15cm (8 inches) down the profile of the topsoil, then you can use a sward lifter to help alleviate this. However, sward lifting is an invasive process from the viewpoint of a grass plant, so it should be done in the autumn. If this is an issue for you, reassess the situation as we come out of summer and into autumn this year.

2. Nitrogen – apply the right product, at the right rate, at the right time.

This is obviously now a more expensive job if we are dependent on purchased fertiliser nitrogen, but there is no getting away from the fact that modern varieties of grass will give us 30kg to 50kg of grass dry matter back over the season for each 1kg of nitrogen fertiliser that is applied. It is clear the return on investment is there, even at high and volatile prices.

But the current (and likely to last) fertiliser price environment means that we need to be as efficient as possible with purchased nitrogen. This means:

◆ Product – Use a product that contains ammonium nitrate or inhibited urea – don’t use straight urea, even in spring. Our springs are now drier and warmer: conditions that mean there is a high chance of losing nitrogen from straight urea by volatilisation of ammonia into the atmosphere. This is not just bad for air quality but is equivalent to throwing your cash away in the wind.

◆ Rate – Apply up to 2kg nitrogen per day, per hectare, between first application and first cut. It’s an old calculation, but it still works. This means that if you have a productive sward that will be cut in the first week of May and you can get on with the first nitrogen application in February, you can go to the Fertiliser Manual maximum of 120kg N/ha for first cut, but remember that this includes the available nitrogen from your slurry that has been applied between last autumn and this spring.

◆ Time – The old approach of T-sum 200 (accumulated air temperature from 1st January to 200oC as an indicator of when to first apply nitrogen) is now really dead due to climate change (we reach T-sum 200 before it’s dry nough to get on with a tractor and fertiliser spreader). So, just go on as soon as ground conditions allow and soil temperature at 10cm depth is steady at 60C. An equally important thing is to split your nitrogen applications by 3 to 4 weeks, particularly if you are applying over 80kg N/ha – this will increase the efficiency of nitrogen uptake the grass, which will benefit yield and crude protein content.

THINKING AHEAD – We need to ask strategic questions about our nitrogen supply with prices set to remain high and volatile, at least over the medium-term. We can think about how to use slurry nitrogen more efficiently (see below) and consider how we can get clovers into our system to replace purchased fertiliser nitrogen. This takes careful planning and a change of mindset, so one for later in the year.

Injecting slurry on John Buckley’s farm in the Tanat Valley on the Powys/Shropshire border.

3. Account for your slurry nutrients.

Leading on from nitrogen, we need to account for the nutrients (N, P, K and S) available for first-cut grass growth from our slurry applications. This is important to save costs and make sure that the grass has what it needs to optimise growth and quality.

The starting point is to get your slurry analysed so that you know what it contains. Then attention should turn to getting the most out of the nitrogen it contains by applying as much as possible within season and using a trailing shoe or injection application, rather than a splash plate.

The table above gives an idea of available nutrients for our first cut crop from standard slurry applications.

* Numbers in the table relate to a ‘typical’ 6% dry matter dairy cattle slurry. ** Nutrients required for first cut, for a sward that will yield over 12t DM/ha over the season, and has soil fertility indexes of 2 for P and 2+ for K. *** 30m3/ha slurry is 2,700 gallons/acre.

It’s important to get the slurry analysed as the book values used above are averages and are only accurate enough for about 30% of farm situations. Average numbers are by their nature just that – averages, and few farms will actually be exactly ‘average’. Your slurry might have higher nutrient contents, which will save more on the purchased fertiliser bill. Or it might have fewer nutrients, in which case we need to make sure that enough is going on to optimise yield and quality for first cut.

THINKING AHEAD – It’s worth considering using a slurry additive that contains bacteria which will enhance the available nitrogen content of your slurry, make it more consistent and easier to work with, and reduce the smell.

Digest-It from Kelvin Cave will do this. Data from research by Kingshay Dairy using Digest-It suggests a 17% increase in the nitrogen content of slurry following treatment.

In the example above, this would mean an extra 9kg N/ha off the need for purchased fertiliser.

4. Don’t forget sulphur.

It is now an old story that we need to replace the sulphur that used to fall out of the atmosphere for free from industrial pollution. The 70 – 80kg SO3/ha that used to arrive by this route is now down to less than 5kg SO3/ha and we have been mining sulphur out of our soil organic matter in the last three decades.


The data on the right comes from a three year replicated trial in Cheshire where 120kg N/ha and 53kg SO3/ha were applied to first-cut silage. The sward was on a medium loam and reseeded a year before the trial started. The average first cut response was an extra 1.2t DM/ha.

The effect of sulphur applications on grass yield at first c CF Fertilisers UK).

Because sulphur is a central part of two ‘essential’ amino acids (the building blocks of proteins), and because proteins are involved as enzymes in the accumulation of sugar within the grass plant, we often get a lift in protein and sugar content when applying sulphur to a deficient situation. In addition to the yield response, this potentially improves the silage fermentation process and the quality of crude protein available to feed out.

THINKING AHEAD – It’s tempting to save on the purchased fertiliser bill by buying straight nitrogen, without sulphur. This is a false economy. The value of the grass yield and quality that you will lose is far greater than the cost saving on the fertiliser. However, there are different options for getting sulphur into the crop nutrient system, not just in conjunction with the nitrogen fertiliser, including applying gypsum (calcium sulphate) and polysulphate (polyhalite).

5. Weeds take grass yield from the clamp.

The perennial problem of dock control is a real headache for those of us who have them on-farm, and some recently published research from Teagasc (2022) brings us some up to date evidence on how to address the issue. Conducted over two sites and four years, the trials looked at the effectiveness of various products that can be sprayed on new leys and established swards. The Teagasc workers came up with the relationship that:

Grass seasonal yield = 11.2t DM/ha – (1.0 x dock yield t DM/ha).

So, basically, for every one tonne of docks you grow, you lose one tonne of grass yield. This pretty much echoes

and confirms the research from SRUC, done decades ago, which showed that for every 10% ground cover of docks you have in a grass field, you lose 10% of the grass yield. And docks make your grass more difficult to ensile, particularly if they have gone stemmy before you make the first cut.

THINKING AHEAD – The Teagasc trials clearly showed that treating new leys for docks was significantly more effective at controlling dock levels than treating established leys. So, if you are reseeding in 2023 and have docks on the farm, make sure you treat the new leys for docks as this will give you the biggest return on your investment in the spray.

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