High quality forages with a low fibre and dry matter content are more prone to slippage after ensiling, especially following poor consolidation when filling the clamp.

Speaking at an industry webinar hosted by Alltech, Dr Dave Davies of Silage Solutions outlined the common causes associated with clamp slippage as well as ways to mitigate the risks.

“Forage density should increase as you move downwards from the top of the clamp to the bottom, as this provides a good foundation to build on” said Davies.

“On farms where slippage has been a problem, the issue is often down to lower forage densities in the bottom half of the clamp.

This becomes a problem when silage is then filled too high, as there is too much weight on top of an unstable base.

Depending on the clamp size, as little as 1 metre in grass height can be as much as 1 tonne pressing on the weak point in the clamp.”

He recommended that for grass silage, the clamp density should be at least 750kg/m3 of dry matter or 250kg/m3 fresh weight.

Density can be measured by coring the pit in a vertical line from the top of the clamp to the base, then dividing the weight of the sample by its volume.

Larger clamps

After visiting NI farms that experienced issues with clamp slippage this winter, silage density was within the optimum target range, as was the average forage dry matter.

This led Davies to suspect that wider pits are being overfilled with low fibre grass, causing them to become unstable and more prone to slippage.

As the side walls of the clamp provide friction that holds ensiled grass in place, when the pit gets wider, this friction effect is reduced.

This increases the risk of slippage towards the centre of the clamp, where grass is usually filled to its highest point.

Rain water leaking down the inside of the clamp wall was also a factor in reducing the friction effect and again increased the risk of slippage.

Chop length

Farmers should liaise with their contractor about chop length, as shorter grass is harder to consolidate and more likely to result in slippage after ensiling.

“For grass silage at 22% dry matter, chop length should be around 10cm. At 23% to 28%, grass should be at least 8cm long. This can be reduced to 5cm for forages around 30%.

"Low dry matter forages are more prone to slippage, so longer chop length is recommended to help with consolidation when filling the clamp.

"A chop length of at least 8cm is vital when ensiling short, leafy grass with an NDF (fibre) content below 45%” said Davies.

Dry matter

He went on to advise farmers that wilting grass to 30% dry matter before ensiling was the best compromise between maintaining forage quality and reducing the likelihood of clamp slippage.

At the outlined dry matter content, grass should be filled in layers around 15cm deep. But when working with wetter silages with dry matter below 25%, the depth of these layers should increase to 25cm.

Filling the clamp

When it comes to filling the clamp, the angle at which grass is ramped in the pit is a huge factor in getting grass rolled consistently.

“Keep the angle of the ramp below 20° where possible, but reduce this angle to 10° if you have regularly experience slippage problems.

"When filling the clamp at an angle greater than 20°, the gravitational drag effect on the tractor comes in to play”.

The first load of grass going in to the clamp should be handled the same as the last load

This means the force pulling the tractor back towards the ground is greater than the force of the tractor compressing grass.

This will result in poor consolidation and greater potential for cracks to appear in the clamp, thereby increasing the chances of slippage.

Davies also advised farmers to follow the same routine throughout the ensiling process. “The first load of grass going in to the clamp should be handled the same as the last load. This means if you start rolling grass with one tractor, then continue to roll with one tractor.

"Any change to the ensiling process will lead to inconsistencies and therefore, create variations in clamp density.”

Offsetting higher concentrate costs with high quality silage

The prospect of concentrate rations costing over £400/t should focus farmer’s minds on the role of high quality silage this summer according to Davies.

“Given the cost of fertiliser, I would have concerns over the protein levels in silages harvested this year as farmers look to reduce nitrogen applications.

Any reduction in crude protein will mean a greater reliance on purchased feed to maintain animal performance.

I would be posing the question to farmers that with cost of rations on the rise, can you afford not to use fertiliser this year.

Any reduction in nitrogen will reduce grass yields, meaning less forage is harvested per acre. Most contractors charge by the acre, therefore making grass silage more expensive on a per tonne basis” said Davies.

Energy increase

Higher feed values in silages made under a multi-cut system will outweigh the increase in contractor costs according to Davies.

He outlined an example whereby a farmer moves from a three cut to a five cut system and raised D-Value by 3% along with energy increasing by by 0.5kg/DM ME.

Energy yielded in grass totalled 18,582 Mj/ha and assuming 5Mj of ME per litre of milk produced, the five cut system supported an additional 3,506 litres of milk/ha farmed.

Using a hypothetical base milk price of 38p/l to reflect current markets, this is potentially worth an additional £1,332/ha.

In terms of beef cattle, the higher energy content would increase weight gain per hectare by 465kg of liveweight, worth £1,162/ha at 250p/kg.

Silage crude protein was also increased in the same example, with the concentrate input reduced by 3t of a 20% ration. Based on a current ration costing £380/t, this equates to a £1,140 saving.


Farmers watching the webinar were advised to spread grass straight after cutting to raise the dry matter of the sward.

“Mow in the morning once dew has lifted as the afternoon offers the best period for wilting grass. In the right conditions, grass will be 28% to 30% dry by the same evening and should be lifted at this point.

There is very little wilting benefit by leaving this grass on the ground overnight. If grass is mowed in the afternoon, keep the wilting period to a maximum of 24 hours.

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