What’s best to compact the clamp – big tractor tyres or train wheels?
The importance of good consolidation is not underestimated by most silage producers. Previously in this series I explored the importance of clamp consolidation but it’s worth looking at this subject in more detail because it has such a huge influence on the losses of your silage. Forage consolidation is critical to all good silage production but it’s especially critical for silage that’s more susceptible to aerobic instability – so silage with high dry matters and silage that’s going to be fed out during the summer months. Good consolidation isn’t just a result of the weight of the tractor driving on top of the clamp; it depends on what is actually in contact with the forage itself.
Tyres are under pressure
In recent years, manufacturers have worked miracles to produce tyres that can operate at full load capacity with air pressures that would have once only been confirmation of a puncture! This is in response to the call to reduce soil damage and compacting caused by farm traffic. Wider tyres and reduced pressures mean today’s bigger tractors still tread lightly – and that’s a problem when it comes to the clamp.
Surely more weight is better?
Not quite, more weight might be the answer – so long as the clamp can take the load – but more weight might really have no benefit whatsoever if it’s on LGP tyres. That’s because what we really need is more pressure, not more weight. The silage to be compacted only really feels the pressure, not the weight.
The old adage was always that the ground experienced the same pressure as that in the tyre (Newton’s third law of reaction between two bodies - every force experiences an equal and opposite reaction etc). It’s not quite that simple because the tyre has some integral strength even if it has no air in it at all. The loads under the edges of the tyre tend to be a bit higher than the centre but effectively the pressure on the silage will be roughly the pressure on the tyre gauge. So a 12t tractor with tyres at 0.7 bar (or 10 psi) will not be as effective on the clamp as an 8t tractor running tyres at say 1 bar (15 psi)
Tracks better than tyres?
Tracks are therefore actually worse than tyres when it comes to consolidating silage on the clamp. Not only is the pressure under the track much lower than under a tyre, but the track has a high velocity at the smaller end wheel that means it tends to pull up or flick the silage surface. That’s not to say tracks are a complete no go on the clamp; the repurposed ski piste basher PistenBully machines are specialist clamp filling tools that can move mountains with ease and safety. But despite their weight they are not specialist compaction machines.
Good vibrations
As every builder will know, the solution to compaction is vibration – isn’t it? For stone and sand and most soil types this rule holds true but for silage the internal friction of the forage means it hardly moves with vibrations. So good consolidation comes down to pure pressure, and that’s where train wheels come in. Just like flat or Cambridge rollers, steel (or some other non compressible material) will result in higher under roller pressures. Train wheels on a frame can do that on silage without the risk of clogging and blocking between the rings. It’s also why the consolidation behind a 4 tonne set of train wheels is so much better than the that behind a 12t tractor that’s three times the weight. What a specialist compactor such a train wheels can do is keep your overall compaction vehicle weight within the design of the walls whilst giving you the best possible compaction.
Choosing your tractor to roll the clamp
So train wheels might be very desirable but if you don’t have the option at hand, then choose the tractor carefully to make the most of your efforts. Biggest isn’t always best and if you have a choice, then go for the one with the narrower tyres at higher pressures. Of course you need to make sure the tyres are clean and the tractor is leak proof as silage is ruined by hydraulic oil.
And if you’re using a contractor to fill and compact the clamp then discuss options for compaction, if the demand is there then maybe they can offer you a solution. For those high risk silage clamps, the benefits in reduced losses at feed out should make good consolidation one of your top priorities.
If you want to discuss how to improve the aerobic stability with better consolidation or to discuss any of the other aspects of silage making covered in this series – contact Jeremy Nash at Jeremy@silageconsultant.co.uk