The length of chop has a huge impact on the quality of the silage you make. There are many benefits from chopping the material but that forager is a costly animal that has a mighty appetite for fuel – so what are we trying to achieve?

• Chopping the material means we can get more into each trailer and reduce the number of loads – and therefore the costs for each tonne harvested.

• Chopping also makes consolidation at the clamp easier and more consistent but these are possibly the least important aspects to consider.

• Chop length has a big impact on the possible losses of feed  value once the clamp has been opened at the feeding stage.

• Chop length also has a huge role to play in the speed and effectiveness of the ensiling process. This is perhaps the most important benefit of chopping the material.

So the benefits are clear but how long or short should you chop?

As ever, there is no simple answer to this and there are implications for going either side of the optimum – so we need to consider a couple of things. Firstly we need to consider what is going to happen to the silage once it’s been fed. Cows have teeth and anaerobic digestion tanks do not. As a general rule, you can feed longer material to cows than you can to tanks.

Secondly  – and most importantly – the dry matter percentage of the material being harvested is the defining factor.

The drier the crop is, the shorter it needs to be chopped to ensure rapid effective ensiling to lock in the feed value. Chopping low DM silage too short will increase the effluent losses and you can watch the sugars just run down the drain. Chopping high DM silage just short enough will expose the bacteria to enough of the plant cells to let them do their work and produce the acids to pickle and protect the crop.

 As a general rule of thumb, for grass silage at 30% DM or above, chop lengths of around 15mm – 25mm should be targeted. Where dry matter falls between 20% and 30% we should be aiming for 25mm – 50mm and for silage below 20% DM then 50mm – 100mm. These figures are dependant on other factors such as grass species but are a reasonable guide.

Maize silage should be harvested at higher dry matter than grasses, typically 28-35%. The chop length again needs to be reduced as dry matter increases, but the range will be much shorter at 10-20mm typically for cattle feed. For AD plants, the chop length needs to but much shorter still – in the region of 4-6mm to increase the surface area. In all cases with maize silage, kernel processing is essential at harvest but this will be covered in another piece.

So why don’t we just chop everything really short?

Well as stated earlier, it burns a whole load of diesel and slows down the harvest process. Chopping too short also increases losses from the clamp and can also make the whole clamp really unstable and liable to slippage. Although this is thankfully rare, it is extremely dangerous and can lead to losses of crop and potential pollution incidents. Chopping wet material really short then compacting too tightly can result in an impervious soggy mat that is unstable and has little feed value.

So how do you control the chop length?

There are only two factors that can alter the chop length in a forage harvester, the speed of the feed rolls and the number of knives on the drum. Altering the forward speed of the machine makes absolutely no difference to the length of chop. To reduce the chop length the driver will need to reduce the speed of the feed rolls feeding the crop to the cutter head. This will reduce the foragers’ appetite, however drivers in a hurry are always tempted to speed up the feed so they can cover more ground. The only alternative is to fit more knives to the drum at the cutting head. Often referred to as Biogas drums or cutters, these can have up to 48 knives to make the most of the 1000+bhp engines.

Technology improvement can take the guesswork out of the equation. John Deere’s HarvestLab was one of the first uses of near-infrared (NIR) spectroscopy to perform real time constant crop analysis on a forage harvester. The system can very accurately measure the crop yield and dry matter (amongst other things) in real time, and store the data. Claas, Krone and New Holland all now offer NIR sensing on their foragers, using the dry matter information to make automatic adjustments of chop length throughout the day and the entire harvest.

For further information on the importance of chop length or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com