Those of you who commonly read the instruction manuals of a new to you machine, may well be familiar with the “adjust the settings to achieve the correct height / depth / width / length etc” advice. Some might suggest this type of statement is of little real help and may actually have led many of us down the road to “ignore the instruction manuals and just crack on”. This route usually ensures the manual remains untroubled until something doesn’t work any more!

So the instructions might tell you exactly how to adjust the height of cut of a machine, but not the height it should be cutting. This begs the obvious question…….

What is the correct height of cut for silage?

Be it grass, whole crop or maize, the obvious answer to this question is cut all of it – as much as possible. For grass you need to consider the speed of regrowth but this is not a concern for whole crop or maize where the cutter should be set as low as possible. Obviously you need to make sure the header doesn’t hit the ground and suck up soil and stones into the silage but that’s all we need to worry about isn’t it?

Maybe it’s a bit more complex than this; in maize it’s important to make sure the header is actually cutting the stalks and not plucking them out of the ground. This is particularly important on sandy soils where the plant roots are more readily parted with the earth. As we all know soil contamination is a massive “No No” in silage, but there are other bits and pieces you want to leave behind in the field rather than take back to the clamp.

The dead and dying leaves around the base of a growing “grass” plant are a case in point This growth pattern is common to both grasses and cereals used for whole crop silage but these decaying leaves can introduce all sorts of undesirable bacteria and moulds that will massively increase the in clamp losses due to poor fermentation. The obvious solution is to set the cutting height a little higher, but it seems crazy to be leaving valuable crop behind in the field!

What is the cost of increasing the cutting height?

So let’s consider the cost of increasing the cutting height a little. Let’s consider a first or second cut of Italian ryegrass. In good conditions, this should be yielding 1.2t of dry matter per acre – or about 3t of DM per hectare. At cutting the crop might be standing two and a half to three feet tall. We can conservatively assume the total plant length of at least 700mm. The crop is therefore yielding at about 1.714kg DM/mm per acre – if that makes any sense. 

Trying to eliminate those undesirable leaves by lifting the cutting height by 5mm will leave 8.6kg of crop dry matter per acre (21.2kg/Ha) in the field. Increasing cutting by 10mm will leave 17.1kg DM (42.4kg/Ha) behind. These are not massive figures but what sort of losses would need to be saved in the clamp to offset what you are leaving in the field?

For a 10mm increase in cutting height you would need a decrease in the clamp losses of about 1.42%. Now if your in clamp losses are below 10%, then it’s pretty unlikely you can get this sort of saving so it’s reasonable to assume your height of cut is already about spot on. For just about everyone else, a 1.5% reduction in losses is really easily achieved, so it’s worth a close look at the crop before it’s cut. 

How to decide the correct cutting height

Take a tape measure, identify level of the dead and dying leaves then measure from the top of these (usually where they attach to the stem) to the ground. Do a few of these measurements across the area and get a feel for the correct height of cut. Different grass species will have different growing characteristics so have a look across all the crops in particular older permanent pasture as this has much different growth pattern to silage leys.

Once you have a target height of cut, set up the mower or header up on a level surface. You might need to adjust cutting height for different fields - this might be a pain but it really is worth the bother. If you are using a contractor, make sure the boss knows your required cutting height, and that the driver knows too. The mower man (or woman) might moan a bit but with modern kit these adjustments can often be done without even leaving the cab.  On initial hearing taking this much effort over cutting height may seem a bit obsessive, but as with everything that really matters, attention to detail is key. And there is nothing that matters more than the quality of your silage.

If you want to discuss the value of correct cutting height or any of the other aspects of silage making covered in this series – contact Jeremy Nash @ jeremynash1@btinternet.com