Silage Advice

Cutting grass silage

When should first-cut grass silage be taken?

There is no fixed time for taking first-cut. It will depend on the type of grass, the weather, and whether you are looking for quality or quantity, etc. Past experience is probably the most valuable guide using stage of growth as your main indicator: for best quality, it is important to cut before heading.

What time of day is best to cut grass for silage?

Although sugars tend to be a bit higher in the afternoon, the difference is not usually very great and should not delay the decision to cut.

Far more important is cutting at a time of day that will allow the crop to be wilted as quickly as possible to the target 28-32% dry matter content (see later). This is important in order to minimise the in-field losses that occur during wilting due to the crop continuing to respire and the action of undesirable microorganisms. If conditions are suitable (e.g. weather conditions, the volume of the crop, and having suitable tedding machinery to speed up wilting) it may be possible to cut the crop in the morning and achieve 28-32% dry matter that same day.

Leaving a cut crop in the field overnight further increases in-field losses and overnight dews can cause the moisture content of the crop increase.

If surface water is present on the crop, this will evaporate faster from a standing crop. However, cut as soon as possible after the crop has dried.

Should grass be analysed before cutting for silage?

This is essential if you believe there may be a problem e.g. high nitrates remaining in the crop from fertiliser application. It will also provide information regarding the ensilability of the crop, which will also aid decisions of when to cut. It is important to get results back within 24 hours for them to be useful as grass composition can change rapidly.

Why are high nitrate levels when cutting grass for silage a problem?

High nitrate levels at harvest (>1000ppm or 0.1%) are an indication that the fertiliser nitrogen applied has not all been converted to protein. This can occur if too much nitrogen (N) fertiliser is applied or if it is applied too near to harvest. It can also occur when there is rain after a prolonged dry spell which will cause rapid N uptake.

Such crops will also have low sugar levels and a higher buffering capacity. Low sugar, high non-protein nitrogen and a high buffering capacity all increase the likelihood of a poor silage fermentation. There is also the risk of toxic silo gas being produced.

How to avoid high nitrate levels in grass silage?

The best way to avoid high nitrate levels in grass is to get the nitrogen fertiliser application rate correct in the first place. However, prolonged periods of dry weather followed by rain, resulting in rapid nitrogen uptake by the plant, cannot be anticipated and can ruin even the best made plans. 

If high nitrates are suspected, be sure to have a representative sample of the grass analysed. If the nitrate level is above 1,000 ppm (0.1%) it should not be ensiled. Instead, wait a few days and analyse again.

If there is some concern that nitrate levels are high, ensiling at a higher % dry matter will help to reduce the risk of a poor fermentation as will application of a silage inoculant designed to make the initial fermentation faster and more efficient

What growth stage should grass be cut for silage?

This will depend on the performance you expect from the silage. The younger and leafier the crop when cut, the higher the D value will be – as after heading the digestibility of grass falls by about 0.5% per day (and a reduction of 3.6 D units requires an extra 1.5 kg of concentrate to be fed to a dairy cow per day). Grass cut after heading also becomes more difficult to consolidate as it gets more stemmy, which can create problems with both fermentation and heating.

What height should grass be cut for silage?

It is advisable to set the mower to leave at least 7-8 cm (3 inches) of stubble. Although cutting lower would increase the yield, the lower material is of poor nutritive value and there is a greater risk of soil contamination. Cutting too low also delays regrowth as it removes the tiller and leaf buds and may even kill some plants.

Leaving a longer stubble can also aid drying as the cut grass is raised higher off the ground, aiding air circulation.

How long an interval should be left between grass silage cuts?

In the UK, five to six weeks is usually required for regrowth if the traditional approach of taking three or four, high-yielding cuts is followed. For improved quality, consider cutting more often (e.g. using a multi-cut approach), which will obviously require shorter intervals.

Although individual cuts with a multi-cut approach will be lighter, the overall yield for the season can actually work out higher than taking fewer cuts (see multi-cut section) as cutting a younger crop means less delay in regrowth. Lighter cuts are also easier to wilt and cutting more often can offer several other advantages compared with cutting at traditional timings, such as improved contractor availability.