Wilting grass silage

Why should grass be wilted for silage?

Wilting has a number of advantages:

• It raises the % dry matter (DM) of the material, which means less water needs to be transported from field to clamp, speeding up that stage of the process and reducing the size of clamp required
• Effluent is reduced; almost completely above 30% DM – something which is especially important with tighter regulations
• It reduces the amount of fermentation required to produce a stable silage so less sugars are required
• It reduces overall protein breakdown – provided it is carried out rapidly
• It reduces the chances of a poor fermentation as some of the microorganisms responsible for bad silage will not survive in higher dry matter silages
• It increases intakes – although production will not necessarily increase by as much

However, it is important not to over-wilt as this increases in-field nutrient losses and higher %DM silages are more susceptible to aerobic spoilage (heating).

What % dry matter (DM) should grass ideally be wilted to?

In normal situations, wilting to 28-32% DM gives the best balance between minimising effluent and minimising field and clamp losses, but only if this can be achieved within 24 hours, and preferably 12 hours or even less.

Dry matters above this may well lead to higher DM intakes but not necessarily to higher animal production. Field DM losses will also be higher and the risk of aerobic spoilage will be significantly increased.

If you suspect the grass may become too dry at harvest, consider leaving a field and pick it up ‘fresh’ to help put some weight on the top of the clamp to help to seal it.

How to wilt grass for silage?

As soon as the crop is cut it starts to deteriorate. Sugars released during mowing will be used by a wide variety of bacteria that can grow in the presence of air. In addition, the grass itself will use up sugar as it continues to respire. Both of these will reduce the sugars available for fermentation. In addition, proteins start to be broken down.

Yeasts and moulds can also proliferate in the dying plant material, increasing the risk of aerobic spoilage later.

For all these reasons, whenever grass is being wilted, it is important to do so as rapidly as possible.

To reduce wilting time, make effective use of mower-conditioners and do not leave cut grass in the swath. Instead, spread it to 100% ground cover as soon as possible by tedding. Grass should be cut in the morning with a mower conditioner and tedded out within 2 hours to allow quick wilting.

This is the time frame that the stomata (leaf pores) are open, so tedding needs to be quick to maximise the moisture loss from stomata. By doing this, it may be possible to reach the optimum 30% DM within a day in good conditions. But make sure tedders and rakes are adjusted correctly to avoid them hitting the ground and contaminating the crop with soil.

Wilting research

Research conducted by Volac’s team of Ecosyl scientists using farm-scale silage machinery has compared the effects of tedding, time of day of cutting, and crop maturity on the rate of crop drying.

Findings from a trial on a light, multi-cut grass crop cut on a warm, dry July day revealed that cutting at 10am and tedding immediately resulted in grass reaching 30% DM in just 4.5 hours. That compared with 7 hours to reach 30% DM if grass was not tedded until 5 hours after cutting, or a full 24 hours if grass was left untedded in rows.

Where cutting was delayed until 3pm, grass reached 30% DM in 5 hours if tedded immediately after cutting, but required a full 23 hours to reach this if not tedded until the following morning. Where the 3pm cut was left untedded, it failed to achieve 30% DM at all – reaching only 24% even after 24 hours.

Overall, the average drying rate for grass cut at 10am and tedded immediately was five times faster than for the grass cut at 3pm and not tedded until the following morning.

This is highly relevant as farms strive to increase milk production from homegrown forage and silage, because the longer that grass is wilted for, the more nutrients it will lose.

In a further trial, the impact of a heavy, mature grass crop and a lighter multi-cut crop on wilting speed when cut in September was assessed. Although drizzle at this site meant neither crop reached the target 28-32% DM, results again showed that tedding increased drying speed substantially.

Even with the wetter conditions, the heavier crop cut at 11am and tedded immediately still reached 22% DM within 5 hours. The lighter crop cut at 1pm reached 24% DM in just 4 hours if tedded immediately, while if left untedded, it had not reached 20% DM even after 20 hours. In both crops, tedding a second time gave an added boost to speed of wilt*.

Summary of actions that can speed up wilting

• Do not cut unless you have a long enough weather window
• Ideally, cut when the weather is warm and windy
• Condition the grass to speed up moisture release
• Maximise exposure to the sun and wind by spreading grass over as much of the field surface as possible and as soon as possible
• Ted twice to speed up drying if needed (*Note: additional tedding may lead to increased field losses)

Even in good conditions it is not advisable to wilt for more than 24 hours because field losses and nutrient losses will probably exceed any benefits resulting from increased dry matter levels. 

How to tell the % dry matter (DM) of cut grass?

To check %DM accurately, one method is to collect a representative sample from the field. Chop it up with scissors to around 1 inch (25mm) lengths, mix it up and take a 50g sample, then microwave this on a low setting in short bursts of maybe 10 seconds. Between bursts, remove it from the microwave, stir it up and re-weigh it. As the grass dries with each burst, the weight will decline. Once it stops declining, divide the final weight by the 50 gram starting weight and multiply it by 100. This will be its %DM.

What to do with cut grass if it rains?

Leaving cut grass lying will increase losses and potentially make it more difficult to ensile due to loss of sugars and an increase in spoilage microorganisms. Provided it is not pouring and any effluent can be safely contained, it may therefore be better to bring in what has already been mown as quickly as possible. If the delay is more than six hours, sheet and seal the silo. If you have a lot of grass down and experience heavy and continuous rain you may find yourself in a salvage situation.

Tips for making silage in wet, catchy conditions

In such conditions it might be best to aim to make good, low dry matter silage rather than attempt to wilt it and still end up with relatively low dry matter silage but with a poor fermentation.

• Don’t attempt wilting
• Get the grass in quickly
• Minimise conditioning
• Lengthen chop
• Minimise rolling
• Use a proven inoculant to improve the fermentation
• If silage is wet, keep clamp height lower to reduce risk of effluent losses and slippage