When making silage, how dry the grass is when we pick it up with a harvester, wagon or baler, impacts on costs, preservation, feed value and effluent production.

How we treat the grass at and after mowing determines its drying or wilting rate and, consequently, its quality, feed value and costs. But Ireland’s big grass yields and broken weather patterns are a particular challenge.

Wilting grass in variable weather conditions can be risky and disruptive.

We need to plan carefully to achieve the necessary wilt, but we also need to have back-up plans. Like many things in life and farming, there are pros and cons with wilting.

Advantages

  • Wilting can help avoid poor (butyric) preservation by concentrating sugars in the grass and facilitating a good lactic acid preservation.

    This is particularly useful with highly digestible grasses cut at a younger growth stage before heading, grasses with higher nitrogen content, or swards with high levels of clover.

    In fact, in many of these situations, wilting is often essential to ensure good preservation. It avoids the need for preservative-type additives.

  • Wilting will tend to increase silage intake in ruminants, but this may not lead to improved performance.
  • Wilting can greatly reduce the weight and volume of silage to be harvested and transported in clamp silage, and similarly reduce the bale number, transport, polythene and wrapping costs in baled silage.
  • Wilting can reduce/eliminate silage effluent production. At 28% to 30% DM, effluent is eliminated.

    • Disadvantages

  • It can be difficult to reliably achieve a satisfactory wilt in Irish weather. Attempting to wilt in broken weather is challenging and risky.
  • Wilting adds extra machinery operations and can interrupt the harvesting schedule, with impacts on seasonal capacity and machinery costs in either a farm or contracting scenario.
  • Leaving grass to wilt in the field can increase forage feed value losses through respiration and microbial activity and physical losses, where the quantity of silage DM harvested reduces.
  • While wilting can increase DM intake, this is often not matched by improved animal performance, possibly linked to a loss in nutritive value during wilting.

    • ‘The faster the wilt the lower the losses’ . Fast wilting, reduces the time for the grass to respire and lose feed value / Donal O'Leary

    Research

    Independent research on wilting was prominent in the 1980s and 1990s, where Irish research (Teagasc) focused on silage quality, animal performance and on wilting rates with different swath treatments.

    The need for wilting depends on the harvesting system, sward type and, particularly, whether high digestibility (high DMD) silage is being targeted. Baled silage will always benefit from significant wilting:

  • It can greatly reduce conservation costs by reducing bale number.
  • It eliminates effluent which can be difficult to capture when bales are stored.
  • If difficult to preserve grass (young leafy grass, high N content, high clover or wet grass) is used, it means a satisfactory preservation will be achieved.

    • Clamp silage is a little different as it depends on the digestibility (DMD) being targeted.

    In dairy systems, for example – in today’s spring-calving herds where cows are frequently grazing grass for some part of the day from immediately after calving – most of the clamp silage is fed to dry cows. In this case, the benefits from very high digestibility grass are fewer, allowing dairy farmers to cut silage at the start of heading, where rising sugar levels make it easier to preserve.

    The extent of wilting needed is consequently less. On these farms, baled silage from younger grass harvested from surplus paddocks during the growing season often provides the high feed value (high DMD) grass needed by cows when they are milking.

    Where a minimum wilt is required, maximising the swath width from the mower may be sufficient. /AR Imagery

    But where winter milk is produced, or in all beef and sheep systems, high DMD silage is beneficial and wilting is more important on these farms, as it allows earlier cuts of high-quality, leafy grass to be more easily conserved.

    Wilting targets

    In simple terms, ‘the faster the wilt, the lower the losses’. Fast wilting reduces the time for the grass to respire and lose feed value (various loss processes), but it greatly reduces the risk of a weather break causing further increased losses. It is much easier get one or two dry days together than three. Dutch research suggests that cut grass should only spend one night on the ground to avoid losses and, consequently, the target for high-quality silage there is to ted quickly and often, if needed.

    How does grass dry?

    Growing grass can typically have a dry matter content of just 14% to 20%. In simple terms, moisture must first move out from leaves and stems, and then it must leave the structure of the swath. Access to dry air and the power of the sun are important.

    For fast drying, spreading out the grass, to have as low a density as possible, is essential.

    A loose, even swath with no lumps is best. Redistribution after drying – where the dry top material is mixed with wetter material will further improve the drying rate. Abrading the grass stems and leaves while mowing can also accelerate drying, but only if the grass is exposed to the drying elements of dry air and the sun.

    Tedding can be carried out quickly after mowing if the mown swaths are only occupying 30 to 60% of the cut area once the surface of the mowed swaths are dry. \ Philip Doyle

    Weather has a huge impact, along with day length. Dry (low humidity) air, wind and sun all contribute to the drying rate. While mechanical interventions will help speed up drying – the drying rate will always be determined by weather.

    How do we speed up wilting?

    Many factors impact on grass drying rate, but we can control only some. These include:

  • Cutting grass dry if possible. Cutting later in the day will leave a higher sugar content, easing preservation, but you lose drying capacity on the first day.
  • Lower-yielding, more frequent cuts are much easier to dry.
  • Use a mower conditioner, but the impact of conditioning is much less than spreading the grass over a wide area, so use both techniques.
  • Spread the grass thinly and evenly over as much of the cut ground as possible. This can be directly from the mower or by separately tedding. Grass spread thinly maximises its exposure to dry air and sun, leading to a much quicker loss of moisture.
  • If necessary, redistribute by spreading/tedding a second time to mix the grass and expose what was under the swath the first time.

    • Swath structure and drying rates

    The impact of swath structure on percentage ground cover and the impact of various swath structures on the drying rate are shown in Table 1 and Figure 1 from Teagasc research. Grouped swaths hardly dry at all, even in good drying conditions, so should be harvested quickly to avoid respiration and other losses.

    Conversely, spread swaths or those with high percentage ground covers, will rapidly lose moisture if weather allows, often allowing an effective wilt within a day or a day and a half.

    Mower conditioners

    Conditioning can speed up the drying rate of the grass by abrading leaves and stems and by ensuring the grass is set up in an airy and even swath, but the speed of drying is still largely dependent on how well spread the grass is, ie the percentage of ground cover achieved. The aggressiveness of the conditioner can be altered by the clearance between the tines and the hood and the speed of the rotor.

    Modern conditioners can be set to produce wide swaths, achieving 60% to 100% ground cover (grass covers the entire cut area at 100%). This 100% cover will give the fastest drying but will involve the tractor driving over the cut grass, which is not favoured in softer ground conditions.

    Tedders: horizontal multi-rotor

    Larger tedders allow for a slower forward speed which is essential with heavy grass crops.

  • Tedders have the biggest impact on drying rate as they can quickly achieve 100% ground cover with a loose, airy swath produced, and they can be used more than once where needed.
  • While some older tedders (particularly dual-purpose tedders/rakes with horizontal tines) had a reputation for forming lumpy swaths that persisted through raking, most modern tedders, when set up properly, can ted evenly.
  • Slow forward speeds are necessary to avoid producing lumps, which is why the wider multi-rotor units are more popular. The angle of the rotor needs to be adjusted properly (usually depending on swath density and how dry it is).
  • Hook type tines, once the preserve of Lely, are well suited to heavy Irish grass crops and are now used by many manufacturers.
  • Physical material losses can occur with high-quality leafy, grass crops, so they must be handled with care once they start to dry (fewer aggressive settings, if a second tedding is needed). Clover is more at risk of loss, so avoid tedding a high clover content crop when it’s partially dry.
  • Tedding can be carried out quickly after mowing if the mown swaths are only occupying 30% to 60% of the cut area, once the surface of the mowed swaths are dry. If a conditioner mower is set to give 80% ground cover (wide swaths), they could be left to dry for a few hours before tedding, but a 100% spread swath will dry quicker, so don’t wait too long if the crop is going to be tedded.

    • Over-the-top tedder

    Over-the-top tedders (swath wilter/wuffler type) are often used in baled silage situations on conditioned swaths and they will redistribute the grass within an airy swath, usually without creating lumps. But the drying rate achieved will still be restricted by the percentage ground cover achieved. Wider units (2.7m to 3m) that can leave a swath the full width of the following baler pick-up will be better than narrower units.

    Practical approaches

  • Plan to wilt rapidly whether using a contractor or wilting yourself and make sure your contractor is aware of your plans.
  • Where a minimum wilt is required, maximising the swath width from the mower may be sufficient, particularly where yields are lower (second cuts or lighter cuts).
  • For a given crop and set of conditions, maximising the surface area by spreading over 100% will achieve the fastest wilt and consequently the lowest risk of losses.
  • Tedders need to be large/wide enough to allow you drive slowly, which is essential with heavy grass crops.
  • You cannot wilt in wet weather. However, modern equipment will allow fast wilts to be achieved in a very short time.
  • Avoid over-wilting, as it may incur losses prior to ensiling, and with clamp silage the less extensive fermentation may lead to less stable silage on feed-out. If the grass is drying too quickly, row it up immediately to reduce the drying rate while waiting for the harvester/baler.