‘Over the years, silage DM content has increased; but nutritional quality hasn’t’

Little improvements have been made in the nutritional quality of Northern Ireland silage over the last decade, according to a CAFRE dairy advisor.

The Agri-Food and Biosciences Institute institute has analysed thousands of silage samples since it was established in 2006.

However, while the dry matter (DM) content of silage increased over this time, in general, there was little improvement in its nutritional quality.

Why is that the case?

“Making good quality silage can be a difficult task. We have changeable weather and so we have short windows of operation for the silage contractors,” Kevin McGrath, a beef and sheep adviser at the College of Agriculture, Food and Rural Enterprise (CAFRE) commented.

The biggest issue can be our need for bulk at the expense of quality. By delaying harvest to achieve bulk, yields are increased but quality declines.

“Most beef and sheep farms will have a mixture of stock each with different feed demands. Common practice on many beef and sheep farms is to produce a large crop of silage of low to average quality, making up the nutritional shortfall with expensive concentrates.

“With tight margins and the fact that stock are housed for up to six months of the year, we need to focus on improving our silage quality to increase output and reduce costs.”

Increasing yield at a lower price, where to start?

McGrath added: “Growing grass whether for grazing or silage needs the correct nutrition. Soils need to be analysed with appropriate levels of lime, nitrogen, phosphate, potassium and sulphur applied depending on results.

A grass plant suffering nutritional stress will naturally grow to head to reproduce, compromising both quality and quantity.

“You must apply fertiliser at the correct time. Apply slurry when grass cover is low to avoid the risk of contamination and poor fermentation.

“Nitrogen fertiliser should be applied at no more than 120kg/ha first cut, 100kg/ha at the second cut and 80kg/ha third cut. Sulphur isn’t stored in the soil. It will raise both sugar and protein levels which can improve both fermentation and production.

“Weeds reduce silage quality, can cause dietary upset and are more difficult to ensile. Docks, for example, are 65% of the nutritional value of grass. At a 10% sward presence, this is the equivalent of one silage bale/10 of docks. Spray 3-4 weeks before harvest.

Types of grass is also important; ryegrasses have higher digestibility with improved yields over poorer swards. Due to a higher sugar content, fermentation and preservation will also be better.

The harvesting process will be aided by the fact that the grass cutting date has the greatest influence on silage quality.

When to harvest?

An earlier cut of higher quality grass may allow reduced concentrate use, but first priority is to ensure an adequate supply of silage to feed all stock over the winter period.

Once the desired quantity is available, the focus then moves to optimising quality.

“Digestibility, measured in D-value, falls by 0.5 D-value a day from when the grass plants start to develop flowering stems,” McGrath stated.

Target D value depends on the class of stock to be fed; 67 D is the minimum target for productive stock and coincides with 50% ear emergence.

McGrath warned that each week delay in cutting could require an extra 1kg of concentrate/weanling/day or 0.25kg/lamb/day to compensate for loss of silage quality.

“As a rule of thumb, one day’s growth should be allocated for each 2.5kg of nitrogen applied. A failure to utilise all the nitrogen can lead to poor fermentation. This has led to farmers delaying harvesting especially if fertiliser has been applied late.

“Pre-cut testing is available indicating both nitrate and sugar levels. If there are risks of residual nitrates at cutting, wilt grass to 30% dry matter (DM) using a quick wilt to reduce the loss of sugars ensuring a good fermentation,” McGrath added.

Ideally farmers should aim to wilt their grass quickly to 25-30% DM. Wilt for 24 hours to increase the concentration of sugars in the grass which in turn improves fermentation and intakes.

Chop length is also an important consideration as it effects consolidation in the pit, clamp or bale. The dryer the grass the shorter the chop length should become.

For a DM of 25-30%, aim for a chop length of 2.5-5cm. For a wetter crop of grass of less than 22% DM, aim for a chop length of 10cm. Chopped silage break down more rapidly in the rumen but if cut too short it can also lead to poor rumen health.

Cut dense swards to a recommended cutting height of 5cm with more open swards increased up to 10cm. This avoids the risk of soil contamination occurring, which may lead to listeria.

Preserving silage

When filling the pit, try to do so as quickly as possible; also roll the pit constantly when filling. Once the pit is filled, do not roll the following day as this can allow air to re-enter and affect the fermentation process.

We can’t forget baled silage as it offers flexibility and the possibility of quality if required. It is important to aim for chopped, well shaped bales to produce fewer bales/ha. This will reduce both baling and wrapping costs.

“Chopping a silage bale has an effect on silage intake with sheep but not cattle. Try to wrap within three hours at the storage site if possible,” McGrath concluded.