Test Grass to Make Better Silage

15 April 2019

Regular testing of silage grasses in the lead up to mowing will help improve timing of cutting, helping ensure feed value is optimised and help take the risk out of fermentation, according to Tom Goatman, Ruminant Nutritionist with Trouw Nutrition GB.

“Forage is typically 50% of the dry matter intake of dairy cows, with grass silage forming a significant proportion of this on most farms,” Mr Goatman comments.  “Yet in most cases the decision about when to mow is based on a visual assessment of the crop which can mask the factors which will affect how well the crop will ferment and ultimately how well it will feed.  Taking a better informed decision will help increase forage quality and production from forage.”

Mr Goatman advises that to understand the maturing of grass and its suitability for cutting, it is important to test grass regularly in the run up to possible mowing.

“Just looking at grass it is impossible to predict the three key parameters that drive the fermentation and feed value, but a simple analysis will provide much better information and allow mowing to be timed more precisely.”

He says that sugar content and the level of free nitrates both affect fermentation quality and these will be highlighted on a test.

A good sugar level is important as it provides an energy source for the lactic acid producing bacteria and will encourage a rapid fall in pH resulting in a more stable fermentation.  In addition, a higher sugar content will help produced a feed with a higher ME content.


“Sugar content will be affected by the weather in the lead up to harvest and by the quality of grasses in the sward.  As grass matures so sugar content tends to reduce so you want to avoid delaying cutting.  We advise looking for a sugar content of 13% and ideally more than 15% in the dry matter of pre-cut grass.”

Free nitrates can significantly compromise fermentation.  Mr Goatman explains that they are nitrogen absorbed from the soil that has not yet been assimilated into protein.  He says they can lead to the production of ammonia during fermentation which increases buffering capacity which can reduce the rate of pH drop, leading to a poorer overall fermentation.

“Free nitrates tend to be higher in cooler seasons and after high nitrogen applications, either as fertiliser or slurry.  Late applications of nitrogen can result in higher free nitrate levels so take care with fertiliser timings.

“High free nitrates can also be a problem if a period of dry weather is followed by rain as we saw in 2018 when free nitrates rose markedly after the summer drought.

“Ideally free nitrates should be less than 1000mg/kg freshweight but interestingly some free nitrates are useful as they inhibit clostridia bacteria which can spoil fermentation.”

Regarding feed value, My Goatman says the key parameter is NDF content.  NDF increases with advancing maturity.  High NDF indicates a crop that is over mature and will not feed as well but conversely low NDF can result in a silage with reduced physical fibre which will reduce rumen function

“Analysis of first cut samples processed through the Trouw Nutrition Analytical Laboratory over the last four years indicate that a threshold of 38-40% should be used as the cutting guide to optimise first cut quality.

“There is no way that you can visually assess the sugar, free nitrate and NDF content of grass by a visual assessment.  A dark green crop may suggest high nitrogen levels but you can’t tell how high.  Similarly, a higher level of ear emergence may suggest NDF is falling with increased lignification but you won’t know the rate.”

He says a pre-cut sample should be a representative sample of the grass in the field, or from all fields being taken for first cut.  The results are back on farm within 24 hours so you can build a picture of the crop.


“Weekly pre-testing of silage fields from mid-April onwards will allow you to monitor the development of fibre in the plant and predict the optimum time to cut.  You will also have information on sugar and free nitrate levels.

“These are important indicators of how a crop will ferment and can help in the decision about which inoculant to use.  Certainly if sugars are low and free nitrates are high, then an inoculant will be essential.”

Mr Goatman stresses that the information provided by a pre-cut test is only a guide but is the best information on which to base cutting decisions.  He emphasises that best practice silage making techniques should always be followed to maximise the prospects of achieving a rapid and effective fermentation, the production of a stable clamp and a high quality feed capable of supporting high levels of milk production.

“However, what a test will do is highlight when particular care should be taken and reduce the risk in silage making,” he concludes.