Understanding the Nutrient Composition of Your Grass

Pre-cut testing of grass is an important tool to improve precision of cutting date and help improve forage quality and the stability of fermentation in the clamp.


Forage forms over 50% of the dry matter intake of dairy cows and more for other ruminant enterprises, therefore, optimizing silage quality is key to profitability on farm and how well animals will perform over winter. By cutting grass at the optimum time you can ensure a better quality, higher energy forage. By monitoring grass development you can make an informed decision on cutting before the plant becomes too mature and less digestible. This reduces the reliance on other, bought in raw materials, helping farms improve income over feed cost and reduce their impact on carbon footprint from feed.

Understanding the sugar content, free nitrates and NDF content of grass will give a much better indication of optimal time to cut for silage, helping to improve quality, fermentation and clamp stability. Target levels are explained in Table 1.

Parameter Target Why is it important?
Dry Matter (DM) Approx. 20% at mowing Intake are optimised with grass silage DM around 30-32%. At standard rates of drying in typical conditions grass cut at 20% DM will achieve this with a 12 hour wilt, minimising respiration losses in the field.
Crude Protein N/A A measure of crude protein levels in the grass. Indicative of then N that has been converted to protein.
NDF <40% DM A measure of fibre levels and indication of plant maturity. Levels above 40% indicate grass is mature, and levels below 38% require very precise silage making techniques.
Sugar Minimum 10% DM
>15% DM Excellent
Sugars are the fuel of fermentation. The higher the initial level of sugar, the more efficient the fermentation.
Nitrates Maximum 2500mg/kg
<1000mg/kg Excellent
Free nitrates are where fertiliser N has not been converted into protein. This can slow the rate of the pH drop causing poor fermentation. Free nitrate >2500mg/kg is poor and should delay cutting where possible – assuming average uptake of nitrates by the plant at 500mg/day.
Importance of sugar levels

To ensure that effective fermentation takes place sugar levels must be above 10% DM and ideally above 15% DM. Sugars are the fuel for lactic acid bacteria required to drop the pH of silage in the clamp preventing the formation of undesirable microbes and bacteria, resulting in a more stable forage. Sugar content increases in warmer conditions, but as grass starts to mature sugar levels drop, resulting in negative implications for fermentation quality.

Dry Matter

Wilting not only increases the % dry matter (DM), but it also helps to reduce losses from effluent. A 24-hour wilt may be suitable in certain circumstances, in order to achieve a target DM of 30-32%. However, it may be too long in very good weather conditions, where cutting in the morning after the dew has lifted and harvesting in the afternoon will be required to get the target DM.

Cutting in the afternoon leads to higher sugar content initially as the crop has been photosynthesising all day, although, it will also lead to increased wilting time which could risk losses of sugars. Its important to think about weather and time of mowing in order to wilt for the correct amount of time to achieve the target DM.

Ensure free nitrates have been converted to crude protein

Risk periods for high nitrate levels are cool growing conditions, high nitrogen applications, and a period of sustained dry weather followed by rain. If nitrogen is not taken up by the plant and converted into crude protein, the nitrate leads to production of ammonia. When ensiled, ammonia reacts with water to form ammonium hydroxide, which is alkaline and ultimately raises the pH of the clamp, resulting in poor fermentation that may then hinder silage intakes.

If free nitrate levels are above 2500mg/kg it is advisable to delay cutting until they have dropped below 2500mg/kg with the optimum target being <1000mg/kg. Under normal conditions a plant will convert around 500mg/day of nitrates into protein. If the uptake from the soil is too great, then the result is a build- up of free nitrates waiting to be converted.