How to Take a Grass Sample

18th May 2018

The weather has finally changed and grass growth has gone into over drive - our thoughts are now turning to summer grazing and making silage.

Hello Again,

David from Devenish here.

The weather has finally changed and grass growth has gone into over drive, our thoughts are now turing to summer grazing and making silage. Grass growth has been recorded at 67 kg DM/ha this week (w/c 14th May) and is predicted to reach 84 kg DM/ha. Taking grass samples can help inform you of the feed value of your grass for grazing and also help in the decision of when to cut silage,– this allows for the optimum use of grass for grazing and silage making.

At time of year we are taking samples for silage, particularly looking at nitrate N and the sugar values in the grass. These are both important as they have a very big effect on how the grass is preserved for the following winter.

Grass Sampling Procedure

Firstly I am going to go through the correct procedure for taking the sample, which is very similar to taking a soil sample. You will need a clean bucket to put the grass into.

  1. The sample taken has to be representative of the grass in the field/paddock in which the grass is growing. When taking the sample use the same W pattern to walk across the field similar to below to ensure it is representative of the grass in the field.
    Grass sample pattern
  2. Make sure you take the sample from the height that the cattle are grazing to and avoid dung and urine patches and other soiled material.
  3. Bring a pair of scissors or shearers to cut the grass down to the height your cows are grazing or you intent to cut silage to, as this will give you a better sample than pulling by hand.
  4. When taking samples to test the sugars in grass, it is better to take at approximately 3 pm in the afternoon, this is when grass is at peak sugar content.
  5. Place all the grass in the bucket, at the end of the field mix up all the grass in the bucket and then take a representative sample out to be sent to the lab.
  6. Grass can is very bulky to be sending to the lab, a rough rule of thumb is to send half a normal size freezer bag to the lab to be tested.
    Grass sample packaging.png

According to a paper by Andrew Dale et al, 2016 from AFBI, the sooner you can get the sample to the lab the better. In the study they looked at the effect different ways of sampling had on the results. They looked at the following ways:

  • Harvesting technique, cut or pluck by hand
  • Storage duration (immediate, 24 hours and 48 hours storage)
  • Storage temperature (average, 15 ˚C or 4˚C)
  • Storage conditions (air present, air excluded and breathable bags)

All samples were analysed fresh using near infra red spectronomy (NIRS) for dry matter (DM), crude protein (CP), water soluble carbohydrates (WSC) (sugars), acid detergent fibre (ADF) and metabolising energy (ME).

The main results were:
  • Hand plucked samples tended to have higher CP (8 g/kg DM) and ME content than cut samples.
  • Changes in grass quality due to storage were small however storing samples under ambient temperatures, in breathable bags for 48 hours led to the greatest sample deterioration.
  • Samples stored for 48 hours had the lowest WSC, DM and ME when compared to immediate sampling.
  • Storage did not seem to effect CP.

What can we take from this study?

I take that we should take our grassland samples early in the week. For testing sugars the best time to do it is at 3 pm in the afternoon. This leaves us with a trade-off for getting the right sugar content, but having to store the sample overnight. I recommend that you store the grass sample in the fridge or freezer overnight and post first class the next morning to the lab. You should squeeze out as much as the air as possible and wrap with elastic bands to keep the air out. Remember when taking samples, use the same procedure and lab so that your results from year to year are comparable.

For silage samples the main parameters you want to get tested are nitrate N and sugar content. Other tests that you can get done as well are, dry matter, crude protein, fibre content, sugars and buffer capacity.

Please get in touch if you would like to discuss grass sampling and how to do it. If you have questions about managing your grass quality or about getting involved in our Soil Improvement Programme, please contact us.

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