When fertilizer and crop fertilizer are applied late, it is important to test the grass before mowing for silage.
Excess nitrogen is a risk on these plots because it reduces grass sugar and increases the protein content of the crop, causing poor fermentation in the clamp.
High ammonia levels in silage result from the breakdown of proteins and clostridial organisms, explains Dr. David Davies of Silage Solutions.
“Protein breakdown occurs when too little acid is produced during fermentation due to a lack of sugar, or when there are too many nitrate residues from fertilizer or manure that buffer the pH drop,” he says.
Also see:First cut silage has a high risk of manure contamination
To avoid these problems, Dr. Davies encouraged farmers to use a simple and relatively inexpensive refractometer – the same device used to measure colostrum quality.
Dr David Davies with a refractometer and grass sample
It gives a direct and relatively accurate reading of the grass sugar content. The nitrate content can also be analyzed on the farm with a meter or test strips.
Watch the video and read Dr. Below is Davies' advice on how to perform the test and what to look out for:
1. Collect and analyze grass
- Walk around the field and take a good representative sample by taking a cutting sample every few meters at the same height as the grass would be cut.
- Collect the grass in a bag.
- Take the shape of the field into account – a small field will have a higher proportion of hedgerow grass and the grass near hedgerows will probably have less sugar and more nitrate because it is in the shade.
- Mix the sample of grass clippings and cut into approximately 1 inch pieces in a bowl, mixing thoroughly again. The mixing process is important because the test requires a representative leaf to stem ratio.
- Place the grass in a pressed clove of garlic and squeeze out a few drops directly onto the refractometer or squeeze it into a small container and pour the required amount of grass juice onto the sensor with a small syringe.
- Hold the refractometer up to the light. You will see a green line at the bottom and a blue line at the top. Where the two lines meet is the Brix score, which is shown as a percentage.
2. Sugar – what to watch out for and what to watch out for
A Brix score of 1% represents approximately 1-2% sugar.
The refractometer overestimates the sugar content by about 0.25% on average, so adjust accordingly.
To ensure good fermentation, this figure should be 2-3% or higher. At 2% an addition is necessary, but at 3% there must be sufficient sugar present.
However, a good additive will always improve the preservation process, and a quick rendering to 30% DM will increase the sugar content on a fresh matter basis as moisture is lost.
3. Nitrate N – what to look for and measure
A nitrate meter measures in ppm, which corresponds to milligrams per liter. Both can be used to provide an estimate of the level of nitrate N, which is essential for silage production.
Take the grass sample using the method described above and then clamp a sample onto the meter's sensor.
The meter will underestimate the nitrate level by approximately 300 mg/kg of fresh matter, so correct accordingly.
The concentration must be lower than 1,500 mg/kg fresh substance or 0.15% FM.
Grass can be easily ensiled with a maximum of 1,500 ppm nitrate, provided there is sufficient sugar present.
Screening the crop at >28% DM helps overcome the effects of high nitrate and low sugar.
Benefits of testing grass on the farm
Laboratory analyzes provide very accurate results, but on-farm testing can immediately provide a more reliable indication of grass sugar and nitrate N levels.
The time between cutting a sample and getting it to the lab can mean the result does not accurately reflect the true contents of the grass, explains Dr. Davies.
“It takes at least a day to reach the lab by mail, and in the meantime the sample changes.
“Three days can then pass between taking samples and getting the results back, which can affect decision-making.
'It is not that the laboratory analysis is wrong, because it is not. It is more accurate than these devices if done correctly, but the time factor is important.”
Dr Davies says a refractometer and nitrate meter are useful because they can be used to analyze multiple samples from multiple fields and different parts of fields – with quick results.
“With a refractometer and nitrate meter you can immediately get a measurement and decide what to do with the crop.
“The numbers are approximate, but they can be a very useful benchmark for silage making.”
Sampling equipment
Refractometer
There are several options available, but the following two are commonly used:
Cole-Parmer RSA-BR32T refractometer. For sale for € 100.80 including VAT
A digital refractometer from Harvest Agri is available for €564.88 including VAT
Nitrate testers
Laqua Twin 8-743 Nitrate meter. €397.59 including VAT
Yara N tester Purchase price £1,350 plus VAT or lease for £350/year. If you decide to buy in year 2, the purchase price is €1,150.
There are other testers available.
For a cheaper option, nitrate rods are manufactured by Quantofix. A box of 100 test sticks costs £52.26 including VAT and is available from Camlab or Cole-Parmer.
