Producers typically analyse the soil to know the requirement for nitrogen (N), phosphorous (P), potassium (K) and sulphur (S) to ensure good forage growth, but they do not always analyse forage prior to cutting. However, forage nitrate content at harvest can impact the ensiling process: fermentation, digestibility and dry matter loss can be negatively affected when nitrate levels are above 0.1% (1,000ppm) before cutting.
Can forage get too much nitrate?
Nitrogen is generally applied to the soil in the form of ammonium nitrate, with the ammonium fraction being converted into nitrate in the soil. Growing plants draw the nitrate from the soil through the roots and either store it or use it to produce proteins.
Utilisation of nitrogen from fertilizers by the plant depends on several factors:
- The climate: During drought periods the growing forage is unable to adsorb nitrogen from the soil, leaving the plant in ‘nitrogen starvation’. When this period is followed by wet weather, the plants will uptake an excess of nitrogen.
- The nutrient profile of the soil (e.g. sulfur ). If the nutrient profile of the soil is not correctly balanced then it is impossible for the growing plant to efficiently utilise the applied nitrogen.
Nitrogen usage by the plant is approximately 2 units per day of plant growth.
How much nitrate is too much?
Plant nitrate levels can be rapidly tested in the field prior to cutting thanks to nitrate test strips. This is done by taking a cross section of grass from a field, cutting it to 2-3cm lengths and then squeezing the juice out with a garlic press onto the test strip and reading the colour scale. This rapid test takes less than one minute and acts as a ‘cut or do not cut’ assessment.
-> Nitrate levels should be less than 0.1% (1,000ppm) before cutting in order to ensure that the fermentation is not adversely effected.
If analysis is not urgent, fresh grass samples can be laboratory tested, which gives an exact nitrate level as well as digestibility and sugar content.
How nitrate affects silage quality and more?
High nitrate can adversely affect the fermentation of the forage and the final silage and thus impact the animal through various mechanisms:
- Nitrate increases the buffering capacity of the forage. This means that during fermentation, more lactic acid needs to be produced in order to achieve the same pH drop.
- Increased levels of nitrate in the forage means that the plant maintains a more rapid growth than optimal and utilises higher levels of plant sugar. This in turn reduces the amount of sugar available for the production of lactic acid.
- During fermentation, the plant nitrate is converted into gaseous nitrogen dioxide and ammonia.
-> 1. The ammonia neutralizes some of the lactic acid produced, slowing down the fermentation.
-> 2. Nitrogen dioxide can be released as dangerous ‘Silo Gas’ or can be converted to nitric acid when it dissolves in free silage moisture. The silo gas is adsorbed into the silage and forms a layer of nitric acid, or is released if the sealing is not perfect as a coloured gas (pictures). This represents a health hazard for humans and animals (inhalation of the gas causes irreparable damage to the lungs).
- Finally, higher than optimal intake levels of nitrate by the cow can affect conception rate (cause of early embryonic death).
In consequence, forages ensiled with higher levels of nitrate will have a slower fermentation, lower digestibility and increased dry matter losses on fermentation. They could also represent a health hazard to both human and animals.