Consider reducing chemical fertiliser applications to align with the growth pattern of white clover in grass-white clover swards. Typically, the clover content of these swards starts to rise in line with the gradual increase of daily soil temperatures and more sunlight as we pass from spring and into summer. Before this rise in clover content, when temperatures are colder and there is less sunlight throughout the day in early to mid-spring, clover content is lower and should be represented by a similar chemical nitrogen approach as you would follow for a grass only sward.
Factors that influence nitrogen fixation rates:
The ability of the clover plant to fix nitrogen is influenced by a number key factors.
- Soil temperatures must be above 8℃ for the clover plant to grow and start to fix nitrogen. Also, the more sunlight that the plant receives, the more fixation will occur. One more factor that affects nitrogen fixation is the amount of chemical nitrogen applied to the sward.
- The amount of nitrogen biologically fixated by the plant is reduced when there is more chemical nitrogen available to be taken up by the clover plant.
- Soil pH, P and K will all influence the productivity and fixation ability of white clover in swards, therefore it is critical to soil sample regularly.
- There is a difference between white clover varieties in their ability to fix nitrogen. The current understanding is that high yielding clovers will fix more nitrogen
Finally, the proportion of clover present in each paddock in April will predict that swards ability to fix nitrogen
How do we score the clover content of our sward?
When considering what rate of chemical nitrogen to apply, it is crucial to know the clover content of each sward so as to prevent a decrease in total dry matter production. Each individual paddock needs to be scored and can be done so using the Teagasc Clover Content Scorecard. Results for each individual paddock should be entered and recorded on to your PastureBase Ireland account. This clover scoring should be done in late April just before we see a real boom in clover content.
Table 1: Nitrogen strategy based on clover percentage
| April clover content % | Mid Feb | Mid March | Mid April | Mid May (2 rot) | Mid June (2 rot) | Mid July (2 rot) | Mid Aug | Mid Sept | Total |
|---|---|---|---|---|---|---|---|---|---|
| Chemical Fertiliser (kg N/ha) | - | - | - | - | - | - | - | - | - |
| Grass sward | 24 | 36 | 20 | 32 | 28 | 28 | 21 | 23 | 212* |
| 5% | 20 | 35 | 20 | 20 | 20 | 20 | 20 | 20 | 175 |
| 10% | 20 | 35 | 20 | 15 | 15 | 10 | 15 | 20 | 150 |
| 15% | 20 | 35 | 20 | 15 | 10 | Soiled water | 10 | 20 | 130 |
| 20% | 20 | 35 | 20 | 15 | Soiled water | Soiled water | Soiled water | 15 | 105 |
- *Chemical N fertiliser can be increased to 230kg N/ha, in paddocks with no clover, as long as whole farm N does not exceed 212kg N/ha
- Soiled water used whenever zero chemical N application
- +25kg organic N applied
A few key takeaways from the table above:
- The amount of chemical nitrogen that is allow allowable under the nitrate regulaltions of nitrogen is spread evenly across the 212 kg N/ ha per year, in most cases this will limit production. One advantage of clover is that nitrogen saved in a high clover paddock can be diverted to a grass only paddock,
- By having a sward with a clover content of at least 20%, the amount of chemical nitrogen applied throughout the year can be reduced by around 50% in these swards.
- Application rates are almost identical no matter the clover content in February, March and April. This is due to the fact that the clover plant is not fixing nitrogen in these months due to the cooler conditions.
- Chemical nitrogen applications get reduced aligning with when the clover plant starts to fix nitrogen in the summer months. The reduction in chemical Nitrogen applied is determined by the clover content in the sward. The greater the clover content in the sward, the greater the reduction in chemical nitrogen needed.
These figures are guidelines only. Of course, farmers have to be flexible with their fertiliser applications. External factors out of the farmers control can also have an impact on fertiliser applications. Weather conditions dictate how much nitrogen we can apply. If we experience cooler temperatures than normal in the month of May, we may need to apply more fertiliser to supplement nitrogen as we would expect to see a lower level of nitrogen fixation due to lower temperatures. Conversely if we had a very warm May with higher temperatures than normal, we could spread less Nitrogen than advised as we would expect to fix more Nitrogen than typical for that time of year.
To learn more about white clover and its benefits on Irish agriculture click here to visit the Teagasc web page on white clover.
