Managing nitrogen fertiliser use
5 min read
Nitrogen fertiliser is a valuable tool for pasture and crop growth in pastoral dairy farms, but it is essential that it is applied at the right time, in the right place and at the right rate. Using less nitrogen fertiliser in this way can contribute to better environmental outcomes and more profitable feed.
The decision to apply nitrogen fertiliser should take into account pasture growth needs, soil conditions, and the cost of growing and harvesting the extra pasture relative to milk price. The profitability of applying nitrogen depends on the extra feed grown (the response) being utilised for animal production. This means that nitrogen either needs to be applied to fill genuine feed deficits, or any surplus feed needs to be identified and harvested to make quality supplement.
It is important to remember that a response from pasture to a nitrogen fertiliser application occurs over 3-14 weeks and therefore should be applied in anticipation of a feed deficit.
Nitrogen fertiliser response is highly dependent on several environmental factors:
Getting the timing and application rate of nitrogen fertiliser right will help increase efficiency and minimise nitrogen loss from your farm. It is important to follow the ‘4 Rs’ of good fertiliser management:
Right place
Right time
Right rate
Right product
Having an annual plan for nitrogen fertiliser use can help maximise the response rate and improve efficiency of use. Contact your local fertiliser rep or farm advisor to help you set up a plan.
Recording and reporting your annual nitrogen use across your farm(s) is also required to ensure you are meeting the nitrogen cap regulations.
The amount of pasture grown in kg DM per kg N/ha applied is the nitrogen ‘response rate’. For example, where 30kg N/ha is applied, and an additional 300kg DM/ha of pasture is grown, the response rate is 10kg DM/kg N fertiliser applied.
The best response to nitrogen fertiliser occurs on fast-growing pasture, when other factors such as moisture and soil temperatures are not limiting growth, and when all other soil nutrient levels are satisfactory.
Results from research projects, including the Forages for Reduced Nitrate Leaching (FRNL) trials, showed that a maximum of 25 to 40kg N/ha are the best application rates. 40kg N/ha application rate was only useful when conditions for pasture growth were optimal.
Pasture growth rate | Pasture growth (kg DM/ha/day) | Response (kg DM/ kg N) | Time to full response (weeks) |
Slow | 10 | 5 | 10-14 |
Moderate | 20-40 | 10 | 6-8 |
Fast | 50-70 | 15 | 5-6 |
Rapid | >80 | 20 | 3-4 |
Late winter/early spring (July-September)
Profitable milk production responses are commonly achieved from nitrogen application in this period where there is a genuine feed deficit. That is when it is applied between the planned start of calving and before balance date – i.e. when feed demand is greater than pasture growth.
The reasons why this is a great time to apply nitrogen fertiliser include:
Note: it takes about 20 days to see a significant pasture yield response to a nitrogen fertiliser application during late winter/early spring.
Spring (October-November)
In spring, temperature and moisture usually don’t limit nitrogen uptake for grass growth, so rapid responses to nitrogen fertiliser can be expected (about 20 days).
There is an increased risk of wastage of the extra pasture generated, reducing the profitability of nitrogen use. Have a plan as to how surplus pasture can be best utilised and practice good pasture management to ensure clovers and spring tillers aren’t affected by canopy closure.
When nitrogen applications are done in early summer, it can stimulate the development and growth of new ryegrass tillers. Nitrogen applications boosted tillering by 37 percent, with an extra 800 kg DM/ha and 66 kg MS/ha produced from December to April after split applications of 100kg N/ha was applied.
Summer (December-February)
At this time of year, clover is fixing nitrogen in the soil, which means mineral nitrogen is likely available. Nitrogen is therefore unlikely to be a limiting factor in pasture growth. If soil tests show nitrogen to be a limiting factor, nitrogen fertiliser applications need to be made before moisture and temperatures also become limiting factors. For farms that have irrigation available, using urease-coated fertiliser can reduce the risk of volatilisation to the atmosphere which occurs in hot and windy conditions.
Autumn (March-April)
Nitrogen has a role in restoring pasture cover after summer or autumn dry periods. After a dry summer, there is normally a considerable pool of nitrogen already in the soil, although sometimes a lot of this nitrogen is unavailable for plant uptake due to immobilisation. In other years, soil moisture is quickly restored through significant rain events that can wash available soil nitrogen below the plant root zone, especially in free-draining soils. It is therefore important to time fertiliser applications well so that plants receive nitrogen while the soil processes recover, and nitrogen doesn’t become a limiting factor for pasture growth. Responses to nitrogen applied in autumn are variable from 4:1 to 10:1, or about 40 days.
There are potential risks when applying nitrogen during this period, including:
Winter (June-July)
Avoid nitrogen applications during this time as the response rate is limited by soil temperature and slow plant uptake and there is a much higher risk of leaching due to high soil moisture.