Spring Nitrogen Use- Do you aspire to be average?
As we’ve previously mentioned, Grasslands has recently been working closely with Andre Van Barneveld, a Dairy Consultant at Graise Ltd. He has great expertise in advising dairy farmers on how to boost profitability without incurring a heavier workload. In this particular blog, Andre has provided his thoughts on the use of chemical nitrogen during the spring period. Thank you Andre for sharing this useful information with us all!
There seems to be a never ending debate about the response you can expect from chemical nitrogen application in early spring.
It is important to understand that nitrogen (N) is a critical limiting element for plant growth and production. It is a major component of chlorophyll, the most important pigment needed for photosynthesis, as well as amino acids, the key building blocks of proteins.
Requirements from optimum nitrogen
The main base of N available to the plant comes from N fixation from the atmosphere. However, for this to happen, the right soil bacteria and enzymes must be active in the soil. Good, healthy soils can provide up to 300KgN/Ha/year organically but the more soil humus is declined and soil structure is compromised through tilling and compaction, the lower the level of organic N available to the plant.
Soils that have been tilled for many years may provide as little as 100Kg organic N/Ha/year. This means that if you apply 200Kg chemical N/Ha, you are only back up to the level that a healthy soil would have produced.
Nitrogen as a growth accelerator
Although N is a fundamental nutrient to plant development and growth, I view chemical N application not as a fertiliser but instead as a growth accelerator. In most cases, chemical N application is a short term boost to stimulate growth in excess of environmental conditions, not applied to correct soil or plant deficiencies. Applying chemical N in this situation accelerates growth. What this means is that if pasture growth rates are very low, there is very little acceleration (response to the N applied) and the higher the growth rates, the higher the acceleration (response).
Irish research data shows response rates to early spring N application of 10 to 1, or even up to 15 to 1 response, meaning you should grow 10 to 15KgDM for every Kg of N applied.
This may be true, but to achieve a response at this level, there are a few factors that need to be right:
- P, K and PH levels in the soil
- Pasture species
- Soil structure/health
- Soil temperature
Soil temperature
Firstly, urea needs a soil temperature of at least 5°C to convert to plant available ammonia N so applying below this would be environmentally and economically irresponsible. Also, as an accelerator, there is little activity in plant growth below 5-6°C so there would be little response at temperatures at, or below this.
If you have very good soil fertility and pasture species, you should get a response of 10:1+ at reasonable soil temperatures but from my experience , the average response is around 7KgDM/KgN at 7°C. I have yet to see research data that exceeds this consistently.
Soil structure
If you have poor soil structure, are recently out of tillage and you find a substantial amount of yellowing in the pasture despite very few frosts or very wet conditions, then you are likely to be suffering from an actual N deficiency in the soil. Chemical N application will be likely to get a response well in excess of the 10:1 by correcting this deficiency. It is fundamental to plant growth and it is the lack of N affecting the chlorophyll levels in the plant that arte causing the yellowing.
It is also important to understand that when growth rates are low, as they are in January/February, the plant is not able to utilise much of the chemical N per day so the response will be spread out over a longer period, up to 8 weeks. This leaves a large proportion of the N exposed to leaching and volatisation /off gassing over that time. Research would suggest over 50% would be lost to leaching and off-gassing in adverse conditions.
It is for this reason that we apply lower rates of N early on as there is a rapidly declining rate of response as application rates rise, I.E. you may get a 15:1 response for the first 10KgN/Ha applied, 10:1 for the next 10KgN/Ha but if you applied in excess of 40KgN/Ha (a bag of urea/acre) in January, the last 10KgN would most likely give you little to no response at all.
Pasture species
My greatest concern, and the reason I am questioning the current application advice, is that it is very general without any of the above explanations to educate the farmer. There is much poor fertility land with poor pasture species that will not achieve anywhere near the quoted response which was achieved under optimal conditions/location. If we as an industry continue to do this, the environmental impact will catch up with us and we run the risk of seeing the signs of N misuse in our waterways and having reduced overall rates of N application imposed on us. This is already happening in NZ.
The recommendations also target each and every farmer although very few farmers are actually able to manage pasture well enough to utilise the extra grass grown. For those with lower stocking rates, with pasture covers nearing 1000KgDM/Ha in early February, the greatest concern may be getting through the grass quickly enough to get area grazed in February and finish the first rotation by balance date. In these cases it would be much more important to get the whole area grazed and follow the grazed area with N to avoid a deficit going into the second rotation.
Managing multiple rotations
Therefore, be very aware that your pasture management will have a greater impact on spring growth then applying high rates of chemical N alone. The greatest stimulant to get the grass growing in spring is for it to be grazed, so ensure you achieve the 30-35% in February guidelines and 2/3 by the 15-17th of March, have a grass budget done to know how much grass you are able to allocate in the first rotation, fill the deficit early with buffer rather than running your first rotation short and having to buffer in the second round.
Blanket spreading high covers that are due to be grazed in February, in my opinion, is questionable at best, especially doing so without knowing your pasture cover and pasture demand.
You will get much greater response from N following grazing when the soil temperatures are more likely to be higher. The greatest threat to your grazing intakes is the second rotation so avoid pasture damage as much as you can, achieve the target residuals in the first round and monitor your grass budget as you go along in March. Don’t aspire to be average, measure, monitor, act and do the basics very well.
Be profitable and enjoy farming.
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