The goal of sidedressing or topdressing nitrogen in season is to supply sufficient N when the crop needs it most, while reducing the risk of N loss. Delaying the application of some of the total N requirement to the point when the crop can use it most allows growers to get higher efficiency of their nitrogen fertilizer.
Interest and uptake in late season nitrogen application has increased in recent years. The expense and environmental impact has made growers think differently about nitrogen application. Extreme weather events have also caused growers to change the method and timing of their nitrogen application to mitigate loses from excessive rainfall. Late N application allows for flexibility when rain events delay a conventional side dress application at knee high corn. Custom applicators are now investing in high clearance sprayers or spreaders that can deliver N into taller corn than traditional sprayers through coulters or a drop pipe system.
It is well documented that a corn plant has the highest demand for nitrogen after tasseling. Most producers are currently applying their nitrogen ahead of planting, with the planter, or as a side dress application at the four to six leaf stage. Many are doing a combination of all three.
Sidedressing has allowed for more nitrogen availability later in season. Since we are unsure of rainfall potential, yield, and the soil nitrate status, splitting nitrogen applications has proven to improve profit potential.
Taking corn yields to new heights is something everyone is interested in, but the economic profile has to fit. If conditions are right for excessive nitrogen loss or you missed sidedressing on smaller corn, applying N later is a great option. Many high yield contest winners state their high yields are due to timing nitrogen closer to tassel. This is an effective way to increase yield by matching plant uptake to nutrient availability.
Do I Need More Nitrogen?
When large amounts of rain come after pre-plant nitrogen applications, early season N losses are likely to occur. Wet conditions through side-dressing will push applications back and many may be looking to rescue applications later in season. Producers must decide if more nitrogen is needed to get to their yield goal. This can be done in a few ways:
- Soil Nitrate Test
- NDVI Imagery (Drone/Satellite/Greenseeker)
- IMAGE Analytics (through partnership with Veritas)
Nitrogen fertilizer will be converted to nitrate in larger quantities as the season progresses and as soil temperatures increases. Knowing what nitrogen source and rate was applied and what the conditions were like after planting will help you make a better informed decision.
Corn Response to In-Season Application
When applying nitrogen late in season, it is important to remember that certain parts of the field where nitrogen was lost (due to ponding, soil saturation, leaching, etc.) have already impacted yield, and it may not be economical to apply much more nitrogen to those areas. Variable rate applications throughout the field can be more economical; putting nitrogen on areas that will see a yield increase and cutting back on areas where yield has already been lost and probably will not recover as well.
In less severe cases of nitrogen loss, many studies have shown that later season applications will increase yields and have an economical return. One thing to always remember is moisture is needed to move the applied nitrogen into the root zone. If dry conditions persist, then the application will not be as effective.
NDVI and GreenSeeker Technology
NDVI (Normalized Difference Vegetation Index) is a way of measuring a crop’s light reflectance, essentially its greenness. This value has shown a very high correlation with crop health and ultimately yield. We are currently able to utilize satellite and drone imagery that can measure this to help determine where the crop has sufficient nitrogen already, where it could use a boost, or where the crop is at the point of no return.
One tool to help us with this is GreenSeeker. GreenSeeker technology has been tested over the past decade through on farm trials and university research. It has produced results of up to 20% yield increase and up to 10% nitrogen reduction. GreenSeeker is a system that is mounted to a sprayer boom that instantly measures and maps crop vigor. The NDVI values that it captures are used as the basis for nitrogen prescription rates. These rates are applied on the fly as the machine is going through the field. This technology can turn standard high clearance sprayers into real time on the go variable rate applicators.
We know that weather is a huge factor in how nitrogen is used and lost in a crop in any given year. Too much rain and we can lose nitrogen through leaching or denitrification. Too little rain and we don’t have much of a crop to worry about using the nitrogen that is available.
Being able to see directly how the field’s yield potential has been affected by the weather, the IMAGE Analytics tool available at Sylvite – in partnership with Veritas Farm Management – gives an inexpensive approach to adopting Variable Rate Application of Nitrogen on any field. The IMAGE Analytics tool allows us to identify the trends in crop potential through the growing season to make a more educated decision on whether to apply, as well as how much N to apply in every area of the field to increase payback and your bottom line.
For growers with yield data, the analytics portion of the tool gives a chance to evaluate how different rates of N applied perform in the high, medium, and low yield potential zones. This information can be used to fine-tune rates in subsequent years to save on input costs as well as produce yield with a more efficient use of N fertilizer.
How much nitrogen does my crop need?
A crop of corn in general removes about 1.2 lbs of N per bushel of yield from the soil. The crop will be able to utilize nitrogen coming from Soil Organic Matter and any residual from cover crop, but some of what fertilizer is applied will be prone to loss. A general rule of thumb is to apply 0.8 – 1 lb of N per bushel of yield goal. If planning on applying nitrogen late in the season, roughly 75 – 100 lbs/ac of N should be applied early to get the crop to the application timing.
What other nutrients can I apply?
The answer to this question could lie in previous soil tests or plant tissue tests. Looking at a plant tissue test before you begin sidedressing can show you what nutrients may be limiting, and will give you a bit of time to figure out which products to add to UAN. There are many fertilizers that are compatible with UAN. If you are seeing a trend appearing on plant tissue tests or from a soil test, you can address potassium, sulfur, zinc, boron or even manganese at this time.
How should I apply my fertilizer?
It is essential to get the nitrogen as close to the roots as possible to ensure uptake with minimal water. Both toolbar and Y-Drop/drop pipes effectively place the nitrogen in the row right by the plants roots when using UAN solutions. If topdressing with a dry nitrogen source, stabilizers are a must as the N is extremely susceptible to volatilization unless we get enough rain to bring it into the root zone.
What is the optimal timing for nutrient application?
A corn plant will uptake around 50-60% of its total nitrogen need from V8-tasseling stage. Nitrogen should be placed before the plants are tasseling at the latest. If drier weather is forecasted, earlier timings are possible as there is less chance for loss, and more time to get the nitrogen into the soil solution where it can be uptaken.
Should I use a stabilizer with my nitrogen?
Much research has shown a benefit to protecting nitrogen from loss. Depending on N source and timing, stabilizers can protect against loss from volatilization, leaching, or denitrification, especially in the event of prolonged dry or wet spells.