By Liz Stahl, Extension Educator – Crops, Jochum Wiersma, Extension Small Grains Specialist, and Fabian Fernandez, Extension Nutrient Management Specialist
The status of nitrogen for this year’s crop, sidedressing tips, and small grains disease and insect pests were discussed on the June 4, University of Minnesota Extension Strategic Farming: Field Notes program. Drs. Fabian Fernandez, Nutrient Management Specialist, and Jochum Wiersma, Small Grains Specialist, discussed these issues with moderator UMN Extension Crops Educator, Claire LaCanne.
Specifically, N losses due to denitrification (a key source of N loss in saturated soils) or nitrate leaching (when nitrate-N is lost through the soil profile, generally with excess water) should have been very minimal. This means that N applied earlier in the season should be there for the crop when it needs it.
Mineralization of N, which is the conversion of organic forms of N to forms that can be taken up by plants, is a major source of N to plants. Little mineralization of N has occurred so far this season, in part due to the temperature fluctuations we have experienced this spring. More steady temperatures are needed for mineralization of N to occur. Once conditions warm up, N mineralization is expected, which will add to the amount of N available for growing crops. Overall, Fernandez states we should be in good shape for N availability this season.
Climate models are predicting wetter springs followed by drier weather, and we are seeing that, particularly in recent years. Keep in mind when N is sidedressed, if conditions dry up, enough water is needed to get N to the root zone where crops can use it.
When sidedressing anhydrous ammonia, applying when corn is at the V4 to the V8 stage is still ideal, but it is best to be on the earlier side of this window than later. Corn roots will start to grow between the corn rows, so you can damage corn roots by later application. It is not expected to pay off to use a nitrification inhibitor when sidedressing N, unless we get radically wet, as the chance of N loss through denitrification and leaching is low this time of the season.
Regarding cover crops, when microbes start to break down the residue, they take up N from the soil (immobilization), so this N is not available to the crop. Fernandez has looked at adding 40 to 80 lbs/a of N in these situations, and found 40 lbs/ac was all that was needed to keep the crop growing until it was time for a full sidedress application.
More details on sidedressing N can be found at “Things to consider when sidedressing nitrogen”.
FHB has a wide host range, and we have learned that the spores are pretty much always around. Consequently, the weather conditions during the crop’s most vulnerable stage drive the need for a fungicide application. If you find that the model has your variety at a moderate to high risk for 3 or more consecutive days before heading, a timely fungicide application is needed.
It is a bit more complicated to predict the need for a fungicide for tan spot, Septoria, and leaf rust. The presence of the spores is not as ubiquitous as it is for FHB. Spores from tan spot and Septoria are produced on last year’s wheat residue and do not travel very far, while spores of leaf rust have to be blown up by southerly winds from states like NE and SD. This means that you need to scout to determine whether the disease is even present in your field. Wheat following wheat is at greatest risk. Luckily, it takes a couple of in-season disease cycles (or generations) for these diseases to reach economically damaging levels, giving you time to combine the information of the disease risk models with the in-field information to make an informed decision.
Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
Nitrogen status
A lack of snow cover, frost, or soil moisture this spring left us with soils with a lot of water storage capacity and little chance of runoff. Any precipitation we have received so far has essentially been staying in the soil, leading to less potential for nitrogen (N) losses.Specifically, N losses due to denitrification (a key source of N loss in saturated soils) or nitrate leaching (when nitrate-N is lost through the soil profile, generally with excess water) should have been very minimal. This means that N applied earlier in the season should be there for the crop when it needs it.
Mineralization of N, which is the conversion of organic forms of N to forms that can be taken up by plants, is a major source of N to plants. Little mineralization of N has occurred so far this season, in part due to the temperature fluctuations we have experienced this spring. More steady temperatures are needed for mineralization of N to occur. Once conditions warm up, N mineralization is expected, which will add to the amount of N available for growing crops. Overall, Fernandez states we should be in good shape for N availability this season.
N sidedressing tips
If you didn’t apply all your N needs yet, corn will be growing very quickly once temperatures warm up. It is better to apply needed N sooner than later, so the N is there when the crop needs it. N uptake by corn increases greatly around the V6 stage of corn, with the sweet spot for sidedressing N being between the V4 to V8 stages of corn. At this point, the potential for N losses diminishes greatly too, as the crop will take up N that is present in the soil. It is important to note that research is not really showing a benefit to applying N past the V12 stage, and corn yield could actually be hurt by delaying applications this late. The goal is to have the N available when the crop needs it.Climate models are predicting wetter springs followed by drier weather, and we are seeing that, particularly in recent years. Keep in mind when N is sidedressed, if conditions dry up, enough water is needed to get N to the root zone where crops can use it.
Sources of N and loss potential
Sources of N vary in their loss potential. If UAN is applied on the soil surface without incorporation, N could be lost through volatilization. Urea, which makes up 50% of UAN, can be lost through volatilization if it is not incorporated by at least 0.25 to 0.50 inches of rain. Knifing in UAN or injecting it into the soil are good practices to reduce loss risks.When sidedressing anhydrous ammonia, applying when corn is at the V4 to the V8 stage is still ideal, but it is best to be on the earlier side of this window than later. Corn roots will start to grow between the corn rows, so you can damage corn roots by later application. It is not expected to pay off to use a nitrification inhibitor when sidedressing N, unless we get radically wet, as the chance of N loss through denitrification and leaching is low this time of the season.
Regarding cover crops, when microbes start to break down the residue, they take up N from the soil (immobilization), so this N is not available to the crop. Fernandez has looked at adding 40 to 80 lbs/a of N in these situations, and found 40 lbs/ac was all that was needed to keep the crop growing until it was time for a full sidedress application.
More details on sidedressing N can be found at “Things to consider when sidedressing nitrogen”.
Small grain pests
Forecasting small grain disease
Wiersma discussed how robust weather-based models help assess the risk of infection for several economically important fungal diseases, including Fusarium head blight (FHB), in small grains. The Small Grain Disease Forecasting Model at https://www.ag.ndsu.edu/cropdisease/ works in conjunction with the North Dakota Agricultural Weather Network (https://ndawn.ndsu.nodak.edu/ ) and the Minnesota Agricultural Weather Network (https://www.mda.state.mn.us/minnesota-ag-weather-network). This system is expanded beyond North Dakota and northwestern MN. To use the model, you input the weather station location closest to you and the growth stage of your crop. This will produce a table that calculates the risk of economic losses caused by FHB and/or the number of days favorable for infection for tan spot, Septoria, and leaf rust over the last 10 days.FHB has a wide host range, and we have learned that the spores are pretty much always around. Consequently, the weather conditions during the crop’s most vulnerable stage drive the need for a fungicide application. If you find that the model has your variety at a moderate to high risk for 3 or more consecutive days before heading, a timely fungicide application is needed.
It is a bit more complicated to predict the need for a fungicide for tan spot, Septoria, and leaf rust. The presence of the spores is not as ubiquitous as it is for FHB. Spores from tan spot and Septoria are produced on last year’s wheat residue and do not travel very far, while spores of leaf rust have to be blown up by southerly winds from states like NE and SD. This means that you need to scout to determine whether the disease is even present in your field. Wheat following wheat is at greatest risk. Luckily, it takes a couple of in-season disease cycles (or generations) for these diseases to reach economically damaging levels, giving you time to combine the information of the disease risk models with the in-field information to make an informed decision.
Small grain insect pests
So far this has been a quiet year for both aphids and armyworms. Like leaf rust, aphids and armyworms migrate north on southerly winds. Wiersma has heard of very few issues in the central plains, so the risk at this point is low and will likely remain low.Listen to the podcasts
Listen to the full program through your preferred podcasting platform or at the Strategic Farming: Field Notes website.Thanks to the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council for their support of this program.
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