Written by: Calvin Trostle, Ph.D. & Jake Mowrer, Ph.D.
In Texas, the timing of topdress Nitrogen applications to wheat and all other small grains is a critical strategy towards optimizing production. Topdressing with nitrogen at the right stage can boost yields, improve protein content and maximize profitability—but knowing when and how to apply it isn’t always straightforward. As growers prepare to topdress their crop, consider reviewing these resources from Texas A&M AgriLife covering common questions and best practices for managing water and nitrogen. Find the answers to these questions here and learn more about water and nitrogen management practices here.
What is the General Nitrogen requirement for wheat grain production?
About 1.25 lbs. of N per bushel is removed from the field. (In contrast the same bushel of wheat grain removes about 0.45-0.50 lbs. of P per bushel.) Applied fertilizer N will reflect some inefficiency.
What are the common rules of thumb for gauging the amount of Nitrogen to apply for wheat grain?
Rule 1A) No soil test: 1.2 lbs. applied (fertilizer) per bushel of yield goal. This is the more common situation in Texas.
Rule 1B) With soil test: 1.5 lbs. N requirement per bushel of yield goal, then reduce fertilizer N according to the level of nitrate-N reported in the soil test. A conventional soil test is likely based on a 6” sample. The Texas A&M AgriLife Soil, Water and Forage Lab, however, can provide recommendations if soil test N data exists for any depth down to 24”. In this case, based on Texas A&M research, all readily available nitrate-N down to 24” is fully credited toward your wheat crop N requirement. This can further reduce expensive N fertilizer costs. Additional information concerning manure or nitrate-containing irrigation water applications can be used to further reduce N fertilizer recommendations.
Rule 2) If grazing then going to grain, fertilize up to 2 lbs. of N per bushel of yield goal at planting. Then when the cattle come off (by first jointing), fertilize again with another 0.75 lbs. N per bushel of yield goal. Cattle must be taken off prior to jointing to ensure that grazing doesn’t remove heads and reduce grain yield. It is important to understand the growing point has already begun differentiating (potential seed number and spikelet number already being determined). Therefore, the critical time for N topdress can be easily missed.
When is the best time to top-dress Nitrogen for grain yield?
All N should be on the field and in the root zone by growing point differentiation during growth stage 5. Jointing will appear shortly. Substantial declines in yield potential may occur if all or even a portion of topdress N is delayed after this stage. Here is a good rule of thumb for topdressing on wheat: “If you are debating whether you should go ahead and topdress N on your wheat, go ahead and do it.” The downside of being late is lost yield potential. It is better to be early with topdress N rather than late.
In dryland wheat production consider the weather forecast. Topdress N applications do not become effective until the N is moved into the root zone by rain or melting snow. Therefore, once you are within about one month of when you would normally anticipate N topdressing based on growth stage, watch the weather forecast for a day or two when precipitation chances are significant (especially if well above 50%). (This assumes the wheat crop you have is worth fertilizing, especially if in a drought.)
If I make good ground application of Nitrogen fertilizer vs. applying through an irrigation center pivot, should I change my timing?
When irrigation is available, it is best to broadcast N applications to coincide with the next irrigation. This dissolves and washes nitrogen into the root zone. If you need to irrigate, but it is a little sooner than you anticipated putting the N on, consider making the N application early. When applying through a center pivot, it is advisable to apply at least some of the needed N early to coincide with necessary/unavoidable irrigation events. Additionally, ensure that your total N is applied by growing point differentiation.
Is there any benefit for Nitrogen applied after jointing?
Late N in some cases may only affect (increase) grain protein content. This is not a factor in most farm wheat sales though the issue has come up in some years. Some grain buyers may balk or will discount if wheat grain protein is less than 11%. However, if a field has a general N deficiency, then it is possible that late N application after jointing is still needed to achieve yield potential. This later application will not have the impact of N that enhances spikelet and seed number. Later N may help you “catch up” to the pre-existing yield potential, but you cannot increase it.
Should I split Nitrogen applications between fall and late winter?
If you are fertilizing dryland wheat in the fall in drier climes of the state (Rolling Plains, High Plains), it is acceptable to apply all N at once. Splitting the N application is agronomically a good idea (historically the split is about ⅓ fall, ⅔ at jointing). This may be impractical for smaller amounts of N. On the other hand, if winter precipitation is good, and the wheat crop looks very good, you may wish to add additional topdress N in the late winter if yield potential appears higher than normal.
Conversely, especially for drier regions of Texas, you may choose to not apply fall N to dryland wheat (especially if you have modest residual N fertility) until you see that you have a satisfactory stand that appears to be worth fertilizing. If you postpone initial N, then consider moving topdress N forward (sooner) in the late winter.
There is a good chance of rain or snow in the forecast. Should I go ahead and apply my topdress Nitrogen?
Yes, especially for dryland. This is especially important for drier regions of the state where average rainfall for March is sporadic and less than one inch. In fact, you should place high emphasis on this, even if it might be up to a month before you would otherwise normally topdress. Rain or snow dissolves the N fertilizer, and moves it into the root zone. You might not get meaningful precipitation for another six weeks in the Rolling Plains and High Plains of Texas. That could be after growing point differentiation and jointing are complete. Furthermore, no applied N is of any value until it is incorporated by water into the soil, whether naturally or through irrigation.
My wheat stand is thin but uniform. Can Nitrogen management help me overcome this thin stand?
Yes. Fall or winter fertilizer while the crop is still tillering, preferably before you begin to see any significant erect growth should help. So, consider advancing some of your N application to drive increased tillering to compensate for the thin stand in the field.
Is there a preferred Nitrogen form to use for wheat topdressing?
Common N forms are all readily soluble, unless they are specifically designated as slow release. N will dissolve and wash into the root zone. Choose your N form based on cost per actual unit of N and/or the method or ease of application.
Is there a limit on how much liquid Nitrogen can be applied with a ground rig without burning the leaves?
Texas A&M AgriLife has not documented the burn potential of liquid fertilizer N on the leaves. Other reports and field observations suggest 50 units of N is safe, and possibly much more. However, this N must be irrigated into the root zone (and we would not gamble on getting rain to wash it in). N from liquid application may be subject to N losses due to high microbial activity on the leaves, especially if temperatures are warm (into the 70s˚F). Ask your fertilizer dealer or applicator for guidance.
Similarly, foliar-applied N products are not recommended to serve the 2/3 N requirement at topdressing. These products are expensive per unit of N. Wheat will respond quickly (2 to 4 days) to soil-applied N if the N can be moved into the root zone. Foliar products with nitrogen are best only for acute N deficiency when yield potential is at significant risk. This pertains to the need to get the first few pounds of nitrogen on the wheat crop per acre while you fertilized conventionally for the rest.
My irrigation water has nitrate-nitrogen (nitrate-N or NO3-N) in it – should I apply this toward my wheat crop?
Yes. This nitrate-N is fully available to the crop, and it should be credited 100% toward wheat N needs. Most irrigation waters in Texas average about 3 to 10 ppm nitrate-N, and some are much higher, even 20 to 50 ppm nitrate-N. Regions within Texas that tend to have higher irrigation water nitrate-N include the Texas South Plains and several ‘hot spots’ in the Texas Rolling Plains aquifers, especially the Seymour Aquifer.
For every 1 ppm nitrate-N in one acre-inch of irrigation, you apply 0.23 lbs. of N per acre. For example, if you have 5 ppm nitrate-N and irrigate 10” then you have applied 11.5 lbs. N per acre, or about 20% of the N requirement for a 50 bu./A crop.
I had a major freeze that may have hurt my yield potential. Will late Nitrogen applications drive compensatory tiller growth to recapture yield potential?
If you have adequately managed your N fertility program for wheat it is doubtful that more N will be required. N is mobile within the plant. If the growing point in the stem is killed, that stem will not grow any more. But, N within that stem can be mobilized to other stems. Furthermore, when freeze damage occurs, producers are inclined to minimize further inputs as yield potential has likely decreased. Texas A&M AgriLife does not recommend changing your N program after a damaging freeze. Added N at this point might in fact drive unwanted vegetative growth that does not contribute significantly to yield.
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