Lower Crop Yields Raise Concerns Over Nitrogen Availability After Dry Weather

Jul 08, 2023

Now is the time to continue with planned nitrogen and potassium applications.

Many first-cutting hay crops did not reach their typical yields due to the recent dry weather across the commonwealth.

But this does not necessarily mean the crops left behind lots of nitrogen that will be available for future cuttings.

Penn State Extension soil fertility specialist Charlie White explains that forages take up the majority of nitrogen for each cutting early in the growth cycle.

With enough soil moisture early in the growth cycle this year, nitrogen uptake levels were probably close to normal in the first cutting.

In droughty conditions, the nitrate taken into the plant is not changed into protein, which can lead to an unsafe buildup of nitrate in the harvested forage. This can lead to nitrate toxicity when feeding the hay to livestock.

To meet the needs of future grass hay cuttings this year, nitrogen applications should still be made between each cutting, based on the expected yield of the following cutting. The standard recommendation is 50 pounds of nitrogen per acre per ton of expected dry hay yield.

Be careful about the nitrogen source and application method to reduce the risk of ammonia losses into the air.

These losses are especially high for fertilizers with urea that are broadcast on the soil surface without incorporation during warm temperatures.

Unfortunately, that is what most hay producers are facing right now.

Volatilization losses are reduced if you can apply within a day or two before a rainfall greater than a half-inch. The current forecast does not call for that amount of rain.

In this situation, use urease inhibitors to help prevent volatilization losses for up to 14 days and allow for a rain to incorporate the urea.

If rain is not expected even within 14 days in your area, consider switching your nitrogen source to ammonium sulfate, which does not volatilize.

Ammonium sulfate also helps meet the sulfur needs of hay crops. But ammonium sulfate should not be the only nitrogen source used over the entire production year.

It is more acidifying than other nitrogen sources and will lead to excessive sulfur applications.

Instead, use the ammonium sulfate only on a cutting where warm, dry weather is expected to last longer than the 14 days of protection provided by a urease inhibitor.

Potassium helps plants deal with water stress, so it is also important to fertilize to meet plant potassium needs now, even in dry weather.

Application rates should always be determined based on a recent soil test, which accounts for the pre-existing stores of K in the soil.

Potassium applications on hay crops should always be split up between cuttings. This reduces the potential for luxury consumption (excessive uptake of a nutrient when it is applied beyond the crop need).

The annual K2O recommendation based on the soil test can be divided into doses applied between cuttings to meet needs for the expected yield in the following cutting.

Potassium will make its way to roots and provide a benefit to the crop in droughty conditions, even though its movement will be slower than normal.

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Heidi Reed is a Penn State Extension educator in York County.

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