Nitrogen considerations for winter pasture

By James Locke

The author is a soils and crops consultant at the Noble Research Institute, Ardmore, Okla.


Before deciding whether to topdress additional fertilizer for a spring grazing turn, you should consider several factors. Below are a few questions that will help you decide whether and how much to topdress.

What is forage demand going to be?

We typically base nitrogen (N) rates on yield goals, in this case dry matter (DM) production. However, we also have to ensure that our yield goals are reasonable according to crop conditions, weather forecasts, and historical production. We can estimate total dry matter demand (DMD), DMD per acre, and N recommendation with the formulas listed in the text box below.

Winter pasture, which for our purposes is going to be a small grain such as wheat, rye, or triticale, will generally produce between 1,000 and 2,000 pounds of dry matter per acre without added nitrogen. This is why we subtract 1,500 pounds from the dry matter demand per acre to estimate the additional production from fertilizer.

If the field history has shown the site to be highly productive without added nitrogen, increase the amount subtracted to as much as 2,000 pounds. Likewise, if the field history has shown it to be a low-production site, reduce the amount subtracted to as little as 1,000 pounds. Divide the result by 18 because about 16 to 20 pounds of additional dry matter is produced per pound of actual nitrogen applied.

An example calculation is shown in the text box. This is for an operation planning to turn out 160 steers weighing an average of 500 pounds on 120 acres of good wheat pasture for a 120-day spring grazing turn (February through May). The steers are projected to gain an average of 2.25 pounds per head per day and come off weighing 770 pounds. Their average weight will be 635 pounds (500 pounds in plus 770 pounds out) divided by two equals 635 pounds.

How much fall-applied N remains?

There are a couple of ways to estimate how much N is still available to credit to spring production. The most accurate way is to collect soil samples in late January or early February. To account for all available N, collect samples as deep as the winter pasture roots will likely penetrate. When collecting subsoil samples, make sure to collect in the depth increments recommended by the laboratory.

In situations where substantial rainfall or irrigation occurred, N could have leached from the root zone or been lost to denitrification. If such events did not occur and the pasture was not grazed in the fall, we can credit all the fall applied N to the topdress recommendation.

If the crop was grazed with a fall turn and soil sampling is not feasible, we can still estimate how much nitrogen the cattle removed. As a general rule of thumb, approximately 30 pounds of N is removed per acre for every 100 pounds of beef produced. For example, if the total gain was 200 pounds per acre during the fall turn, we can estimate that approximately 60 pounds of nitrogen per acre was removed from the system.

What is the economic return?

For graze-out systems, compare the value of the potential additional gain with the cost of the N fertilizer needed to produce the additional forage. A general rule of thumb, as stated above, is that 1 pound of N will produce an additional 16 to 20 pounds of high-quality dry matter forage, which with 85 percent grazing efficiency, provides 14 to 17 pounds of dry matter forage consumed. It requires about 8 pounds of consumed forage to produce 1 pound of gain.

Therefore, for each pound of N, we expect to produce enough usable forage to result in approximately 2 pounds of gain. At the time of this writing, nitrogen costs approximately 43 cents per pound ($400 per ton for urea) and the estimated value of gain for the spring turn is 76.44 cents per pound (BeefBasis.com). If N costs 43 cents per pound and the anticipated value of 2 pounds of gain is $1.53, the marginal return would be $1.10 per pound of N fertilizer applied.

Evaluating potential economic return, anticipating forage demand, and taking credit for residual nitrogen are some of the most important considerations for deciding whether and how much nitrogen to topdress on winter pasture. Still, when making fertility management decisions, make sure to also consider other factors such as weather, winter pasture species, variety, soil type, soil pH, other nutrient levels, and pest pressure.

Determining nitrogen application rates for winter pastures

  • (Cattle number) x (average cattle weight in pounds) x (0.026 [percent DM intake]) x (number of expected grazing days) = total forage demand in pounds of dry matter (total DMD)
  • (Total DMD divided by 0.85 grazing efficiency) divided by number of acres = DMD in pounds per acre (DMD per acre)
  • (DMD per acre minus 1,500 pounds) divided by 18 = recommended pounds actual N per acre

Example:

  • 160 head x 635 pounds x 0.026 intake x 120 days = 316,992 pounds total DMD
  • (316,992 pounds total DMD divided by 0.85 grazing efficiency) divided by 120 acres = 3,108 pounds DMD per acre
  • (3,108 pounds DMD per acre minus 1,500 pounds) divided by 18 = 89 pounds actual N per acre recommended


This article appeared in the February 2019 issue of Grower on page 12.

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