University of Nebraska-Lincoln Extension, Institute of Agriculture and Natural Resources

October 27, 2006

What’s new in nitrogen fertilizer use for grain sorghum?

 Grain sorghum following soybean

We conducted 39 trials across southeastern Nebraska for grain sorghum following soybean in rotation. We determined an equation for estimating the economically optimal nitrogen rate (EONR) for grain sorghum following soybean based on expected yield and sorghum grain price relative to the price of fertilizer nitrogen.

EONR (lb/acre) = (0.121 x EY x PGPN) – (0.164 x EY) – (5.417 x PGPN)

where

EY = expected yield (bu/Ac) estimated as 1.05 x average yield, and
PGPN = the price of sorghum grain ($/bu) divided by the price of fertilizer N ($/lb).
Therefore, if EY = 130, PG = $2.20/bu, and PN = $0.30/lb, then

PGPN = 2.20/0.30 or 7.33, and EONR = (0.121 x 130 x 7.33) – (0.164 x 130) – (5.417 x 7.33) = 54 lb N/acre.

If soil organic matter is less than 1.8%, increase the nitrogen rate by 20 lb/ac. If the weighted average nitrate-nitrogen concentration (ppm) in the root zone at 0-2 or 0-3 feet is greater than 6 ppm, decrease the nitrogen rate by 8 lb/ac for each increase of 1 ppm in nitrate-N. Give credit for applied manure and for nitrate-nitrogen in irrigation water.

Grain sorghum following sorghum, corn or another cereal crop

The basic equation is not changed and is based on information in the table on page 98 of Nutrient Mangement for Agronomic Crops in Nebraska can be used. However, we are offering a price adjustment factor (PGPN x 0.11) for determining the EONR with varying sorghum and nitrogen prices. The equation for grain sorghum following a cereal crop is:

EONR (lb/acre) = [70 + (1.1 x EY) – (20 x OM) – (14 x NO3-N ppm) – other credits] x (PGPN x 0.11).

where

EY = expected yield (bu/ac) estimated as 1.05 x average yield of the past five sorghum crops grown,
OM = percent soil organic matter to a maximum of 3%,
NO3-N ppm = average nitrate-nitrogen concentration for the 2 – 3 foot depth in parts per million, and
PGPN = the price of grain ($/bu) divided by the price of fertilizer N ($/lb).

Other credits include nitrogen from manure or other applied organic material and from irrigation water.

Charles Wortmann
Extension Soils Specialist

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Copyright 2006 by the University of Nebraska Board of Regents. All rights reserved.
Published by University of Nebraska-Lincoln Extension in the Institute of Agriculture and Natural Resources Cooperating with the counties and the U.S. Department of Agriculture.
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