A manure management plan is a tool
for animal farmers that describes how they plan to place and use their manure
nutrients for crop production. The process of filing out a manure management
plan makes producers identify the amount of manure they expect their farm to
produce, estimate the nutrient concentration in the manure, determine the
number of acres that are required for land application, and then detail the amount
of manure that will be applied to each available acre. In Iowa these plans are
based both on the nitrogen needs of the crop as well as the phosphorus index
for each field.
Current Iowa law states,
“Nitrogen-based application rates shall be based on the optimum crop yields and
crop nitrogen usage rate factor values or from other credible sources.
Nitrogen-based manure rates shall account for legume production in the year
prior to growing corn or other grass.” Therefore, I’m going to take a minute to
compare these options to try to better understand what they mean for manure
nutrient management. The first approach to calculating the nitrogen application
rate desired to support crop production is the yield goal method. In this
approach, the farmer determines the optimum yield of the crop and then
multiplies the yield times a crop nitrogen usage rate factor for the specific
crop. The required nitrogen rate is then adjusted for ammonia losses during
application, the nitrogen availability of the manure, and any previous legume
crops grown in the field. The optimum yield for each crop may be set to either
the average of the last 5-year county yields plus 10 percent or the average of
the highest 4 out of the last 5-year county average.
The other approach is the maximum
return to nitrogen. This approach uses economic return to N application found
in research trials as the basis for the suggested N rate. The average of N
responses accumulated from a population of N rate trial sites is used to
estimate the point where net return is the greatest (an example of yield
response curves shown below). That is, it identifies the nitrogen the point
where the next added unit of fertilizer results in a yield increase that based
on the value of corn is equal to the price of that unit of nitrogen. This may
sound complicated, but a tool to help with these calculations is available at: http://cnrc.agron.iastate.edu/.
Input a corn price, a nitrogen fertilizer price, select your crop rotation
(corn-soybean or continuous corn), and select which state or fertilizer region
you are in, and the program generates an estimate of what fertilizer rate will
give you your maximum return to nitrogen.
So now we get to the important part,
how do these two methods compare.
To determine the maximum return to
nitrogen we have to estimate a nitrogen price (I used $0.36 per pound) and a
corn price (I used $2.82 per bushel) for both a continuous corn rotation and a
corn soybean rotation (fertilizer recommendation for the corn phase).
Recently, the Corn Nitrogen Rate
Calculator split the state of Iowa into two regions – Main Iowa area, which
includes majority of the state, and a Southeast Iowa area, which includes Soil
Regions 21, 22, and half of 17 (generally south of highway 92). In the main
Iowa area, the results indicated in a corn soybean rotation my maximum return
to nitrogen would occur at an application rate of 126 (112-140) lbs N/acre and
at 176 (159-190) lbs N/acre in the
continuous corn rotation (numbers in parenthesis produce a profit estimated to
be within $1 per acre of the maximum return to nitrogen). In the Southeast Iowa
region, my maximum return to nitrogen would happen at 142 (129-159) lbs N/acre
in a corn soybean rotation and at 192 (179-211) lbs N/acre in a continuous corn
rotation. I compared this to the amount of nitrogen that would be applied if
the yield goal method was used (the ideal yield was set to the higher of the
average of the last 5-year county yields plus 10 percent or the average of the
highest 4 out of the last 5-year county average). The manure application you’d
pick based on yield goals is summarized in the table below (end of the post).
When I compared the results of the
maximum return to nitrogen and yield goal approaches for nitrogen application
rates, I found that in 82 of Iowa’s 99 counties the maximum return to nitrogen
resulted in a lower nitrogen application rate than the yield goal method in a
corn soybean rotation. In these cases, the yield goal method resulted in a
nitrogen application rate within $1 of the profit produced at the maximum
return to nitrogen in 18 of the counties, a lower nitrogen application rate in
11 of the counties, and a higher application rate in 70 of the counties as
compared to the maximum return to nitrogen approach. Similarly, in the
continuous corn rotation the results showed that 15 counties produced
application within the $1 max profit nitrogen application bracket, a lower
nitrogen application rate in 15 of the counties, and a higher application rate
in 69 of the counties as compared to the maximum return to nitrogen approach.
These comparisons are summarized in the figure of Iowa shown. In the counties
left white, the maximum return to nitrogen recommended nitrogen application
rate was less than that calculated for the yield goal approach for both
continuous corn (CC) and corn soybean (CS) rotations. In the green-shaded
counties, the yield goal method predicted a nitrogen application rate within
the range provided by the maximum return to nitrogen concept for either the CC,
CS, or both, depending on the shade of green as specified in the figure.
Finally, in the counties shaded orange or red the maximum return to nitrogen
approach suggested a higher nitrogen application rate than what would be
estimated based on the yield goal approach.
So which method should you use? I
think both methods have some strengths and limitations. Conceptually the yield
goal method seems really nice as it’s essentially a mass balance approach where
we try to supply the amount of nitrogen we will be removing with the harvested
portion of the plant as well as what we might be losing to other places.
However, in practice, its accuracy is limited by our ability to accurately
estimate increased nitrogen cycling resulting from a rotation effect with a
legume (especially soybean in the corn-soybean rotation) and in general it
doesn’t predict the application rate that will maximize our profit in using
nitrogen. On the other hand, in the maximum return to nitrogen approach we have
a much better chance of applying at the rate that maximizes the nitrogen value
of the manure. However, in addition to just nitrogen, manure also supplies
other nutrients like phosphorus, potassium, and organic matter which may impact
how we value this nutrient source. Additionally, in some high yield cases, the
maximum return to nitrogen approach may recommend nitrogen application rates
below what is being removed with the harvested corn grain (about 175 bu/acre in
a corn soybean rotation and 232 bu/acre in a continuous corn rotation).
So that’s a fair amount of
discussion with no solid answer about which method to use. I think that the
maximum return to nitrogen concept is preferable as it attempts to make better
use of our nitrogen resources. In general, I’d prefer to write my manure management plan with using the yield
goal method, but when it comes time to apply my manure, I’d collect my manure
sample, determine its nitrogen content, and adjust my application rate to try
to achieve an application within the range the maximum return to nitrogen
approach suggests. One benefit of doing this is if it looks like a great growing season we'd still be able to consider side dressing some nitrogen onto our crop and being within the bounds we set in our manure management plan.
Additional resources for selecting
your nitrogen application rate can be found at:
PM-1714 “Nitrogen Fertilizer
Recommendations for Corn in Iowa” available at: https://store.extension.iastate.edu/Product/Nitrogen-Fertilizer-Recommendations-for-Corn-in-Iowa
PM-2015 “Concepts and rationale for
regional nitrogen rate guidelines for corn” available at: https://store.extension.iastate.edu/Product/Concepts-and-Rationale-for-Regional-Nitrogen-Rate-Guidelines-for-Corn
|
County
|
Corn
|
Soybean
|
N use factor
|
Nitrogen Application -Corn-Soybean
|
Nitrogen Application - Continuous
Corn
|
|
(bu/acre)
|
(bu/acre)
|
(lb N/bu)
|
(lb N/acre)
|
(lb N/acre)
|
|
|
Adair
|
163
|
50.8
|
1.2
|
146
|
196
|
|
Adams
|
165
|
50.6
|
1.2
|
148
|
198
|
|
Allamakee
|
185
|
57.1
|
1.2
|
172
|
222
|
|
Appanoose
|
139
|
42.3
|
1.2
|
125
|
167
|
|
Audubon
|
181
|
55.4
|
1.1
|
149
|
199
|
|
Benton
|
181
|
57.3
|
1.2
|
167
|
217
|
|
Black Hawk
|
184
|
55.6
|
1.2
|
171
|
221
|
|
Boone
|
189
|
53.4
|
1.2
|
177
|
227
|
|
Bremer
|
191
|
57.2
|
1.2
|
179
|
229
|
|
Buchanan
|
189
|
56.9
|
1.2
|
177
|
227
|
|
Buena Vista
|
190
|
54.1
|
1.2
|
178
|
228
|
|
Butler
|
187
|
51.6
|
1.2
|
174
|
224
|
|
Calhoun
|
183
|
51.6
|
1.2
|
170
|
220
|
|
Carroll
|
161
|
54.5
|
1.2
|
143
|
193
|
|
Cass
|
181
|
54.0
|
1.1
|
149
|
199
|
|
Cedar
|
195
|
62.0
|
1.2
|
184
|
234
|
|
Cerro Gordo
|
181
|
52.7
|
1.2
|
167
|
217
|
|
Cherokee
|
201
|
60.8
|
1.1
|
171
|
221
|
|
Chickasaw
|
186
|
53.7
|
1.2
|
173
|
223
|
|
Clarke
|
138
|
44.4
|
1.2
|
121
|
166
|
|
Clay
|
196
|
56.1
|
1.1
|
166
|
216
|
|
Clayton
|
196
|
61.0
|
1.2
|
185
|
235
|
|
Clinton
|
196
|
62.1
|
1.2
|
185
|
235
|
|
Crawford
|
188
|
57.0
|
1.1
|
157
|
207
|
|
Dallas
|
181
|
53.2
|
1.2
|
167
|
217
|
|
Davis
|
143
|
42.6
|
1.2
|
129
|
172
|
|
Decatur
|
146
|
43.1
|
1.2
|
132
|
175
|
|
Delaware
|
192
|
58.1
|
1.2
|
180
|
230
|
|
Des Moines
|
183
|
56.3
|
1.2
|
170
|
220
|
|
Dickinson
|
186
|
53.1
|
1.2
|
173
|
223
|
|
Dubuque
|
201
|
62.6
|
1.2
|
191
|
241
|
|
Emmet
|
192
|
52.3
|
1.2
|
180
|
230
|
|
Fayette
|
194
|
58.1
|
1.2
|
183
|
233
|
|
Floyd
|
183
|
54.1
|
1.2
|
170
|
220
|
|
Franklin
|
195
|
54.6
|
1.2
|
184
|
234
|
|
Fremont
|
180
|
53.2
|
1.1
|
148
|
198
|
|
Greene
|
180
|
51.0
|
1.2
|
166
|
216
|
|
Grundy
|
197
|
61.6
|
1.2
|
186
|
236
|
|
Guthrie
|
169
|
51.0
|
1.2
|
153
|
203
|
|
Hamilton
|
181
|
52.1
|
1.2
|
167
|
217
|
|
Hancock
|
191
|
53.8
|
1.2
|
179
|
229
|
|
Hardin
|
192
|
56.5
|
1.2
|
180
|
230
|
|
Harrison
|
182
|
51.3
|
1.1
|
150
|
200
|
|
Henry
|
174
|
55.4
|
1.2
|
159
|
209
|
|
Howard
|
187
|
54.5
|
1.2
|
174
|
224
|
|
Humboldt
|
189
|
53.2
|
1.2
|
177
|
227
|
|
Ida
|
199
|
59.7
|
1.1
|
169
|
219
|
|
Iowa
|
190
|
57.4
|
1.2
|
178
|
228
|
|
Jackson
|
186
|
58.7
|
1.2
|
173
|
223
|
|
Jasper
|
192
|
57.5
|
1.2
|
180
|
230
|
|
Jefferson
|
161
|
49.4
|
1.2
|
144
|
193
|
|
Johnson
|
189
|
55.9
|
1.2
|
177
|
227
|
|
Jones
|
186
|
60.1
|
1.2
|
173
|
223
|
|
Keokuk
|
175
|
54.3
|
1.2
|
160
|
210
|
|
Kossuth
|
195
|
54.3
|
1.2
|
184
|
234
|
|
Lee
|
156
|
49.4
|
1.2
|
138
|
187
|
|
Linn
|
184
|
56.5
|
1.2
|
171
|
221
|
|
Louisa
|
185
|
56.5
|
1.2
|
172
|
222
|
|
Lucas
|
138
|
45.6
|
1.2
|
120
|
166
|
|
Lyon
|
199
|
61.2
|
0.9
|
129
|
179
|
|
Madison
|
160
|
49.0
|
1.2
|
143
|
192
|
|
Mahaska
|
186
|
55.7
|
1.2
|
173
|
223
|
|
Marion
|
170
|
53.3
|
1.2
|
154
|
204
|
|
Marshall
|
192
|
60.4
|
1.2
|
180
|
230
|
|
Mills
|
179
|
51.9
|
1.1
|
147
|
197
|
|
Mitchell
|
188
|
53.5
|
1.2
|
176
|
226
|
|
Monona
|
170
|
51.4
|
1.1
|
137
|
187
|
|
Monroe
|
142
|
44.9
|
1.2
|
126
|
170
|
|
Montgomery
|
173
|
51.8
|
1.1
|
140
|
190
|
|
Muscatine
|
185
|
59.1
|
1.2
|
172
|
222
|
|
O'Brien
|
204
|
61.9
|
1.1
|
174
|
224
|
|
Osceola
|
202
|
57.9
|
1.1
|
172
|
222
|
|
Page
|
166
|
51.2
|
1.1
|
133
|
183
|
|
Palo Alto
|
193
|
53.9
|
1.2
|
182
|
232
|
|
Plymouth
|
188
|
56.9
|
1.1
|
157
|
207
|
|
Pocahontas
|
196
|
53.5
|
1.2
|
185
|
235
|
|
Polk
|
181
|
54.4
|
1.2
|
167
|
217
|
|
Pottawattamie
|
188
|
54.8
|
1.1
|
157
|
207
|
|
Poweshiek
|
193
|
57.1
|
1.2
|
182
|
232
|
|
Ringgold
|
139
|
44.8
|
1.2
|
122
|
167
|
|
Sac
|
185
|
55.7
|
1.1
|
154
|
204
|
|
Scott
|
190
|
63.5
|
1.2
|
178
|
228
|
|
Shelby
|
192
|
56.9
|
1.1
|
161
|
211
|
|
Sioux
|
199
|
63.7
|
1.1
|
169
|
219
|
|
Story
|
179
|
52.6
|
1.2
|
165
|
215
|
|
Tama
|
188
|
59.1
|
1.2
|
176
|
226
|
|
Taylor
|
158
|
47.1
|
1.2
|
143
|
190
|
|
Union
|
153
|
48.4
|
1.2
|
135
|
184
|
|
Van Buren
|
155
|
46.9
|
1.2
|
139
|
186
|
|
Wapello
|
159
|
49.1
|
1.2
|
142
|
191
|
|
Warren
|
158
|
50.9
|
1.2
|
140
|
190
|
|
Washington
|
183
|
58.3
|
1.2
|
170
|
220
|
|
Wayne
|
145
|
43.4
|
1.2
|
131
|
174
|
|
Webster
|
191
|
53.9
|
1.2
|
179
|
229
|
|
Winnebago
|
190
|
53.0
|
1.2
|
178
|
228
|
|
Winneshiek
|
191
|
55.4
|
1.2
|
179
|
229
|
|
Woodbury
|
187
|
54.2
|
1.1
|
156
|
206
|
|
Worth
|
187
|
53.6
|
1.2
|
174
|
224
|
|
Wright
|
194
|
53.7
|
1.2
|
183
|
233
|
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