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://extension.agron.iastate.edu/soilfertility/nrate.aspx.
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 $0.44 per pound) and a corn price (I used $3.75
per bushel) for both a continuous corn rotation and a corn soybean rotation
(fertilizer recommendation for the corn phase). The results indicated that my
maximum return to nitrogen would occur at a nitrogen application rate of 132
(120-145) lbs N/acre in the corn-soybean rotation and at 184 (172-197) 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). I then
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.
However, if your fields yields are consistently larger than the 175 bu/acre in
a corn-soybean rotation and 232 bu/acre in a continuous corn rotation, I’d
probably switch to using the yield goal method. 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.
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
|
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|
|
|
Corn-Soybean
|
Continuous
Corn
|
|
County
|
Corn
(bu/acre)
|
Soybean
(bu/acre)
|
N use
factor
(lb
N/bu)
|
Nitrogen
Application
(lb
N/acre)
|
Nitrogen
Application
(lb
N/acre)
|
|
Adair
|
157
|
47.2
|
1.2
|
141
|
188
|
|
Adams
|
158
|
47.2
|
1.2
|
142
|
190
|
|
Allamakee
|
186
|
51.4
|
1.2
|
173
|
223
|
|
Appanoose
|
115
|
39.4
|
1.2
|
99
|
138
|
|
Audubon
|
176
|
52.5
|
1.1
|
144
|
194
|
|
Benton
|
180
|
52.5
|
1.2
|
166
|
216
|
|
Black
Hawk
|
181
|
50.4
|
1.2
|
167
|
217
|
|
Boone
|
183
|
48.8
|
1.2
|
171
|
220
|
|
Bremer
|
192
|
52.1
|
1.2
|
180
|
230
|
|
Buchanan
|
184
|
50.1
|
1.2
|
171
|
221
|
|
Buena
Vista
|
189
|
49.3
|
1.2
|
178
|
227
|
|
Butler
|
188
|
51.5
|
1.2
|
176
|
226
|
|
Calhoun
|
174
|
46.2
|
1.2
|
163
|
209
|
|
Carroll
|
177
|
50.2
|
1.2
|
162
|
212
|
|
Cass
|
175
|
51
|
1.1
|
143
|
193
|
|
Cedar
|
194
|
56.3
|
1.2
|
183
|
233
|
|
Cerro
Gordo
|
176
|
47.1
|
1.2
|
164
|
211
|
|
Cherokee
|
200
|
54.1
|
1.1
|
170
|
220
|
|
Chickasaw
|
185
|
49.5
|
1.2
|
173
|
222
|
|
Clarke
|
118
|
39.8
|
1.2
|
102
|
142
|
|
Clay
|
195
|
51.2
|
1.1
|
165
|
215
|
|
Clayton
|
192
|
56
|
1.2
|
180
|
230
|
|
Clinton
|
191
|
54.1
|
1.2
|
179
|
229
|
|
Crawford
|
189
|
52.1
|
1.1
|
158
|
208
|
|
Dallas
|
170
|
49.3
|
1.2
|
155
|
204
|
|
Davis
|
118
|
38.9
|
1.2
|
103
|
142
|
|
Decatur
|
124
|
41.3
|
1.2
|
108
|
149
|
|
Delaware
|
188
|
54.4
|
1.2
|
176
|
226
|
|
Des
Moines
|
162
|
50.4
|
1.2
|
144
|
194
|
|
Dickinson
|
189
|
48.9
|
1.2
|
178
|
227
|
|
Dubuque
|
195
|
55.8
|
1.2
|
184
|
234
|
|
Emmet
|
193
|
49.3
|
1.2
|
182
|
232
|
|
Fayette
|
186
|
52.4
|
1.2
|
173
|
223
|
|
Floyd
|
182
|
49.8
|
1.2
|
169
|
218
|
|
Franklin
|
188
|
49.3
|
1.2
|
176
|
226
|
|
Fremont
|
174
|
48.7
|
1.1
|
143
|
191
|
|
Greene
|
177
|
47.6
|
1.2
|
165
|
212
|
|
Grundy
|
195
|
56.1
|
1.2
|
184
|
234
|
|
Guthrie
|
160
|
47.3
|
1.2
|
145
|
192
|
|
Hamilton
|
174
|
47.6
|
1.2
|
161
|
209
|
|
Hancock
|
187
|
49.1
|
1.2
|
175
|
224
|
|
Hardin
|
189
|
52
|
1.2
|
177
|
227
|
|
Harrison
|
184
|
48.4
|
1.1
|
154
|
202
|
|
Henry
|
150
|
49
|
1.2
|
131
|
180
|
|
Howard
|
186
|
48.9
|
1.2
|
174
|
223
|
|
Humboldt
|
183
|
48.6
|
1.2
|
171
|
220
|
|
Ida
|
201
|
53
|
1.1
|
171
|
221
|
|
Iowa
|
179
|
53.4
|
1.2
|
165
|
215
|
|
Jackson
|
175
|
52.6
|
1.2
|
160
|
210
|
|
Jasper
|
180
|
52.4
|
1.2
|
166
|
216
|
|
Jefferson
|
135
|
45.9
|
1.2
|
116
|
162
|
|
Johnson
|
180
|
50.6
|
1.2
|
166
|
216
|
|
Jones
|
185
|
54.1
|
1.2
|
172
|
222
|
|
Keokuk
|
155
|
49.6
|
1.2
|
136
|
186
|
|
Kossuth
|
196
|
50.4
|
1.2
|
185
|
235
|
|
Lee
|
140
|
45.3
|
1.2
|
123
|
168
|
|
Linn
|
184
|
51.8
|
1.2
|
171
|
221
|
|
Louisa
|
166
|
50.6
|
1.2
|
149
|
199
|
|
Lucas
|
116
|
39.8
|
1.2
|
99
|
139
|
|
Lyon
|
204
|
54.6
|
0.9
|
134
|
184
|
|
Madison
|
150
|
45.7
|
1.2
|
134
|
180
|
|
Mahaska
|
169
|
51.6
|
1.2
|
153
|
203
|
|
Marion
|
155
|
48.3
|
1.2
|
138
|
186
|
|
Marshall
|
187
|
56.1
|
1.2
|
174
|
224
|
|
Mills
|
178
|
50
|
1.1
|
146
|
196
|
|
Mitchell
|
186
|
48.2
|
1.2
|
175
|
223
|
|
Monona
|
180
|
49.7
|
1.1
|
148
|
198
|
|
Monroe
|
117
|
41.7
|
1.2
|
99
|
140
|
|
Montgomery
|
174
|
49.1
|
1.1
|
142
|
191
|
|
Muscatine
|
174
|
52.2
|
1.2
|
159
|
209
|
|
O'Brien
|
205
|
54.4
|
1.1
|
176
|
226
|
|
Osceola
|
203
|
51.2
|
1.1
|
173
|
223
|
|
Page
|
162
|
47.4
|
1.1
|
131
|
178
|
|
Palo
Alto
|
193
|
49.4
|
1.2
|
182
|
232
|
|
Plymouth
|
187
|
53.7
|
1.1
|
156
|
206
|
|
Pocahontas
|
192
|
48.3
|
1.2
|
182
|
230
|
|
Polk
|
170
|
50.4
|
1.2
|
154
|
204
|
|
Pottawattamie
|
185
|
52.2
|
1.1
|
154
|
204
|
|
Poweshiek
|
182
|
54.4
|
1.2
|
168
|
218
|
|
Ringgold
|
119
|
42.4
|
1.2
|
100
|
143
|
|
Sac
|
183
|
49.7
|
1.1
|
152
|
201
|
|
Scott
|
179
|
55.9
|
1.2
|
165
|
215
|
|
Shelby
|
192
|
53
|
1.1
|
161
|
211
|
|
Sioux
|
201
|
56.8
|
1.1
|
171
|
221
|
|
Story
|
175
|
50.3
|
1.2
|
160
|
210
|
|
Tama
|
185
|
55
|
1.2
|
172
|
222
|
|
Taylor
|
141
|
42.3
|
1.2
|
127
|
169
|
|
Union
|
138
|
47.4
|
1.2
|
118
|
166
|
|
Van
Buren
|
132
|
43.8
|
1.2
|
115
|
158
|
|
Wapello
|
138
|
44.1
|
1.2
|
122
|
166
|
|
Warren
|
142
|
47.2
|
1.2
|
123
|
170
|
|
Washington
|
166
|
51.6
|
1.2
|
149
|
199
|
|
Wayne
|
117
|
38.6
|
1.2
|
102
|
140
|
|
Webster
|
181
|
47.9
|
1.2
|
169
|
217
|
|
Winnebago
|
188
|
49
|
1.2
|
177
|
226
|
|
Winneshiek
|
192
|
50
|
1.2
|
180
|
230
|
|
Woodbury
|
181
|
48.9
|
1.1
|
150
|
199
|
|
Worth
|
185
|
47
|
1.2
|
175
|
222
|
|
Wright
|
186
|
48.2
|
1.2
|
175
|
223
|
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