Wednesday, January 7, 2015

Iowa Manure Management Plans: Yield Goal Method versus the Maximum Return to Nitrogen

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





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