Winter Manure Application – What’s the Science Tell Us?

Certainly one discussion point when we talk about using manure as fertilizer is that we want to try to match manure nutrient availability to crop nutrient need and demand. At first glance this might seem like well nothing is growing in the winter, why would we put manure on then? Although some reasons may exist, such as well its better than having a storage overflow, or it reduces some compaction risk especially if we’ve had a wet fall or may have a wet spring, but I think we can all recognize that it some ways the risk of nutrient loss may be higher. However, that means we need to really know and understand what causes the risk on how to best minimize it if winter manure application is necessary. So what science is out there?

Let’s start with this, if it is risky why might we still practice winter manure application? In my mind there are probably three driving factors for this: 1. Reduced manure storage structure size/management of the storage structure, 2. Time available for manure application, and 3. Compaction risk. Constructing a storage is an expense, it may offer vale in that it improves our manure application timing, but accounting for this value can be difficult. Spring and fall are busy times on the farm and in sometimes we find ourselves in a situation with more hours than work and as winter generally has less demands on our time, it becomes a manure application window. Finally, frozen soils offer more support for heavy equipment and thus reduce compaction risk.

The downside of winter manure is risk of nutrient transport. Nutrient movement is always driven by water movement – so what makes winter application riskier? Well, given the last week winter might seem cold, but generally winter is a mix of freezing and thawing, and these freeze-thaw cycles affect soil structure, infiltration, and water movement. Often what you see happen is freezing and thawing break up surface soil aggregates increasing crusting (or even getting the top layer of soil filled with water and freezing solid).  This make is more difficult for water to move through the soil and makes it harder to resist erosion. But it’s really not as cut and dried as that, way back in 1955 they identified four types of frozen soil structures: concrete, honeycomb, stalactite, and granular. Of these structures, it’s the concrete structure that really slows water infiltration. Unfortunately, this is also one of the more common structures to develop especially if soils are wetter when they freeze. This is one of the reasons that if winter manure application is necessary, earlier application (in the winter) is safer, because after several freeze-thaw cycles soils tend to be wetter from the snow water they have infiltrated.

One thing that all the studies of manure application to frozen ground have in common is variability. Every situation is different – the weather, the soil, the manure characteristics all play a role. However, what we do know is that the response is typically driven by the hydraulic conditions. If rapid snowmelt or rainfall is imminent, don’t apply. Runoff will move manure nutrients.  Anticipate problems early in the winter and get manure on then, especially before snow cover develops or before freeze-thaw cycles cause the soil to get wetter and refreeze. If applications must be made later in the winter, choose fields that get the manure closer to the soil surface and look for weather conditions when rain and melt is not imminent.

IMPACT: Utilizing Manure Value

Manure - An important part of the fertility puzzle. ISU is working hard to and hopefully help make a positive impact on how farmers are using manure.

Update! Yield Goal compared to Maximum Return to Nitrogen

A few years ago I wrote a post discussing both of these methods for determining nitrogen needs. Since then we've had a few more crop years. This means county yields have been updated and there are newer records in the Maximum Return to Nitrogen calculator so I figured it was time for another look.

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