Wednesday, May 17, 2017

Manure Application Uniformity



Maybe you have seen some tweets and pictures about manure application uniformity testing over the past years, or hopefully seen some information about upcoming field days where uniformity is going to be a key topic. You might be wondering why we are making a big deal about this, but when it comes to using manure as a fertilizer effectively, understanding how uncertainty impacts your decisions is critical for making the best management decision.

So why should we worry about manure application uniformity? Nitrogen for crop growth can come from multiple sources, the soil organic matter can mineralize and in doing so release mineral nitrogen for the plant. The remaining nitrogen needed to support crop growth comes from applied fertilizer. Years of research have gone into characterizing how crops would respond.  If you take a look at the Nitrogen Rate Calculator it will give you an idea of how a corn crop (in this case corn following soybeans) responds to the addition of nitrogen. What this graph (figure 1) shows is how the addition of nitrogen causes corn to respond. What the figure demonstrates is that we want to apply somewhere around 150 lb N/acre, if we are less than this the yield goes down, if we apply more than this we see minimal yield improvement.


Figure 1. Corn response to added nitrogen for Iowa conditions for a corn crop following soybean.

So what does this all have to do with manure application uniformity? When we are trying to hit 150 lb N/acre does that mean we just need to average that for the field? Probably not, it’s about getting a condition where it’s uniform over the whole field (yes, there might be some soil variations and every field has a bit different response to nitrogen, and weather conditions matter so some year’s crop response to nitrogen is much more drastic than others, but for the sake of argument, let’s work with this curve to see what it means).

For fun as we think about the math behind this problem, let’s assume we have corn planted on 30-inch spacing, our manure toolbar also has 30-inch spacing, and that corn roots only get their nitrogen from the manure application band that was placed next to that corn row. Then let’s figure we applied 150 lb N/acre from liquid swine manure that tested 50 lb available N/1000 gallons, so we were applying 3000 gallons an acre.
Now think about two pieces of equipment, one has a knife-to-knife coefficient of variation of 35% at this application rate, the other has a coefficient of variation of 10%. In both cases let’s figure an 8-knife setup. To give you an idea what this looks like in terms of nitrogen application rates achieved by the different knives and the impact different levels of uniformity have on crop yield let’s run through an example. Both of the tools in this example hit the right application rate on average, but how they do it, in terms of evenness across the toolbar is very different. What I want you to start thinking about is what would this mean for your crop yield from row-to-row and nitrogen leaching.

Table 1. Nitrogen and manure application rates for two pieces of application equipment that that achieve different levels of manure application uniformity.
Knife #
N Application
(lb N/acre)
N Application
(lb N/acre)
Application Rate
(gallons/acre)
Application Rate
(gallons/acre)
% of
Desired Rate
% of
Desired Rate
1
100
150
2000
3000
67
100
2
135
160
2700
3200
90
107
3
165
170
3300
3400
110
113
4
210
160
4200
3200
140
107
5
220
150
4400
3000
147
100
6
180
140
3600
2800
120
93
7
120
120
2400
2400
80
80
8
70
150
1400
3000
47
100
Average
150
150
3000
3000
100
100
St. Dev.
53
15
1060
302
35
10
COV
35
10
35
10
35
10


So let’s do a nitrogen example, using figure 1 (the nitrogen response curve) you can make an estimate of the corn yield that would be achieved from each of the knives (and if you assume a maximum yield of around 200 bushels an acre) can figure out what the field level yields would be.  So if you work through this math, you can find a few interesting results (table 2). Even though we were putting on the same amount of nitrogen on in both cases, because of variation from knife-to-knife we get different average yields per acre. In the case of corn following soybean, yields increased by 2 extra bushels per acre yield from the improved distribution and in the case of continuous corn about 4 extra bushels per acre. But there are other things to notice, the coefficient of variation in corn yield is always much lower than that in the nitrogen application rate. The soil supplies some of the nitrogen and this dampens out the response making everything a bit more uniform, but one thing to keep in mind is that early in the growing season the response might be more visually drastic than what final yields end up showing. Overall I think this asks an interesting question -how good is uniform enough, how does application uniformity uncertainty compare with other uncertainties in crop production, and how does this information help us make better manure decisions?

Table 2. Impact of nitrogen application uniformity on crop yield in corn-soybean (CS) and continuous corn (CC) rotations for machines with coefficients of variation in their application of 35% (left) and 10% (right)
Knife #
Corn Yield (CS)
(bu/acre)
Corn Yield (CS)
(bu/acre)

Corn Yield (CC)
(bu/acre)
Corn Yield (CC)
(bu/acre)
1
187
194
162
179
2
193
195
175
182
3
196
196
183
184
4
198
195
190
182
5
198
194
191
179
6
197
193
186
177
7
191
191
170
170
8
178
194

146
179
Average
192
194
175
179
St. Dev.
7
2
16
4
COV
4
1

9
2


To learn more about this topic and how you can get the most benefit from your manure make sure you register for one of our four field days, get a free lunch, get your manure questions answered, and learn how to set yourself up to maximize the benefit of manure on your farm this fall.