On a drive through the countryside earlier this spring, you may have noticed some stripes or streaking in fields of corn, with the tell-tale signs of darker strips of green mingled with yellow right near it. While there are potentially numerous causes, they can be in fields that get synthetic fertilizers as well as manure.
So, let's talk about what we are seeing and looking for, and perhaps why this year it appears a bit more vividly. This is always most pronounced early in the growing season — especially in years with a cooler spring.
When it comes to nitrogen supply, typically there are multiple sources, the fertilizer we add to support crop growth and mineralization from the soil. Early in the growing season, especially when soils stay cool, plants are more dependent on the added nitrogen (additionally, the less formed root system means they are not collecting from larger areas of soil). As temperatures get warmer and soil mineralization speeds up, often times things will start to look a bit more even.
So, let's start with liquid manure. Here we are talking about two things; knife-to-knife variability and the nitrogen application angle relative to the planting angle. The first is a machinery question — how uniform is the flow with different distributors and sizes of the tool-bar? The second means the corn will be planted different distances from the nitrogen source so it will be different sizes when the roots finally make it to the nitrogen.
So, what are we looking for to tell if this is an issue in our field? Is the striping that occurs at the spacing of the knives and in the direction of travel of the manure application?
I've often approached this topic for liquid manure with a discussion on how much variability we are seeing from knife-to-knife. 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 has 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. Table 1 shows how the addition of nitrogen causes corn to respond. What the figure demonstrates is we want to apply somewhere around 150 pounds of nitrogen per acre. If we are less than this, the yield goes down; if we apply more than this, we see minimal yield improvement.
So what does this all have to do with manure application uniformity? When we are trying to hit 150 pounds per 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, let's think about the math behind this problem. 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 pounds of nitrogen per acre from liquid swine manure that tested 50 pounds available nitrogen per 1,000 gallons, so we were applying 3,000 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, and the other has a coefficient of variation of 10%. In both cases, let's figure an eight-knife setup. To give you an idea of 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 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.
Let's do a nitrogen example. Using the nitrogen response curve, you can make an estimate of the corn yield 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 1). Even though we were putting on the same amount of nitrogen, because of variation from knife-to-knife, we get different average yields per acre. In the case of corn following soybeans, yields increased by two extra bushels per acre yield from the improved distribution, and in the case of continuous corn, about four extra bushels per acre.
There are other things to notice, though. The coefficient of variation in corn yield is always much lower than 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 interesting questions: 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?
What about on the solid manure side? Again, we can see the same thing if solid manure isn't uniformly spread, but it can be a bit more complicated than that. What type of manure are we talking about and was it the only fertilizer source?
If the manure had a high carbon-to-nitrogen ratio (such as straw rich bedded pack manure), the heavier application spots may look deficient, not those that didn't get as much manure, but because as this material breaks down, we may experience short-term nitrogen tie-up. Learning to watch and diagnose these spots is important for learning what it has to teach you.