Land Use and Manure

We all know that our land resource base is finite. No matter what, Iowa is only going to have an area of 55,857 square miles (or just under 36 million acres); of this, about 26.2 million acres, or about 73% of the state, is farmland. How we choose to use this land can have profound impacts on our ability to produce food, fuel, and fiber and the impacts we have on the environment. Our goal here is not to discuss the different impacts choosing alternative land uses can have, but rather to look at how we are currently using our land, especially farmland use, and how it varies throughout the state.

As we do this we are going to focus on two maps of Iowa, the first gives the percent of land in each county in Iowa that is classified as farmland. As stated above, as a state, Iowa has 73% of its land in crop production, but this changes pretty substantially throughout Iowa with a low of 34% in Appanoose to a high of 94% in Palo Alto and Grundy counties. This map is shown below, each county is color coded with darker green counties having a lower percentage of its land in cropland, and more red counties having a higher percent of its land in cropland. What we see should be pretty expected, both the Des Moines lobe soil formation and the loess soils of western Iowa. Much lower percentages of our land are used for cropland in southern Iowa, especially south central and southeast Iowa.

Figure 1. Percent of land that is cropland (based on 2012 Census of Ag).

As a follow-up I was also interested in what percent of our cropland we are using for corn production. As a state Iowa averaged 54% of its cropland planted to corn, but again this varies throughout the state. In this case the lowest value was 25% corn, which occurred in both Lucas and Appanoose counties, while the high was 73% which occurred in Delaware county. In this case the pattern indicates that the northern 2/3 of the state have more corn in their rotations, as these counties typically had more than 50% of their cropland planted to corn, while the southern 1/3 of the state utilized different crops, as they often had less than half their cropland planted to corn.

Figure 2. Percent of cropland that is planted to corn (based on 2012 Census of Ag).

So, what does this have to do with manure? Well, growing corn requires a fair bit of nutrient, both nitrogen and phosphorus, so it is often an excellent crop to utilize our manure resources to grow. In fact, I have read several pieces of scientific literature that state we find more continuous corn rotations around swine farms, potentially this could be try if we were trying to find crops to use our manure nutrients on a few of acres as possible to limit the distance we have to haul the manure. Alternatively, it might be true if our manure was proving serving as a super-cost effective fertilizer that its use as or fertilizer source made continuous corn rotations more profitable than the corn-soybean rotations. (I have looked at this question a bit, and will soon share with how estimated profits for continuous corn and corn-soybean rotations compare if you are using manure as the fertilizer or if you are using synthetic nitrogen fertilizer and what I think this means for rotation selection).

So that got me wondering, is animal farming (or manure production) driving our crop selection patters at the landscape level? That is, are our animal farms in Iowa driving a land use change where we see more of our land being used for corn production, which would be indicative of continuous corn rotations). This could be an important question, as Iowa State research has generally showed greater nitrate leaching potential in continuous corn rotations than in corn-soybean rotations, probably due to the greater fertilization requirements in continuous corn situations as compared to corn-soybean rotations. What I found indicated that there was no relationship between manure production and the choice to plant more corn (at least at the county level and across all animal species). This is shown below in figure 3, what the data shows is random scatter around a flat line describing the relationship between how much manure nitrogen is available in a county and the amount of that county’s cropland planted to corn. When I focused just on swine farming, I saw a very similar pattern, i.e., no relationship between animal production and the percent of crop ground planted to corn. This technique provides pretty good evidence that at the county level, animal farming doesn’t seem to be driving our crop rotation choices.

Figure 3. Relationship, or lack thereof in this case, between the amount of manure produced in a county and the percent of cropland planted to corn within a county.

So… what does this all mean, and why was I doing it anyway? Well, next time you hear someone say animal farming is causing us to grow too much corn, you can say that having animals in your county doesn’t appear to increase how much corn is grown.

The reason I started this analysis was far different than what I’ve shown today. I was interested in determining when a nutrient partition (nutrient separation) manure treatment technology would be cost feasible to implement. To do this I was evaluating how a farm’s costs of manure application changed with differing levels of treatment effectiveness. Performing this analysis requires information on how the manure will be distributed, i.e., the transport distance from the farm. This is dependent on the availability of land for manure application and the amount of manure (in terms of total nutrient need) that can be applied. These values most certainly vary from farm-to-farm as they depend not only on items including topography, road placement, locations of streams and drainages, and variables of the like, but also on how much land the farmer owns, the willingness of neighbors to either accept or buy manure nutrients, the crop rotations being utilized, and the yield potential of the crop. Hence an interest in understanding how much cropland and in particular corn ground is available for manure application as this will impact our result, so yet again, something for you to look forward to in the future.