How will waters of Iowa be impacted by expansion?

Iowa State University manure specialist explains how manure can be used as an effective fertilizer resource.

May 14, 2018

5 Min Read
How will waters of Iowa be impacted by expansion?

By Daniel Anderson, Iowa State University agricultural engineering specialist
Recently an article was published in The Storm Lake Times titled Nitrate levels expected to rise with hog numbers. The topic of nutrient management and the role animal production is multifaceted, rarely having any easy answers, and almost always having complex interactions that need to be carefully considered. Here we will take a look at some of the implications animal agriculture has on nutrient management and as a result, its impact the concentrations of nitrate and phosphorus in the Raccoon River. As animal agriculture looks to expand, an important question is, “How will waters of Iowa be impacted?”

An important starting point for any conversation on manure nutrients is, how effectively they can be used as a fertilizer resource. Most manure in Iowa is managed this way. In terms of nutrient losses, most university studies have indicated when appropriate application timing guidance is followed, and when similar nitrogen application rates are selected, nitrogen losses via leaching will be similar to those from commercial fertilizer sources. For example, the Iowa Nutrient Reduction Strategy science assessment team summarized all the research comparing both swine manure and poultry manure to spring applied commercial fertilizer and found nitrate-N leaching losses were similar as were corn yield. Similarly, in terms of phosphorus management, given the same soil conditions and runoff shortly after application, Iowa State University research shows using manure, instead of commercial phosphorus fertilizers, actually reduces phosphorus loss from that first runoff even by about 50%! The best research we have says, under appropriate management strategies and rate selection, the nitrogen use efficiency is not largely impacted fertilizer source. Rather, it is more controlled by crop rotation selection, soil properties, and weather conditions of a particular growing season and it will actually improve phosphorus management.

A second important question is, “Will the addition of new confinement operations result in too much manure?” While there are many ways to define and assess what too much manure is, an important starting point to the conversation is, “What cropland is available to which the manure could be applied as a beneficial fertilizer?” As a state, Iowa currently obtains between 25-30% of the nitrogen, phosphorus, and potassium we need for crop production from animal manures, with the rest of the required fertility coming from other commercial fertilizer sources. This does vary by county, but currently, all counties harvest more nutrients in the crops they produce that are available in the manure produced within that county. Figure 1 below shows how the amount of manure nitrogen available for crop production compared to the amount of nitrogen removed with non-legume harvested crops (i.e., doesn’t account for nitrogen removal with soybean or alfalfa).


In the recent article, one of the comments made was that CAFOs can change an area’s nutrient balance by importing feed for the livestock. While this does have the potential to create nutrient imbalances, it is only a piece of the puzzle. The Manure Management Plan (MMP) was developed to make sure, even if feed for the livestock was being imported, sufficient land was available to appropriately use the manure. While it can still serve as an indicator, it is one of many indicators of how nutrient management is working. Manure management plans ensure the same thing in a more measured and complete way.

We all know 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, fiber, and the impacts we have on the environment. Because of this, it was suggested adding animals would cause more row crop production to produce the feed for these animals. Yet, as a state, Iowa counties show very little correlation between the amount of land that is planted to corn and the amount of manure produced in that county.


Figure 2: 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.

In terms of nutrient management, this is only a piece of the puzzle as there are some additional differences between manures and other commercial fertilizers that may make their nutrient losses a bit different. In particular, manure is a complete fertilizer, in that it contains all the macronutrients crops need, but not necessarily a balanced fertilizer. That is, the nutrient ratios in manure may not be balanced to crop removal. Historically, manures have been relatively high in phosphorus as compared to plant available nitrogen. The Iowa Phosphorus Index has been used in Manure Management Plans (bill passed in 2002 and implemented in 2008) and uses information about how much phosphorus is currently present in the soil, how much will be added, and its risk of transport to an Iowa water body to determine if manure application should be limited by supplying nitrogen or phosphorus. This is a risk-based approach that focuses on water quality in making a manure management decision. With this said, changes in farming and feeding practices in the swine industry have reduced the amount of phosphorus in swine manures relative to its nitrogen content making its nutrient content approximately balanced for corn-soybean rotations and reducing the risk of phosphorus build-up.

So what’s all this mean? While there are certainly challenges to managing manure that make it a unique fertilizer option. Farmers strive to get value from this manure in their operations, and in so doing typically make application decisions that result in nutrient losses to water similar to those of other fertilizer option. Though it may seem like Iowa is livestock rich, it’s important to remember that adequate land to utilize our manure exist. Livestock operations play a vital role in Iowa’s agriculture economy and continue to strive to do so in ways that decrease environmental impact, that is more sustainable, and more importantly, these farms continue to strive to do better.

Source: The Real Scoop on Manure is written by Anderson and brought to you by Iowa State University Extension and Outreach.

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