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 crop land 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 than 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).
Figure 1. Percent of nitrogen available in animal
manures relative to the nitrogen removal with harvested, non-legume crops.
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 researches 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 are more sustainable, and more importantly these
farms continue to strive to do better.
