Are hog farms the leading cause of nitrate-nitrogen in water supplies?

In a recent discussion, I was asked how much of Iowa’s fertilizer need was being met by land application of swine manure. At the same time, I’ve been driving across much of Iowa and looking at a corn crop that appears to have run out of nitrogen because of potential leaching of water soluble N due to the wet summer. I have also started thinking about community water supplies and the potential in future years for the pork industry to be cited as the cause of elevated nitrate-N levels in drinking water supplies.

This led me to do the following math. In 2007, Iowa had 13,842,282 acres of corn harvested for grain, with an average yield of 166 bu/A as reported by the 2007 USDA Census of Agriculture. If I assume that today’s hybrids are very efficient users of nitrogen, it takes about 0.8 lb of nitrogen per bushel of corn. This means that the Iowa corn crop in 2007 needed 1,838,255,000 pounds of nitrogen.

On December 1, 2006, there were 16,220,000 pigs in the kept for market inventory in Iowa. These pigs would have represented the average population that produced manure in 2006 that was land applied in the fall of 2006 for use by the 2007 corn crop. Note that I’m ignoring the manure production from breed-wean facilities. Although they produce a large amount of manure, relative to wean-finish their contribution to the following math is low, especially given the fact that approximately 50% of the wean-finish inventory in Iowa is imported from another state or Canada.

For ease of computation, I said all of the pigs would be housed in the equivalent of wean-finish facilities for an average stay of 165 days from weaning to slaughter. If the average manure production in wean-finish is 0.9 gal/pig/d and the manure in the pit contains 50 lb of total N per 1000 gal of manure, and there are 2 turns of pigs per space/year, this becomes 240,867,000 lb of nitrogen applied to Iowa cropland in the fall of 2006, assuming no losses from land application, etc.

The nitrogen from wean-finish swine manure applied to Iowa cropland had the potential to supply 13% of the total corn crop need in the 2007 year. Yes, some of the manure would have been over applied with the potential of leaching to water supplies. However, this analysis suggests that the swine population in Iowa is not nearly large enough to be the major source of nitrogen for the Iowa corn crop, or the major contributor to the nitrate-nitrogen that is increasingly reported from public water supplies.

In the above example, I used Iowa as it now has over 30% of the pigs in the nation. You could just as easily do the math for your local county or region using the USDA Census of Ag data for crop acres and inventory of pigs numbers. While Iowa no longer reports the pig inventory by county, Minnesota still does an estimate each December. Even in Martin county Minnesota (Farimont), the pig population is not large enough to produce enough manure to come close to meeting the fertilizer needs of the corn produced by farmers in that county.

Pork producers across the US have made remarkable progress in responsibly applying swine manure to crop land as a fertilizer resource. As communities struggle with meeting EPA nitrate-nitrogen levels in their drinking water, we need to use computations such as the above estimates to help them understand the impact of pork production in this issue. If we don’t speak up, we’ll become the bad guys once again.

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