Determining the Environmental Impact
of Irrigating with Swine Effluent

Charles A. Shapiro, Crop Nutrition
Bill Kranz, Biological Systems Engineering
Mike Brumm, Animal Science
Northeast Research and Extension Center-Norfolk

Bruce Anderson
Agronomy
University of Nebraska-Lincoln

Nebraska swine annually produces manure containing 40 million pounds of nitrogen. The trend toward increased concentration of animals in large production units makes it difficult to find enough available land for economical manure distribution at agronomic application rates. In Nebraska, pigs per farm have increased from 250 in 1982 to 507 in 1997. As the number of pigs per enterprise increased, there has not been a corresponding increase in the number of acres per enterprise available for land application and crop utilization of the stored swine manure.

The goal of our research is to evaluate alfalfa as a nitrogen sink for swine effluent. Data from our experiment has shown that alfalfa receiving 600 pounds of swine effluent nitrogen per acre removed about 100 pounds more nitrogen per acre than alfalfa receiving no swine effluent. Established, irrigated alfalfa can remove more than 700 pounds of nitrogen per acre in the harvested hay (Table 1). The implication is that producers can reduce the land base for effluent distribution by over 50% when compared to the 200-pound removal rate for corn followed by winter rye (Table 1). This could be beneficial to producers who do not have sufficient land to apply effluent at agronomic rates to corn or other row crops.

Additional advantages to alfalfa are: it covers the ground all year round which reduces the erosion potential; the nitrogen use curve is more constant through the season than for annual crops; effluent application can occur at times that are not possible in a corn system; and alfalfa is deep rooted and can scavenge nitrogen from deeper in the soil than most other corps grown in Nebraska.

A line source sprinkler system is used to distribute a range of effluent rates to both alfalfa and corn. Figure 1 shows the distribution of the effluent and of fresh water. The experiment is designed so that the distribution patterns of both the fresh and effluent waters produce an even amount of water application. Therefore, only effluent rates change. Rates of effluent are chosen that provide from 0 to 140% of the predicted nitrogen harvest for the corn-winter rye and alfalfa treatments. Irrigation of each crop can be controlled and is applied based on soil moisture and crop nitrogen needs.

Laboratory analysis shows that the effluent contains about 80 lbs total nitrogen, 100 lbs K₂O, and 10 lbs P₂O₅ per acre-inch of water. The goal is to apply sufficient effluent so that at the end of the growing season both the corn and alfalfa will have plot areas with an excess of applied N. Sampling soil, leachate and crop harvest takes place at five equally spaced areas across each cropping system plot for a range of 0 to 140 percent of nitrogen application versus estimated harvest removal.

At each sampling site a porous cup extractor is installed 6.5 feet in the ground. The soil water solution passing the cup is sampled and analyzed for nitrate. Other instruments determine the rate of water flow at that point. This information is used to determine the amount of nitrate leaching at each sampling site.

The original alfalfa stand was planted in the fall of 1992. In 1996 the corn-rye and alfalfa areas were switched. However, the gradient of increasing levels of swine effluent remained the same. In 1996, a non-nodulating alfalfa variety was planted along with the conventional variety, Saranac. Unlike the conventional variety, the non-nodulating line can not use atmospheric nitrogen for crop growth needs. Use of the non-nodulating alfalfa allows a determination of how much nitrogen harvested in the alfalfa came from the soil and applied effluent versus nitrogen fixed from the atmosphere by symbiosis.

In 1996, the non-nodulating alfalfa nitrogen harvest was 70 percent of the nodulating alfalfa at the zero effluent rate, but equal to the nodulating alfalfa at the higher nitrogen rates. Due to sufficient rainfall and the use of irrigation scheduling the maximum nitrogen applied in 1996 was 75 lbs total nitrogen/acre. A severe winter in 1996 caused winterkill in the experiment, so the alfalfa was replanted in 1997.

Documenting the environmental effects of swine effluent application is the major objective of this research. Two indicators have been monitored 1) soil nutrient levels in the spring and fall and 2) nitrate leaching.

Soil samples taken in the spring of 1997 indicated that a buildup of both phosphorus and potassium at the higher application rates is occurring (Table 2). Continued buildup of soil potassium could cause soil structure problems in the future. At some point, effluent might need to be reduced until soil phosphorus and potassium levels decrease.

Leaching of nitrate may occur when drainage through the soil profile occurs. When irrigation-scheduling techniques are used correctly, drainage is held to a minimum. When rainfall is greater than crop use, drainage is inevitable. In addition, manure storage capacity considerations may necessitate land application regardless of soil moisture availability, thus, increasing the risk of a drainage event. In 1994 there was drainage. Porous cup data summarized for the season indicated nitrogen leaching from all treatments (Table 3). If 10 ppm nitrate-N concentration is the limit for acceptable nitrate concentration then 340 lbs of nitrogen in the effluent can be applied per acre. This would result in less than 15 lbs nitrate-N leached per acre per year. Drainage was greater in the zero effluent treatments because less alfalfa dry matter was produced and total water use per acre was less. Alfalfa is an excellent target crop for swine effluent since it harvests more nitrogen then the corn-rye cropping system studied and drainage nitrate levels can be controlled by irrigation scheduling and application rates.

Research funded by Burlington Northern Endowment. The non-nodulating alfalfa was donated by Joanne Lamb at the USDA Dairy Forage Laboratory in Minnesota. Previous funding for this project included support form the Nebraska Pork Producer's Council and the UNL Water Center.

Figure 1. Field layout, water distribution and porous cup installation. Concord. NE.

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