Effects of Swine Manure Management Options
on Groundwater Quality and Crop Response

Thad Hardeman
Graduate Research Assistant in Agricultural Engineering
Iowa State University

Mickelson, S. K., Baker, J. L., and Kanwar, R. S.

Introduction

One of the primary concerns of the growing hog industry is the issue of swine (Sus spp.) manure application. Manure, a valuable source of nitrogen and phosphorus, can contaminate both surface water and groundwater. Runoff from applied fields can cause eutrophic conditions in lakes, ponds, and estuaries. The effects of manure application on groundwater quality are somewhat different than that of surface water. Groundwater pollution often directly affects drinking water and can occur as contamination by bacteria, nitrogen, and sometimes phosphorus. The largest nutrient contamination concern for groundwater, however, is that of nitrogen leaching. Nitrogen leaches primarily as nitrate (NO₃), an extremely mobile anion. High nitrate in drinking water has been documented to cause methemoglobinemia, or blue baby syndrome, in infants under 6 months of age (Johnson et al., 1987). Furthermore, Spalding and Exner (1993) note that consumption of nitrate contaminated drinking water may be linked to hypertension, birth defects, cancers, and infant mortality.

The level to which groundwater becomes contaminated depends greatly on the circumstances in the crop production system. Crop, rainfall, soil, and nutrient management are variables included in such a system. Within the nutrient management scheme, three variables, rate, method, and timing are important in terms of manure application. The objective of this research and paper will address the effects of rate, method, and timing of the land application of swine manure on groundwater quality and crop response.

The study, which began in January of 1996, was conducted on the Iowa State University Agricultural Engineering Research Farm near Ames, Iowa. Soils at this site are primarily Clarion Loam located on 2-4% slopes. Twenty-seven plots, three replications of nine treatments, were surveyed and laid out. A randomized block design was used for appropriate statistical analysis. The study was set up to examine surface water quality, groundwater quality, and crop response.

A list of the treatments is given in Table 1. The application rate was based on N recommendation for corn with a yield goal of 160 bu/acre. Considering losses, availability, and soybean credit, the rate was calculated and rounded to 150 lb N/acre. Treatments 4 and 5 are noted as "new" because of the utilization of an injector knife recently designed for low residue disturbance.

The treatments allowed for examination of manure application rate, method, and timing. Two rates were being examined; a single rate and a double rate. To account for method and timing, four systems of application were used - fall inject, fall new inject, late-winter broadcast and spring inject. For background comparative purposes, a control plot was included. The control plot received 150 lb N/ac of commercial inorganic fertilizer. Because the project began in the winter of 1996, all fall treatments received inorganic fertilizer in the 1996 season at the same rate as the control.

Figure 1 shows tile water nitrate concentrations for the manure applied and control treatments. The common trend in the figure is the apparent flushing of nitrates through the tile lines as the season progresses. Figure 1 shows the double rate winter broadcast as consistently having the highest concentration of nitrate over most of the season. Season total nitrate-nitrogen losses and flow-weighted concentrations are shown in Table 2.

In terms of yields in 1996, the corn yields were higher for the manure application treatments than all other treatments (Table 2). Examining the treatments that received inorganic fertilizer, all appeared to be similar in terms of corn yield. The double rate winter broadcast is higher than the single rate winter broadcast as well as all other treatments. Comparing between spring inject treatments, there does not appear to be a major difference. Soybean yields do not show great differences among the treatments.

Stalk sample data, acquired in a fashion as noted by Blackmer and Mallarino (1997), is shown in Table 2. The double rate winter broadcast and the spring inject treatments had significantly (at the 1% level) higher mean nitrate concentrations than the other treatments. Stalk test results also indicated a significant (at the 1% level) difference between double rate and single rate treatments as well as between spring inject and winter broadcast.

Weekly examination of the 1997 tile water nitrate concentrations generates similar conclusions to those of 1996. Figure 2 shows tile water nitrate concentrations for the control and the fall applied treatments. Figure 3 shows tile water nitrate concentrations for the twice manured and control treatments. Season total nitrate-nitrogen losses and flow-weighted concentrations are shown in Table 3. Concentrations of nitrate-nitrogen in the tile water samples were lower in 1997 than 1996.

As shown in Table 3, yields in 1997 indicate the effect of double rate applications over single rate applications. For average yields, all double rate treatments performed better than the respective single rate treatments. The spring inject treatment had the greatest average yield for single rate treatments. For double rate treatments, again the spring inject treatment had the greatest average yield.

Careful examination of interactions among treatments leads to a conclusion as to which management system may be deemed "best," that system which minimizes environmental impact and gives optimal crop response to nutrients in manure. The treatment that best fits the goal of manure application, minimal environmental impact with optimal nutrient utilization for crop response, is the single rate spring inject. Compared to the double rate spring inject, the single rate treatment gave similar yields without requiring "excessive" nitrogen, as indicated by the nitrate stalk test. Additionally, for both years, the single rate spring-inject treatment had lower losses than the double rate winter broadcast. In 1997, the single rate spring inject had lower losses than the double rate spring inject.

Table 1. Treatments

Table 2. 1996 Yield, stalk, and flow nitrate-nitrogen data

Table 3. 1997 Yield, stalk, and flow nitrate-nitrogen data

Figure 1. Weekly nitrate-nitrogen concentration during 1996 season

Figure 2. Weekly NO₃-N concentration for fall applied and control, 1996

Figure 3. Weekly NO₃-N concentration for twice applied and control, 1996

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