The answer may lie in the beneficial attributes of manure. Because animal waste consists of organic matter, applications can improve a variety of soil characteristics, including increased water infiltration rates and water-holding capacity that help reduce runoff. One study, for example, showed how cropland treated with poultry litter took eight times longer than untreated cropland to begin runoff after rainfall. Using manure to replace highly soluble chemical fertilizer may also explain why manure treatments experience decreased rates of runoff.
But we only get these benefits if manure is applied correctly. Nutrients from animal waste can similarly enter surface water if manure is managed poorly. Examples might include spills, winter spreading and excessive application. (Manure may also be harming well water. However, our study looks at surface water quality only.)
Furthermore, our result is stronger evidence for the effectiveness of manure spreading when considering that our data includes farmers with poor manure management practices. If our aggregate analysis includes over-appliers and winter-spreaders, and our result still indicates a positive effect, it stands to reason that manure usage’s effect on water quality would be even greater if all farmers had spread at correct rates.
Thus our analysis suggests that substituting manure for chemical fertilizer can significantly improve water quality. Policies that encourage such a change could reduce the eutrophication of rivers and lakes, thereby leading to a number of environmental and economic benefits. Our research simulates these economic gains for some home owners in Dunn County. If a policy caused five percent of farm acres contributing to nutrient loading in the county to switch from fertilizer to manure, lakefront property values would increase by 1,587 dollars. Larger changes of 10 and 15 percent could push up values even more—by 7,935 and 12,696 dollars, respectively.
However, considerations for deciding whether to adopt manure are complex. While it may be better for water quality, there are legitimate concerns about animal waste’s variable nutrient content, weed seeds, and its costs of transport and application. Further work is needed on policies that will make manure usage more cost-effective and appealing to farmers. Moreover, efforts are needed to encourage correct rates of nutrient application that are critical for reducing water pollution. It is clear that cleaning up our water is no easy task. But for now, knowing manure may be more effective than chemical fertilizer for reducing runoff is a step in the right direction.