Dairy manure and synthetic fertilizer: A meta-analysis of crop production and environmental quality

Authors: O'Brien, Peter; Hatfield, Jerry

Submitted to: Agrosystems, Geosciences & Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2019
Publication Date: 6/20/2019
Citation: O'Brien, P.L., Hatfield, J.L. 2019. Dairy manure and synthetic fertilizer: A meta-analysis of crop production and environmental quality. Agrosystems, Geosciences & Environment. 2(1):1900027. https://doi.org/10.2134/age2019.04.0027.
DOI: https://doi.org/10.2134/age2019.04.0027

Interpretive Summary: Dairy farms are an important part of the world economy and play and vital role in long-term food security. The demand for dairy products continues to increase, but land available for dairy farms is decreasing. To meet that demand, dairy producers must find ways to improve efficiency and reduce costs in all parts of the farm, including managing animals, animal feed, and animal manures. Many dairy farms now rely on traditional row crops, especially corn, to provide animal feed, which can produce more food on less land when compared to pasture-based grazing dairy farms. One method that can improve efficiency is to use dairy manure as a fertilizer to grow crops. While dairy farmers have been doing this for years, some researchers have shown that manure application can have negative environmental impacts, harming soil, air, and water quality. This meta-analysis combined the findings of 58 published studies to determine that using manure as fertilizer can produce the same amount and same quality of crop as using synthetic fertilizer. Manure applications increased soil organic matter, which is a good indicator of long-term soil health. However, soil and water quality may be negatively affected because the manure also increased soil P levels. Other measures of environmental quality, such as nitrate leaching and nitrous oxide emissions, were too variable make any conclusions. Overall, these results show that dairy farmers can use manure as a fertilizer to produce high quality crops for animal feed. This finding is important because it may reduce the costs of both purchasing other fertilizer and disposing of the manure. However, the manure must be applied with caution and at the appropriate rates so that it does not negative impact soil, air, or water quality

Technical Abstract: Management of dairy production systems, including animals, forages, and manure, must be optimized to meet rising demand for dairy products. Many dairy producers now rely on row crop agriculture to produce feed because animal density continues to rise, while land allocated to dairy farms is decreasing. To improve efficiency, producers often apply dairy manure to meet crop nutrient demand, both reducing the need for synthetic fertilizer and the cost associated with manure disposal. However, repeated manure applications have been linked with environmental concerns, such as soil nutrient loading, increased greenhouse gas emissions, and decreased water quality. The objective of this research was to synthesize the literature comparing dairy manure fertilizer with synthetic fertilizer to determine impacts to both crop productivity and environmental quality in row crop systems. The meta-analysis showed that, when applied at rates providing comparable levels of plant-available N, manure fertilizer matched synthetic fertilizer in crop production, quality, and digestibility. Manure application increased soil organic matter, which may have an overall positive effect on long-term soil fertility, but it also increased soil P, suggesting a threat to water quality. Soil nitrate leaching and nitrous oxide emissions were highly variable across studies, so no clear trends were evident. Overall, these findings suggest that manure is a suitable fertilizer in row crop agriculture, but it should be applied with caution to limit risk to environmental quality, which can vary with site-specific factors like climate, soil texture, and application practices.