Dairy Manure and Synthetic Fertilizer: A Meta‐Analysis of Crop Production and Environmental Quality

Abstract

Core Ideas Dairy manure matches synthetic fertilizer in yield and quality of row crops. Manure application increases soil organic matter and soil P levels. Appropriate application rates are critical to avoid negative environmental impacts. Dairy production systems rely on row crop agriculture to produce feed because animal density continues to rise, whereas land allocated to dairy farms is decreasing. Management of the entire dairy system, including animals, forages, and manure, must be optimized to meet production goals while remaining economically viable. Many producers 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 in row crop agricultural production systems to determine impacts on both crop productivity and environmental quality. Fifty‐eight studies were included in a meta‐analysis that 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.