Manure is a nutrient-packed by-product with the ability to help fertilize fields. Liquid dairy manure contains a variety of beneficial macronutrients, but its micronutrient profile deserves equal attention. This includes secondary nutrients such as calcium, magnesium, and sulfur, and micronutrients such as boron, copper, zinc, manganese, and iron. Micronutrients are vital for crop health, and liquid dairy manure provides a natural, cost-effective source.
Years of data
In a collaborative effort between the University of Wisconsin-Madison Division of Extension, the Nutrient and Pest Management (NPM) Program, and AgSource Laboratories, liquid dairy manure micronutrient lab test data was reviewed by a team of manure experts. Liquid manure samples are those with a dry matter (DM) content less than or equal to 11%. This analysis included three data sets covering 28 years.
Big data sets can be powerful in identifying trends in variables of interest. Broad data analysis is beneficial for dairy studies because farms are diverse, varying across climates, soil types, management systems, sizes, dairy breeds, crop management, and feed rations. Once trends are identified, they can be investigated more closely on each farm. For this analysis, the exact age of each manure sample was unknown. Samples obtained represent likely scenarios for dairy manure management and ensure a representative and random subsample of dairy manure.
Dive into the micronutrients
Recent data sheds light on the micronutrient composition of liquid dairy manure. While most of our results reflect general trends reported previously, a few new observations were made (see table).
1. Over the 28-year period, three data sets of dairy manure samples from Wisconsin showed micronutrient values that were trending higher for calcium, magnesium, iron, zinc, copper, and manganese.
2. The overall highest rise across the 28 years was in iron. The reason for this jump was not determined. Yet, agronomically, slightly higher iron may make more iron available in soils with a pH over 7.5. This includes soils with naturally high pH that are found along Lake Michigan in Wisconsin.
3. Sulfur and boron levels dropped across the 28-year data sets.
Two long-term trends were especially noteworthy. First, manure content of sulfur and boron declined over the data sets, presumably reflecting “cleaner” processing of both dairy trace mineral and crop fertilizer micronutrient products and air quality. Dairies are no longer obtaining any “free” micronutrients as clean air means little to no sulfuric acid deposition from acid rain as in the past. According to the Environmental Protection Agency, the reduction in total sulfur deposition (wet plus dry) in the Midwest was 81% from 2000 to 2002 and 2020 to 2022, a value of similar magnitude to that of wet sulfate deposition of 71%.
Second, the copper content of liquid manure samples continues to rise, reflecting continued or greater use of copper sulfate foot baths. This raises concerns about any long-term soil loading and potentially higher plant levels.
It’s best to test
We graphed the data and found trends for rising and falling levels of the secondary micronutrients for which Wisconsin liquid dairy manure is tested for in the lab (see figure). The high variability of manure content, however, indicates that these average values are not reliable for nutrient management planning purposes. What they do is emphasize the need for on-farm manure data to determine application rates.
It has long been thought that dairy manure typically contains enough key micronutrients, making it a beneficial amendment. However, the long-term data from this analysis underscores the need for farmers to check their own manure sources for nutrient value. Iron and zinc are usually present in adequate amounts to support crop growth. Manganese and copper levels are typically sufficient but may require monitoring to prevent toxicity. Boron and sulfur are present in smaller amounts, emphasizing the need for testing manure, soil, and plant tissue in high demanding crops like alfalfa and silage.
Liquid dairy manure is more than just a waste product; it’s a reservoir of vital crop nutrients, including often-overlooked micronutrients. By integrating it thoughtfully into crop conversations, farmers can improve soil fertility, enhance crop yields, and contribute to a sustainable agriculture.
This article appeared in the November 2025 issue of Journal of Nutrient Management on page 20.
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