How much does a footbath solution affect mineral concentrations in the soil once manure is applied? That is the question regional dairy educator Aerica Bjurstrom and her colleagues at the University of Wisconsin have tried answering.
Bjurstrom presented preliminary research findings of her study during the educational seminar, “Impact of copper sulfate footbath use on eastern Wisconsin’s manure, soil and forage copper concentrations,” during the 2023 North American Manure Expo in Arlington, Wisconsin.
“Copper is naturally occurring in soils, and it is rarely deficient in Wisconsin,” said Bjurstrom, prefacing the study’s purpose and results. “It binds with the clay, so really once it’s on the farm, it stays on the farm.”
The project followed nearly 12,000 dairy cows for digital dermatitis (DD) on 45 eastern Wisconsin dairy farms. Then in 2022, Bjurstrom and her team evaluated copper concentrations in the soil, alfalfa plants, manure and cattle livers from 20 farms in 12 Wisconsin counties that were known to use copper sulfate as a footbath solution. Herd sizes ranged from 190 to 4,600 cows.
The domino effect of copper on farms
Results from a university survey in 2016 discovered that copper sulfate was the most commonly used disinfectant solution to manage DD in Wisconsin herds. Nearly 40% of survey respondents used a 4% to 6% copper sulfate solution one to three times per week, Bjurstrom noted. Industry standards are to use a disinfectant at 2% to 5% concentration and change the solution after 150 to 300 cow passes, offered three times a week to the entire herd.
“We know copper sulfate is a low-cost solution for footbaths in cold temperatures,” Bjurstrom said. “But we also know less is more, not more is more. It’s most effective if we use it like antibiotics and don’t overuse it.”
It is also known that plants absorb copper from the soils which – depending on the concentration – can impact the lateral root and seedling growth. A plant’s tolerance for copper presence varies, as pasture grass typically has a low tolerance while alfalfa can withstand more.
The same is true for copper concentrations in dairy cattle. Concentrations above 15 parts per million (ppm) for dry and lactating cows, and above 25 ppm for calves may cause oxidative liver damage.
Preliminary results indicate judicial amounts of copper in forages
Bjurstrom’s research results came from several samples of alfalfa in various stages of growth as well as soil types and depths. For the alfalfa plant, samples were taken from the top of the plant and the whole plant before first crop, third crop and after third crop, if manure was applied. The soil was sampled at 6 inches below the surface, 1 foot, 2 feet and 3 feet below.
The results indicated levels within industry standards. Concentrations of copper in the alfalfa were at 11.1 milligrams (mg) of copper per kilogram (kg) from the top cuttings and 12.1 mg per kg in the whole-plant samples.
“Comparing those results to industry surveys, they were right in the range of what we want to see,” said Bjurstrom, noting the National Resource Council suggests a safe concentration of 9 mg of copper per kg for legume silages.
Differences in results were noticed at first crop and after third crop with copper levels at 11 ppm and 13 ppm, respectively. Industry standards are at 9 ppm for silage and 13 ppm to 15 ppm for other feed.
“We could reasonably assume the reason we saw higher concentrations in third crop was that the plant was shorter at harvest,” Bjurstrom said. “It was also hotter when we harvested that third crop in July.”
Surface soil was tested for copper on 19 of the participating farms. Bjurstrom shared the results from the top 6 inches of soil with an average copper concentration of 6.1 ppm, but the samples ranged from 1 to 13.5 ppm.
Additionally, manure samples were taken on six participating farms with the average copper concentration at 373 ppm; industry guidelines suggest a safe range of 60 to 847 ppm.
The results from the study indicated copper from copper sulfate footbath solutions does influence soil and forage concentrations after manure is applied to the fields. However, the results were not indicative of a large influence, but something to be aware of.
“When using copper sulfate in a footbath, we need to know how much is used,” Bjurstrom said. “Applications are less than 2 pounds per year, but that does gradually build up over the years.”
For example, Bjurstrom shared that a 5% solution in a 96-square-foot footbath is 25 pounds of copper sulfate. If this is on a 1,000-cow dairy that provides four changes per offering, that results in 15,600 pounds of copper sulfate per year.
“That’s a lot of copper,” Bjurstrom said. “To address copper presence, the nutritionist should monitor it in the forage, the agronomist should monitor it in the soil and manure, the veterinarian should monitor it in the footbath. Dairy producers should maintain their hoof-trimming schedules, spot treat and consider replacing copper sulfate.”
Alternatives for copper sulfate include zinc sulfate, which creates similar environmental troubles, and formalin, which is a known carcinogen.
“Nothing is perfect,” Bjurstrom said. “They’re all evil necessities.”
Bjurstrom’s research will continue with a footbath management survey for participating farms and liver biopsies of cattle on those farms or in commercial slaughter plants.
