Is Horse Manure Safe For Organic Gardens? PDF Print E-mail
Written by Maria Gaura   
SANTA CRUZ (January 2009) - It may be stinky and attract flies, but nothing makes a garden grow like a steaming pile of horse manure. Horse droppings make an excellent garden fertilizer and, better yet, can be collected for free at many stables. But is horse manure as natural as it smells?
Horses are prone to a host of parasites, and most horse owners regularly dose their animals with vermicides – medications toxic to intestinal worms and other insect pests. These medications pass through the digestive system, prompting some gardeners to ask whether tainted manure may be harming their crops, their families, or the environment.
Vermicides kill intestinal worms, but are they also fatal to earthworms? Do vermicide toxins build up in the soil over time? Can vegetables be considered ‘organically grown’ if they were fertilized with vermicide-laced horse manure?
REASSURING RESULTS
Fortunately for gardeners and animal owners, most research to date indicates that Ivermectin, the vermicide most frequently given to horses, cattle and sheep, breaks down quickly once it is excreted. Several studies have shown that Ivermectin degrades rapidly when manure is hot-composted or exposed to sunlight, and somewhat less rapidly when manure is simply piled up and left to decompose.


Many horse stables give away manure for free
A study of Ivermectin-treated sheep found the half-life of the chemical in sheep manure to range from seven to ten days. Because of quick decomposition, Ivermectin preparations given to animals have not been found to build up in manure-amended soil.
And a three-year experiment by a team of Swedish scientists showed that manure from Ivermectin-treated cattle had no impact on the numbers or diversity of nematodes living in pasture soil, compared to a control pasture stocked with untreated animals.
STILL TOXIC
This doesn’t mean that vermicides are completely safe, however. Ivermectin can be fatal to certain breeds of herding dogs and to numerous aquatic creatures, and can cause nausea, headache and heart irregularities if splashed onto human skin. In addition, tainted manure can kill worms in compost bins, according to Karin Grobe, worm composting specialist for Santa Cruz County.
Some studies have shown that very high concentrations of Ivermectin can be fatal to earthworms and soil creatures, though such high exposures would likely be the result of a spill or careless disposal of medicine containers.
And there are several newer types of vermicides now in use that have not been as well studied as Ivermectin, which was introduced in 1981.
Scientists at the Food and Drug Administration’s Center for Veterinary Medicine are currently studying the effects of six veterinary vermicides, including Ivermectin. The results of this multi-year study will not be available for another year or two, according to CVM spokeswoman Siobhan DeLancey.
COMPOST REGULATION
The results of the CVM study may help organic farming organizations decide whether new regulations for compost are needed to protect the integrity of organically-grown foods.
At present, certified organic farmers are allowed to use manure from non-organic livestock on their crops, according to Jane Sooby, research specialist with the Organic Farming Research Foundation in Santa Cruz. However, farmers must test for chemical residues if contamination is suspected.
The transformative powers of the composting process have been proved by other studies. A pilot study by the USDA’s Agricultural Research Service found that as much as 75 percent of the antibiotics administered to beef cattle is excreted in urine and manure. But the antibiotic concentrations in cattle manure dropped by 91 to 99 percent after 28 days in a compost pile made of layered straw and manure. The antibiotic load in a manure-only pile decreased by 75 to 90 percent over the same period of time.
FEED THE WORMS
The best advice for gardeners concerned about vermicide residue is to compost fresh horse or cattle manure before using it, or to expose it thoroughly to sunshine. The most effective way to compost fresh manure is to alternate layers of manure with dry straw, building a pile at least 2 to 3 feet tall. Six weeks of composting should provide plenty of time for vermicides or antibiotics in the manure to degrade.
Worm specialist Grobe hopes fear of contamination doesn’t dissuade gardeners from making their own compost. “I just tell people that the vermicide wears off,” Grobe said. “Probably the best way to increase worms in your yard is to put out a load of manure. Worms need something to eat, and when you see thousands of them thriving out there, you know this stuff isn’t harming them.”

 

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