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Cocoa shells are a popular garden mulch because they're attractive and smell good. The mulch also enriches soil and uses material that would otherwise be wasted. But be cautious with this mulch if you have a dog. Many dogs are attracted to chocolate smells and may scarf down the mulch like candy. And like chocolate, it could seriously sicken them.
Here's the science. Cocoa shells are high in the substances theobromine and caffeine, which are methylxanthines. When cocoa beans are processed, methylxanthines “migrate from the bean into the shell," an article in the science journal Molecules states. And a large dose of methylxanthines can make dogs ill.
Part of the reason is that dogs’ bodies readily absorb methylxanthines, according to the Merck Veterinary Manual.
Furthermore, “dogs and horses appear to be far more sensitive” to methylxanthines, particularly theobromine, reports N.A. Adamafio in the Journal of Biological Sciences. Eating cocoa mulch can cause vomiting, diarrhea, blood in urine, high heart rate, and loss of movement control. Some animals have died as a result.
Cocoa bean hulls have more methylxanthines per ounce than milk chocolate and sweet dark chocolate, according to a report on chocolate intoxication in dogs. To put this in context, "one ounce of milk chocolate per pound of body weight" could be deadly, according to Merck.
Cocoa mulch can contain 10-30 milligrams per gram of theobromine, a Vetstream report on chocolate poisoning says. Dogs can become sick after eating 20 milligrams per gram of theobromine and have died after eating “80-300 mg. of theobromine per kilogram of body weight."
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Are people overreacting? The American Veterinary Medical Association (AVMA) said in 2006 that processing technology produced cocoa mulch with “lower chemical residues."
The AVMA also went on to report that the ASPCA's Animal Poison Control Center advised pet owners not to use cocoa shell mulch or at least be careful using it around dogs that eat anything (like every dog ever?). "The odds of dogs dying from eating the fresh mulch are low,” but pups who've eaten it should be taken to a veterinarian.
Bottom line? If you're a dog parent, check with individual cocoa mulch manufacturers to find out how the mulch was processed. Or, err on the side of caution and choose a different type of mulch for your yard.