Weed killers may go from plant to pooch

Many people treat their lawns with weed killers — also known as herbicides — to rid themselves of unwanted plants, such as dandelions. Most people know to keep small children away from the grass after it’s been sprayed. That’s because these chemicals can be dangerous if children touched the treated lawn and then put their hands to their mouths. New data show that herbicides also can end up in dogs. The evidence: It comes out the other end in the animals’ urine.

Angus Murphy studies plants at the University of Maryland in College Park. He began to wonder if dogs might be exposed to herbicides when he saw neighborhood signs that warned a lawn had been sprayed with weed killers. “I would see the dogs running through the yards when the grass was still wet,” he recalls. “I looked at the signs and they said don’t re-enter [the lawn] for 24 hours or until the treatment was dry.”

So he teamed up with other scientists to investigate how much weed killer might come off the grass while it was wet — and whether those chemicals might go from plant to pooch.

First, Murphy and his group had to find out how long herbicide sprays can be brushed off with casual contact. They applied the same amount of three different kinds of weed killers to different patches of grass.

But the amount of chemical coming off grass might change if the grass was wet or dry. So the scientists added herbicides to green grass that was wet (to simulate a recent rain) or dry. To find out if it made a difference whether the plants were dead or alive, the researchers also applied weed killer to brown grass.

Afterward, the scientists placed pieces of cloth on wooden blocks. They dragged the blocks across each patch of grass to see how much herbicide came off. They first did this a few minutes after the spraying. Then, they dragged clean cloth blocks across the area one hour, one day, two days and three days after the weed killer had been applied.

In green grass, two of the weed killers rubbed off on both tries the first day, but not after that. A third chemical known as 2,4-D — for 2,4-dichlorophenoxyacetic (Di-KLOR-oh-fen-OX-ee-uh-SEE-tic) acid — rubbed off onto the cloth for two full days after application. And on dry, brown lawns, 2,4-D was still coming off the grass even three days later. That was long after the blades of grass were dry.

“I was pleased to see there was data on brown lawns,” says Mark Carroll. A plant scientist, also at the University of Maryland, he was not involved in the new study. “We often don’t think to do that stuff” — check for a late effect of something on different types of targets. But the researchers did that and turned up a surprise, Carroll notes. That signals that “if you apply these products to a brown lawn, it’s going to hang around a little longer.”

From lawn to dog

So the herbicides were coming off the grass. But unless the chemicals get into animals, it might not pose risks. So Murphy and his group recruited 33 dogs and their owners. They included people who sprayed weed killer on their lawns and those who did not. Then, before and after lawns had been treated with herbicides, the researchers collected the dogs’ pee. (“I’m happy to say I wasn’t there to do it,” Murphy says. “It was one of the students from the vet school.”)

Most dogs — including half of those whose owners did not treat their lawns — had herbicides in their urine. Among dogs whose owners did spray weed killers, 14 out of 25 animals had chemicals in their urine before the latest spray of their lawn. After spraying, 19 out of 25 dogs were excreting the chemicals.

“The herbicides move into the animals and it’s detectable,” says Murphy. And as might be expected, “the greatest uptake and highest levels were where the homeowners were applying [these chemicals],” he notes.

“What surprised us the most was the extent to which there was uptake in the animals when [their lawns] weren’t having treatments,” he says. These animals appear to get exposed during walks in the neighborhood. This can include grassy areas where others have used weed killers. Murphy and his colleagues published their findings July 1 in the journal Science of the Total Environment.

“I think it’s a neat combination of work, where you’ve got half the team looking at the fate of the herbicide and the other half looking at the potential to affect dogs,” says Carroll.

The dogs may lick lawn chemicals off of the wet grass. It’s also possible the herbicide collects on their fur and that the pets lick it off later. But if the chemicals are ending up in the animals’ pee, then they’re definitely being exposed. Murphy does not yet know if this is a problem. His group did not look for signs of harm.

Still, the scientists say, the new data suggest that if the label on the weed killer says to keep off the lawn, that should apply not only to people but also to pets. “Anything you apply to the environment, you’ve got to have some respect for it,” Carroll says. Those instructions on the label are important. They are usually required by law to keep people and their belongings safe. “But a lot of people don’t read the label,” Carroll notes. “If you’re going to make the choice to apply the product,” he says, any responsible owner also will “keep the dog off the yard.”

Power Words

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canid  The biological family of mammals that are carnivores and omnivores. The family includes dogs, wolves, foxes, jackals and coyotes. Members of this family are known as canines.

chemical     A substance formed from two or more atoms that unite (become bonded together) in a fixed proportion and structure. For example, water is a chemical made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H₂O.

2,4-Dichlorophenoxyacetic acid  Also called 2,4-D, this is a man-made chemical meant to mimic a plant hormone. These plant hormones, called auxins, are toxic to some plants at high doses, so 2,4-D is commonly used as a weed-killer or herbicide.

excrete  To remove waste products from the body, such as in the urine.

herbicide   A weed killer. Some herbicides kill all types of plants, but others are “selective.” That means they are designed to kill certain unwanted plants (considered weeds) but leave desirable plants, such as lawn grasses or crops, untouched.

toxic  Poisonous or able to harm or kill cells, tissues or whole organisms. The measure of risk posed by such a poison is its toxicity.