Get Off My Lawn!

As spring transitions to summer, many pet parents are concerned for their lawns as dandelions, crabgrass and other weeds attempt a hostile takeover of the backyard. The application of post-emergent herbicides to kill off the undesirable plants in a home that includes pets can lead to uncertainty and controversy over safety. Just how concerned should pet parents be?

IMPS has found quite a few alarming blog posts on this topic, some citing studies to support that alarm. Often mentioned is a paper in Science of the Total Environment ("Detection of herbicides in the urine of pet dogs following home lawn chemical application") that reports some chemicals remain on treated lawns for at least 48 hours.
Another study cited is from 2004 that points to a possible link between one of the most popular chemicals and incidents of bladder cancer in some Scottish terriers. IMPS is typically skeptical of alarmist reporting thus took a deeper look into this subject.


Herbicides come in two types - pre and post emergent. There are also two subcategories - those that kill all plants and those that are selective. A lawn maintenance firm or diligent homeowner will usually apply a pre-emergent in the very early spring to knock out the bad plants before they can germinate. Later in the year (and perhaps more than once) a post emergent may be applied to kill off those weeds and other undesirable plants that pop up during the summer. A prime example are dandelions which spreads by seed over great distances.

The two primary ingredients in many selective, post emergents are 2,4-Dichlorophenoxyacetic acid (2,4-D) and Mecoprop (MCPP). 2,4-D degrades in soil relatively quickly - it has a half-life of 6 to 10 days and is broken down by soil microbes. MCPP lingers a bit longer and has about a 21 day half-life.

These herbicides may come as granules, dust or in liquid form. When applied correctly and mixed as directed (in the case of concentrates), 2,4-D and MCPP should pose low toxicity risk to both humans and most pets. The problem, however, is that chemicals are can be misapplied, not correctly mixed or handled recklessly.

Given that regulators and the industry quote a half-life of at least six days, it should not be suprising that a study can find evidence of the active chemicals 48 and 72 hours after application.


The primary absorption path is gastrointestinal and much less so by the lungs. The skin provides a very resilient barrier to prevent entry of the chemicals into the blood stream, though some forms of 2,4-D can cause mild skin and severe eye irritation. Excreted in the urine, very little of these chemicals undergo any transformation in the body and does not build up in soft tissue.

In the case of humans, the in-body half life for 2,4-D is 13-39 hours and for MCPP about 17 hours. However, the pH of the urine can have a marked effect on how long these chemicals linger in the body. In the case of acidic urine, the half-life can be extended to as much as 90 hours in humans.


The most serious effects of 2,4-D (and likely MCPP too) occur when large quantities are ingested in a liquid form. Vomiting, diarrhea, headaches and confused or aggressive behaviour may occur in humans. In addition to those symptoms, pets who ingested 2,4-D also had symptoms of drooling, staggering, convulsions and loss of appetite.

Humans and animals who ingest a significant quantity of 2,4-D or MCPP can suffer skeletal muscle damage, acidosis, electrolyte imbalances and even kidney failure. In some cases they may lose consciousness.

Studies on 2,4-D have not found any clear causual or indirect link to cancer in humans. Fittingly, the WHO’s International Agency for Research on Cancer (IARC) has concluded that 2,4-D, is “possibly” carcinogenic to humans. In the IARC assessment exposure to the chemical in animals and people was evaluated by over 20 scientists from 13 countries. Evidence that linked 2,4-D exposure to cancer in experimental animals was “limited” and “inadequate” in human studies. Though a few studies found an increased risk of leukemia and non-Hodgkin's lymphoma in humans exposed to very high levels of 2,4-D, the direct cause and effect remain in question as most were exposed to multiple pesticides.


If you think your pet has had significant exposure, especially in liquid form, to one of these herbicides, you should go to the vet immediately and let them know your suspicions.

If necessary, there are tests to detect the presence and magnitude of the chemicals in the blood and urine. However, immediate treatment measures are critical and should not wait for confirmation if exposure to significant amounts of 2,4-D or MCPP is reasonably certain.

It is highly likely your vet will administer intravenous fluids to speed the excretion in the urine and reduce the concentration of the toxic chemicals in the kidneys. Potassium chloride may be added to offset increased losses of potassium.

The vet will also consider treating very acidic urine with sodium bicarbonate ("alkalinizing the urine"). As there are no controlled studies, this treatment has been called controversial by some but reasonable by others given the criticality of the situation.

Your pet's BUN, creatinine and electrolytes will need to be closely monitored to balance fluid overload against renal impairment. As is the case of pets with kidney disease, heart conditions can further complicate treatment. Depending on your pet's unique situation as well as the availability of the necessary equipment, your vet may instead chose to use hemodialysis.

Conclusion and Precautions:

The above information on the effects and treatment are certainly scary and should be taken seriously. However, pet parents need to remember this describes a worst case scenario of exposure/consumption of large amounts of 2,4-D and MCPP in liquid form. While application instructions may not be followed precisely, the resulting variability in the quantity of chemicals applied to a lawn is not likely to lead to such dire consequences, if any at all.

The cancer risk does not appear to be significant in the amounts and frequency that most pets are likely to be exposed to under ordinary circumstances.

Pet parents should take common sense precautions reflective of how the chemicals will be applied, the frequency of application and the size of their pets. To start, if the herbicide is applied as a liquid, pet parents MUST keep their pets indoors during the application process and until it has completely dried on the grass. Pet parents should also inspect the application area for any puddles of contaminated material.

Next, respect the half-life of the chemicals applied and keep pets off the grass for six days. In a perfect world, this would be unnecessary but it does give the opportunity for a mild over application to be reduced to a safe level, even with no water to speed absorbtion and decomposition.

Pets do lick their paws and fur and thus can ingest residual 2,4-D and MCPP orally. If your pet does go on the grass the next day, don't freak out - rinse their paws off or give a bath if they have really been rolling around in it. Remember, the maximum risk is immediately after a liquid application, before it is fully dry.

Pet parents also need to be aware, as best possible, of what is going on in surrounding areas. Speak with your neighbors and ask them to alert you if they or a landscaper will be applying lawn treatments. On windy days, herbicides in liquid spray or dust form can easily drift into your yard.

Likewise, look for warning notices on private and public lawns (often too few are put out). When in doubt, avoid the area. Likewise, avoid lawns that are still wet from rainfall or irrigation if you are uncertain if they have recently been treated.

Any exposure to herbicides your pet does receive is likely to be low and infrequent and not a significant health risk if these precautions are followed. [Note: if your pets live on an active farm, additional precautions may be required]

THREAT LEVEL: Moderate immediately after application, decreasing to low after a few days. In most cases, don't panic, grab a hose.

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