A problem with the indiscriminate use of weed killers is that they may kill other plants in addition to those you wish to eliminate. Toxic herbicides wash into streams and rivers, contaminating the water supply and potentially damaging crops grown further downstream. Weed killers can be toxic to children, pets, or wild animals and birds that may ingest the chemical by eating or chewing on the plants to which they were applied. Chemical residue may remain in the plants and soil for years, and the residue could be linked to cancers and other serious illnesses. Alternatives to herbicides are hand-pulling small quantities of weeds, leaving some weeds to grow and even eating weeds you know to be safe, such as dandelion greens and pigweed.
Broadleaf herbicides like 2,4-D target a range of plants with broad leaves, which includes weeds along with many desirable vegetables, herbs and flowers. Spraying may be the quickest way to apply weed killers, but the mist will cover everything in its path. Roadside spraying of weeds can also kill adjacent trees, shrubs and any plants the chemical contacts. Spraying on windy days is particularly dangerous because the weed killer will drift across a much larger range than intended.
Chemicals that are applied to plants eventually drain into the sewer system or ground water, ending up in the water supply. Atrazine, which is among the most commonly used weed killers in commercial agriculture, was shown in a 2002 study from the University of California at Berkeley, published by the "Proceedings of the National Academy of Sciences," to cause hermaphroditism in male tadpoles. Its effects on mammals are being studied, but atrazine has been banned in Switzerland since 2001.
Children love to pick dandelions and other common weeds, playing with them and often putting them in their mouths. Pets may chew on or eat plants that have been sprayed with herbicide, such as grasses or catnip. Dogs and cats also clean their paws by licking them and so will ingest chemicals that were applied to plants they walk on. Wild mammals, birds and insects may also browse or feed on plants that have been sprayed. Depending on the concentration of herbicide, ingestion of toxic herbicides can be lethal or cause serious illness to animals and people. A 1997 study from researchers at the University of North Carolina, Chapel Hill, published in the "American Journal of Epidemiology" studied farm families in Ontario, Canada. The study found increased risk of miscarriage following exposure to glyphosate, atrazine, organophosphates and other yard herbicides. "Occupational and Environmental Medicine" published a study in 2003 linking higher incidences of non-Hodgkin lymphoma on farms to exposure to the herbicides atrazine, glyphosate and sodium chlorate, as well as several pesticides.
Clopyralid is an herbicide that is used in pastures. Residue from this herbicide may be found in straw that is used for mulch in the garden, as well as manure that is added as a soil amendment. Even in very small amounts, clopyralid residue from straw or manure can kill beans, lettuces, tomatoes, potatoes, peppers and eggplant. Studies on long-term effects on humans from herbicides like clopyralid may be inconclusive, but many people choose to grow and eat food that was not grown using herbicides or other chemicals.