Why does my dog eat grass?

The best-known study comes from Karen Sueda, Benjamin Hart and Kelly Cliff at UC Davis, published in 2008. In one group of 47 owners at the university's teaching hospital, 79 percent had seen their dogs eat grass or other plants. A larger internet survey produced 1,571 usable responses: most dogs ate plants daily or weekly, grass was the plant eaten most often, and few owners reported that their dogs looked ill beforehand or vomited afterward. The authors concluded that plant eating is a normal behavior of domestic dogs.

The details matter. In Hart's summary for veterinarians, 8 percent of owners said their dogs frequently showed signs of illness before eating plants and 22 percent said their dogs regularly vomited afterward, and vomiting appeared to be incidental rather than caused by the grass. Diet type made no difference, and neither did fiber. Younger dogs ate plants more often and were less likely to seem ill. Hart's hypothesis is that wild ancestors ate plants to help purge intestinal worms, an idea that still needs testing.

Hart's advice to vets ends with a plain instruction: keep grass-eating dogs away from chemically treated lawns and toxic plants. The National Pesticide Information Center, a partnership between Oregon State University and the U.S. Environmental Protection Agency, advises keeping pets off treated areas until the pesticide is completely dry and the area well ventilated, and says granular lawn products may need 24 hours or longer. The product label gives the specific interval for each product.

Fertilizer and mulch matter too. The ASPCA notes that fertilizers commonly contain ingredients dogs find tasty, and that cocoa mulch attracts dogs with its sweet smell and, like chocolate, can cause problems. On a walk past a row of manicured lawns, a grazer will go for the greenest strip, so steer it to a patch you know. If you think your dog ate a treated plant or product, call your vet or ASPCA Animal Poison Control at (888) 426-4435, which may charge a fee.

Puppies and young dogs graze more than older dogs, the UC Davis survey found, and they put everything else in their mouths as well. Watch a puppy's grazing for sticks, stones, mulch and garden plants, and check any plant against the ASPCA's toxic plant list before you put it where your puppy plays. Teach leave it early. It will save you a great many trips across the yard, and it carries over to the beach and the trail.

An older dog that suddenly starts eating grass is a different story. A new habit in a senior, especially with weight loss, vomiting, a change in appetite or stool, or lip licking at night, deserves a vet check rather than a wait-and-see. Bring a note of when the grazing started, how often it happens and what follows it. The appointment is also a good moment to ask whether your senior's diet still suits it.