Why Do Dogs Eat Grass?

A common, if seemingly strange, behaviour in our beloved pet dogs is their penchant for eating grass. Eating grass or other plants is regularly documented in dogs, as it is in their genetic predecessor, the wolf. But why do they do it?

Few dogs are fully carnivorous; their digestive systems are designed to be able to cope with the omnivorous diet of a scavenger. This lifestyle was one of the characteristics that brought their ancestors into close proximity with humans thousands of years ago and saw the initiation of the domestication process. However, grass can still sometimes seem an unusual choice for a species named after their canine teeth - which are not necessarily designed for grazing!

Hypotheses for Grass Eating

Researchers have spent time exploring this curiosity in our four-legged friends and have come up with a few hypotheses as to why they carry out this behaviour. Some of these ideas include a dog feeling sick and needing to vomit, dietary deficiencies, an innate predisposition inherited from wild canid ancestors, possibly to help purge intestinal parasites (Hart, 2008).

Grass Eating for Self-Medication

Sueda et al (2008) carried out a series of surveys with pet owners to see if they could find out more information on why dogs may eat grass. They surveyed veterinary students with pet dogs about their dog’s grass-eating tendencies and whether the students observed signs of sickness before grass-eating or vomiting afterwards. 

All the students reported that their dogs ate grass however none reported seeing signs of illness before their dogs ate grass, and only 8% said their dogs were likely to vomit afterwards. When dogs’ owners were asked the same thing, a similar pattern in results was reported, suggesting that for most dogs, grass or plant-eating did not appear directly linked to the hypothesis that dogs eat grass when feeling unwell or to be sick.

The same researchers carried out a wider survey with dog owners and found that 68% of owners reported their dog(s) ingesting plants on a daily or weekly basis. Of the plant-eating dog population, younger dogs ate plants more frequently than did older dogs. Younger dogs were also less likely to appear ill beforehand or to vomit afterwards, however, any dog that showed signs of illness before eating plants was more likely to vomit after plant ingestion (Sueda et al., 2008).

Grass Eating for Dietary Deficiencies

The aforementioned study also failed to find links between a dog’s diet and the tendency to eat grass. For example, there was no indication that dogs receiving less fibre in their diets tended to eat plants more often (Sueda et al., 2008). Therefore, it is unlikely the cause of grass/plant-eating is related to dietary deficiencies. However, one case study has been recorded of a reduction in plant-eating behaviour when a higher fibre diet was provided to a poodle with chronic daily grazing/vomiting tendencies (Kang et al., 2007).

Grass Eating for Parasite Control

The final hypothesis, that plant-eating serves a biological purpose such as ridding canids of gastrointestinal parasites (i.e., worms) was developed based on observations of plant-eating in wild chimpanzees (e.g., Huffman et al., 1996). Chimps eat leaves from a variety of plants, which is thought to increase intestinal motility and physically help “sweep” the gut clear of worms (Hart, 2008). However, this idea requires further research to gain evidence of its likelihood.

Grass Eating for Pleasure

Alongside these scientific explanations, there is also the more informal idea that dogs perhaps just like the taste! Young green shoots and spring grass often has a higher sugar content than older, courser stems and so for some dogs, it may be enjoyable or act as a palate cleanser. 

Many dogs, especially young dogs, explore the world partly through their mouths to learn about textures and tastes, what smells taste like, etc., which all engage the gustatory system. 

So rather than grass-eating being a case of “why?” it may more be a case of “why not?”. Work by Bjone et al. (2007) suggests that hunger and satiety may also play a part in dogs deciding to nibble on some greenery. 

Grass Eating for Attention?

While all the reasoning so far is related to feeding and physiology it should not be overlooked that for some dogs, taking a bite of grass or plants* in the house or garden may also be linked to gaining them some attention from owners. 

Whether good or bad attention, if a dog finds such attention rewarding then plant-eating could become a learnt behaviour as a method to get their owners’ interest or to relieve boredom. If you suspect this is the case then ensure your dog has other options to get your attention or to prevent boredom, for example, by providing them with suitable toys for mental stimulation.

*It should also be highlighted here as to not allow dogs access to ingest any form of plant that may be toxic to them.

Advice to Owners

While “pica” (the act of eating foreign objects) is an abnormal animal behaviour that should warrant concern, the commonality of grass eating among pet dogs is not related. If excessive grass-eating followed by vomiting is occurring frequently then speak to a vet because it could be a sign of other problems.  

If your dog is a “grazer” then ensure they stay away from any grasses that may have been treated with chemicals or may be contaminated with slug pellets. It would also be advisable to ensure they do not have access to poisonous plants in the house or garden (some common garden examples include foxgloves, ivy, wisteria, hydrangeas, rhododendrons, lupin, nightshade, daffodil or tulip bulbs but a simple internet search can reveal more). Conkers, mushrooms and acorns are also poisonous.  It is worth clearing them from the garden regularly as they may be accidentally ingested during grass eating. 

Whatever your dog’s tastes providing them access to a complete, balanced diet, and worming them regularly can help ensure they stay healthy and maintain gastrointestinal health. 

References
Bjone, S. J., Brown, W. Y., & Price, I. R. (2007). Grass eating patterns in the domestic dog, Canis familiaris. Recent advances in animal nutrition in Australia, 16, 45-49. 
Hart, B. L. (2008). Why do dogs and cats eat grass? Veterinary medicine, 103(12), 648.
Huffman, M. A., Page, J. E., Sukhdeo, M. V., Gotoh, S., Kalunde, M. S., Chandrasiri, T., & Towers, G. N. (1996). Leaf-swallowing by chimpanzees: a behavioral adaptation for the control of strongyle nematode infections. International Journal of Primatology, 17(4), 475-503. 
Kang, B. T., Jung, D. I., Yoo, J. H., Park, C., Woo, E. J., & Park, H. M. (2007). A high fiber diet responsive case in a poodle dog with long-term plant eating behavior. Journal of Veterinary Medical Science, 69(7), 779-782.
Sueda, K. L. C., Hart, B. L., & Cliff, K. D. (2008). Characterisation of plant eating in dogs. Applied animal behaviour science, 111(1-2), 120-132.