Lots of dog owners scratch their heads about feeding their dogs veggies and fruits and I get it. After all, dogs are canines and canines are carnivores...but not totally. In fact, according to a Scandinavian study analyzing the composition of wolf scat (conducted between 1992-2005), up to 12% of their diet was composed of berries and vegetation (higher in summer).

The question remains: Why do canines need to consume berries and vegetation?

According to another study, (MECH & BOITANI 2003), The consumption of these foods is supposed to be effective as a purgative and to wipe the intestine from parasites and hairs. Makes sense, after all a predominately carnivorous diet is a rather grimy affair.

In any case, I'll let the findings of the study speak for themselves and explain why your dog needs fruits and veggies. 

Methodology of the study

Seasonal diet composition of wolves (Canis lupus) was studied on the Scandinavian Peninsula by analyzing 2063 wolf scats. The scats were collected during summer and winter of the years 1992 to 2005. A reduced data set (n = 1594) was used to describe the seasonal feeding pattern of 10 Scandinavian wolf territories in detail.

A relatively high occurrence of plant material was observed in the analysed scats both in summer and winter. Both vegetation and berries were consumed more frequently in summer than in winter due to the higher availability of plants during the summer months.

Although it was assumed that vegetation was partly consumed by chance in consequence of the feeding behavior of wolves and the habit to lick blood from the ground, vegetation that appeared in well-ordered bundles and with more than 5% of the dry volume of the scat need to be regarded as voluntarily consumed. Similar data was also observed in Yellowstone wolves in 2006.

In addition, plant matter is prevalent in wolves' summer diet, with 392, (74%) of 530 scats analyzed containing some type of plant material, largely grass (Graminae). This is consistent with summer observations of wolves consuming grass and other plant material. The prevalence of vegetation found in summer scats indicates that consumption of these food types is intentional

Note that since this is all scat analysis, it is clear that these plants were not eaten to induce vomiting (which is what is often said of domestic dog and cat plant eating behaviors).  

Sueda, Hart and Cliff (2008), who surveyed over 1500 dog owners about their plant eating behavior and found that "68% of dogs were reported to eat plants on a daily or weekly basis with the remainder eating plants once a month or less. Grass was the most frequently eaten plant by 79% of dogs. Only 9% were reported to frequently appear ill before eating plants and only 22% were reported to frequently vomit afterwards. "

Hart added:

Our current hypothesis is that grass eating mostly occurs in normal dogs and in normal cats, and is not associated with illness or a dietary deficiency, but reflects an innate predisposition that was inherited from wild ancestors and that had an ongoing intestinal parasite purging effect that served as prophylactic treatment for the animals in nature (who were always exposed to intestinal parasites).

So don't forget to include fruits and veggies in your dog's diet, it's something canines have been eating for millennia.