The pet food industry claims more than $28 billion in sales in the United States alone. That's almost three times the size of the US movie theater industry for those counting.

Pet food is also one of the most-ordered items at both Amazon and Walmart. We track sales rank data for products sold at both e-commerce giants, and with dogs on our brains this fine spring Friday, we dug into the data that is the last year's sales-ranks at the top-two e-commerce sites to reveal some nuggets of delicious information.

We looked at pet-foods that were top-ranked for at least 100 days in the past year, and then ranked those by their average category rank. This revealed consistent best sellers and solved for trends that came and went.

Purina wins at Walmart while Taste of the Wild rules Amazon

When it comes to dog food, Purina Beneful Originals With Real Beef Dry Dog Food in the 3.5lb. Bag took top spot at Walmart.com. In fact, it ranked #1 at the vendor for 167 days in the past year. The price is right, too - the whole bag goes for just $5.48 (at the time of posting). Right behind it at #2 is the Real Chicken version which goes for the same price.

At Amazon, it seems buyers go a bit more upmarket for their canine friends with Taste of the Wild Grain Free High Protein Natural Dry Dog Food. Its best-selling iteration is the $48.99 30lb bag. Taste of the Wild takes spot #2 at Amazon as well with its Canine Formula. This Schell & Kampeter brand impressively beats out Purina at Amazon, whose top-seller at Amazon was Purina's Pro Plan Savor Adult Shredded Blend Chicken & Rice Formula Dog Food.

Interestingly, Greenies Pill Pockets Treats for Dogs was the consistent pet-supply category best-seller at Amazon, but since it's not a staple, every-day dog food, we didn't give it top honors. That said, it ranked top-10 at Amazon for 154 days. Greenies Original Teenie Dog Dental Chews Dog treats ranked an average #15 for the bulk of the past year - 252 straight days.

All dogs were good boys in the making of this article.