Millennials are opting to buy more expensive pet food, and it's causing problems for some of the industry's household names.

According to a recent report by The Wall Street Journal, pet-food brands such as Mars' Pedigree, Nestle's Purina, and Smucker's Gravy Train and Kibbles 'n Bits are suffering because of this shift in spending behavior.

The average price of pet food increased from $1.71 a pound in 2011 to $2.55 a pound by the end of 2017.

Millennials are feeding their pets with expensive food, and it's causing problems for some of the industry's household names.

According to a recent report by The Wall Street Journal, legacy pet-food brands such as Mars' Pedigree, Nestle's Purina, and Smucker's Gravy Train and Kibbles 'n Bits have all seen sales sag as pet owners shift toward premium-pet-food products.

Many millennials are waiting longer to get married, buy a house, or have children, and they're instead choosing to become pet owners.

"They treat them like it was their firstborn child," Beverley Petrunich, owner of DoGone Fun, a dog-day-care center in Chicago told The Journal.

Read more: Pets are like children to many Americans — here's how much pet owners spend on average each month

According to Nielsen, annual household spending on pet food among pet owners increased 36% between 2007 and 2017.

This has led to a flurry of new brands entering the market. Many of these new brands are selling more premium food and "human-grade" snacks, including Rachael Ray's Nutrish brand, which uses high-end ingredients and even sells gluten-free meals for pets.

According to data analytics firm GfK, more than 4,500 new pet-food products were introduced in 2017, which was a 45% increase from the year before. The majority of those new products were premium, according to Gfk.

This shift has led to prices rising substantially. According to GfK, the average price of pet food increased from $1.71 a pound in 2011 to $2.55 a pound by the end of 2017.

Each year, the total amount spent on pets has increased as owners pamper these creatures more. According to the American Pet Products Association (APPA), US pet owners spent $69.5 billion on their pets last year, up from $66.75 billion in 2016 and $41.2 billion in 2007.