MillerCoors has fired WPP's Cavalry three years after the holding company created the Chicago shop for the sole purpose of servicing Coors brands and new products. The brewer has put Cavalry's brands into an invitation-only review -- including Coors Light, Coors Banquet, Redd's Apple Ale and Smith & Forge Hard Cider -- marking yet another agency shift in the increasingly tumultuous beer market.

David Kroll, who is one month into the job as the brewer's chief marketing officer, said MillerCoors has invited three agencies to pitch for the business. He did not name the shops, but said none of them are within WPP. He declined to say if the brewer is considering its other roster agencies, which include Omnicom's TBWA Worldwide, Los Angeles for Miller Lite and Publicis Groupe's Leo Burnett in Chicago for Miller High Life. The winning agency is expected to be chosen by mid-to-late September.

'Time for a change'

The plum assignment is Coors Light, which is the second-largest beer in the U.S., but has been stuck in a recent sales slump. The brewer spent $155.9 million on measured media on the Coors family of brands last year, according to the Ad Age Datacenter, which uses figures from Kantar Media.

"It's frankly just time for a change," said Mr. Kroll, who replaced longtime MillerCoors CMO Andy England on July 2. "My expectation for MillerCoors is that new media and consistently great creative become hallmarks for us as a company. And given that, I felt a fresh communications approach was required for Coors Light, our largest brand."

The move immediately throws future of Cavalry into doubt. The shop was created by WPP in mid-2012 shortly after MillerCoors fired DraftFCB (now called FCB). Cavalry, which employs roughly 60 people, has gained other work, including projects for PepsiCo and Tyson Foods. But the agency -- which WPP on its website describes as a "full-service agency specializing in marketing to men" -- still relies on MillerCoors for the overwhelming majority of its revenue.

Cavalry CEO Marty Stock said in email that "we're incredibly disappointed that a relationship that goes back three decades with so many successful brands has come to an end. Regime change is a fact of life in this business. We wish MillerCoors well wherever they go next."

While Cavalry has been with MillerCoors for three years, some key executives in the agency have been on Coors brands for many years. Mr. Stock, who previously oversaw the MillerCoors account while at DraftFCB, has worked on the Coors brands for 26 years.

WPP's client-specific agency model suffers loss

WPP executives have described Cavalry as part of its client-specific agency strategy, following the model of shops like Team Detroit for Ford. Cavalry handled creative, digital and Hispanic advertising for Coors brands, drawing on talent from WPP's VML, Y&R and Bravo.

Mr. Kroll said Bravo would stay on to handle Hispanic, but that the winning agency would field creative and digital.

Mr. Kroll signaled that Coors Light would get more aggressive on digital media, saying the brewer has been "too TV-centric in our approach." TV will "remain a part of our marketing mix, as it should, but we will be shifting to much more of a balance of 'media first' versus creative first," he said.

"The way consumers are interacting with brands is changing literally literally by the minute," he said. The brewer is "well behind the curve in really engaging drinkers in that unplugged world, and must find ways to connect with them in a way that is relevant to their lifestyle."

The change underscores the pressure MillerCoors is under to once again grow Coors Light. The so-called Silver Bullet had been a star player in the brewer's portfolio, growing sales every year from 2005 to 2012. By the end of 2011, the brand passed Budweiser to become the nation's second-largest beer brand by market share, a position it still holds. But Coors Light has been in decline since 2013, as it faces the same challenges vexing other big beer brands that have struggled to capture new drinkers as millennials turn to craft brews and spirits.

Coors Light sales dropped by low-single digits in the second quarter, the brewer reported on an earnings call on Thursday. Yet the brand still gained share in the so-called premium light segment, which includes brands like Bud Light and Miller Lite.

The brand has stuck with its Rocky Mountain "cold refreshment" positioning for years, but there have been some marketing stumbles of late. Earlier this year the brewer ditched a planned campaign by Cavalry because ads did not go over well with distributors. The shelved ads involved a man who dispensed Rocky Mountain wisdom about Coors Light. The brand this summer has run ads called "Born in the Rockies" that use breathtaking mountain imagery that emphasize the brand's Rocky Mountain roots.

During an interview in mid-July, Mr. Kroll pledged to "get back to a much more consistent meaningful message around Rocky Mountain cold refreshment," suggesting that the brewer's marketing across the board has been "too scattered."

He said at the time that he did not have immediate plans to make agency changes. Asked this week what had changed, he said he was "making this move to put in place for Coors Light specifically an agency that really sets this brand up back for growth," adding that he is "committed to building long-term agency relationships."

Smaller brands performed well

Beyond Coors Light, the other MillerCoors brands at Cavalry have performed well. The Redd's franchise reached double-digit growth in the second quarter. Coors Banquet grew low-single digits and is on track for its ninth-straight year of growth, the brewer reported. Smith & Forge, which launched last year, is a "fantastic story," Mr. Kroll said on the earnings call, noting that it is the No. 2 brand in cider despite only being available in cans.

Asked why MillerCoors stripped all the brands from Cavalry -- and not just Coors Light -- Mr. Kroll said "the scale of that business just wouldn't justify Cavalry being able to retain the depth of talent that we would require on those businesses." He added that Cavalry "delivered us some very good work" on the smaller brands.

The review marks the latest chapter in what has been a dizzying time for beer accounts. Brewers seem more willing than ever to quickly terminate agency relationships as they struggle to find the right marketing formula to lure drinkers who are increasingly enticed by smaller craft brands and spirits.

In July, A-B InBev made its fourth agency change in roughly four years on Bud Light -- the nation's largest brew -- moving it to Wieden & Kennedy from BBDO. Just a couple weeks earlier, W&K lost the Heineken account as the brand consolidated globally with Publicis Worldwide. Late last year, MillerCoors moved Miller Lite to TBWA Worldwide in Los Angeles. Lite has shuffled between various shops in recent years, including Saatchi & Saatchi and DraftFCB.

Mr. Kroll said he does not have plans to make near-term changes on the brewer's other agencies, which include Interpublic's Initiative for media and CommonGround/MGS, which leads multicultural but has also been brought in on some general-market assignments.