"I'm filling mouthwash bottles, you're filling milk bottles," Brunner told a dairy farmer at a meet and greet event in Springfield last month.
Brunner, who hails from urban St. Louis and lacks rural bona-fides compared to rival Sarah Steelman, made efforts early in his campaign to seek support from the agricultural community, which appear to be paying off.
Brunner has secured the endorsements of the influential Missouri Soybean Association, Missouri Pork Association, and John Deere PAC, and is beginning to pull support from rural lawmakers like state Sen. Brian Munzlinger, chairman of the Missouri Senate's agriculture committee.
But all of that work has been under fire from Missouri Democrats and Brunner's Republican rivals, alike, ever since farm issues became a constant topic of discussion during last month's Senate debate on the 2013 farm bill.
Steelman said at the time that "no real conservative" could support the bill, which led to a slew of questions over whether Brunner supports the bill. Brunner has repeatedly stayed quiet on the $500 billion legislation, much of which goes to food aid programs.
The debate over the farm bill led to the latest clash between the two, which peaked this week with a new television ad from Steelman highlighting a $10,000 contribution to the Humane Farming Association from a foundation established by Brunner and his wife.
The Brunner campaign denounced the ad as a "personal attack" on Brunner, and said it was Brunner's daughter, Ginny Becker, who chose - on her own - to allocate the funds to the group in lieu of a Christmas gift one year.
The Steelman campaign charged on Thursday that regardless of how the funds were allocated, Brunner and his wife are the only trustees to the foundation and are therefore the only people who could write the check.
Steelman, herself, on a campaign swing through southwest Missouri this week, continued to make the charge while speaking to a group at a pet supplier in Goodman, Mo.
"One of my opponents in the primary has supported Humane Farming, which is way further to the left than HSUS," she said.
Steelman's ad, coupled with her campaign tour, prompted the Brunner campaign to release a memo Thursday criticizing Steelman's "anti-agriculture record." Brunner's campaign notes that in 2003, Steelman accepted the maximum campaign contribution at the time of $1,175 from Nancy Grove, president of the animal welfare group, "Missouri Alliance of Animal Legislation."
The group touts itself as a major supporter of the Humane Society of the United States, and Grove - in 2010 - was a major contributor to efforts to put greater requirements on Missouri dog breeders.
The Brunner campaign also noted state Senate votes from Steelman in favor in 1999 - then against in 2001 - legislation that had farmers switch to selling animals based the USDA's quality of meat formula, as well as legislation in 1999 that could have raised property taxes on farm land.
"With a damaging voting record for Missouri agriculture, it's no wonder Sarah Steelman has earned zero endorsements from agricultural leaders or organizations," wrote Todd Abrajano, spokesman for Brunner.
The continued clash between the two leaves their third rival, U.S. Rep. Todd Akin, relatively unscathed on the issue as the campaign nears closer to the August 7 primary. The three are facing off to challenge incumbent Democrat Claire McCaskill in November.