OWINGS MILLS, Md. -- Baltimore Ravens coach John Harbaugh said referees wouldn't be rusty in the preseason if they spent more time at training camp.

Referees are required to be at only three training camp practices a year, and Harbaugh has proposed that the NFL increase the requirement. However, according to Harbaugh, the league said the refs union won't allow that.

"Gosh, you're doing something at the highest level possible," Harbaugh said after practice Saturday. "You're at the top of the field. You'd think you'd need more than three days of practice a year to really be good at it. So I don't think it's really an outrageous thing to ask for."

Harbaugh made this point after noting that officials missed an intentional grounding penalty in the Ravens' preseason opener against the New Orleans Saints on Thursday.

John Harbaugh thinks refs would be more prepared for games if they were required to attend more than three training camp practices. Rick Osentoski/USA TODAY Sports

"If they got more practice time in training camp, I think they'd do a little bit better," he said.

Harbaugh is in favor of having officials at every one of his practices.

"We think that makes us better and certainly makes the officials better, too," he said.

Harbaugh doesn't understand why officials aren't at more practices. He said every referee he's spoken with has been in favor of attending more practices.

"My proposal is that they get together and get on the same page," Harbaugh said. "Maybe talk to one another and solve a problem once in a while instead of creating a problem."