It doesn't take long to clean your combine between varieties How good is good enough when you're cleaning out your combine to prevent contamination between crops?

That really wasn't an issue until identity-preserved (IP) crops were added to the crop mix. But now, even clean grain is a contaminant if you're selling IP grains on contract.

"Some sources of contamination, like cross pollination, are beyond your control," says Wally Greenlees, Iowa State University ag engineering field specialist. "Things you can control include your combine, augers and other grain handling equipment."

Greenlees experimented with a John Deere 4420 combine last fall to test how much grain you can clean out of a combine in the least amount of time.

"We figured that 45 minutes was the longest time most farmers would be willing to take to clean grain out of their combine," he says. "We tried two methods.

- "The first was an intensive procedure in which the grain was removed via gravity, hand cleaning and vacuum cleaning.

- "The second was a less intensive method in which the grain was removed by gravity and hand cleaning only."

During the intensive procedure, Greenlees and ISU field specialist Shawn Shouse spent 15 minutes to remove grain by hand and by operating the combine with all access doors opened. That included removing visible ears and stalks from the corn header, opening the clean grain elevator and tailings elevator doors, opening and clearing the rock trap and feeder house by hand, sweeping off edges and corners in the grain tank, operating the threshing and separation mechanisms in place at high idle for one minute with all doors open, and operating the unloading auger to clear grain from the auger joint. The last 30 minutes were used to remove grain from the combine with an industrial vacuum.

For the shorter procedure, the researchers repeated the 15-minute process, but didn't vacuum the combine.

Following both cleaning procedures, Greenlees and Shouse harvested an additional 60 bu of grain. Samples were taken as the grain was unloaded to test for contamination. Yellow corn and red corn were harvested alternately so any contamination was readily visible.

"The results suggest that there really wasn't much difference, in terms of contamination, between the two procedures," says Greenlees. "While the data is going to be different for different machines, it looks like using the 15-minute cleanout procedure and then flushing the combine with 60 to 75 bu of grain reduces contamination levels to 2% or less."

So, is the 15-minute cleanout good enough?

Shirley, IL, farmer Terry Schneider isn't so sure. He grew several IP crops last year that caused him to clean out his John Deere 9600 combine 17 times. Each time he took two hours to carefully clean and vacuum the unit before he harvested the next crop.

"Maybe we're too cautious, but with the IP crops we've always been told that we needed to be as precise as we can be," Schneider says. "How much time you take depends on how good of results you want."

Schneider notes that newer combines have more edges and crevices to hide grain and may be more difficult to clean out than the combine used in the ISU study.