For decades, Idaho,s sugarbeet growers have been rotating their crops with potatoes. Now, a University of Idaho weed scientist says they may be unintentionally growing as many as 211 sacks of potatoes while they're raising sugarbeets.

According to Don Morishita, so many leftover potatoes from the previous year's harvest can sprout among the current year's sugarbeets that sugarbeet root yields can be sliced by 25 percent to 61 percent.

"It was really an eye-opener for me," Morishita says. "I think what really makes the potatoes so competitive is that they have a jump on the sugarbeets early in the season and it's just hard for the beets to catch up after that.

"The sugarbeet roots and the potato tubers are competing for underground space, and there?s just a certain amount of space that?s available for them to grow."

Morishita measured the potential effects of volunteer potatoes on sugarbeet crops in 2005, after learning that Washington State University scientists had found an average 9,985 leftover potato tubers per acre in fields they had surveyed.

At the University of Idaho's Kimberly Research and Extension Center in 2005 and 2006, he deliberately planted potatoes in seven different densities--between 2,728 and 16,336 plants per acre--among sugarbeets.

In a 100-foot crop row, Morishita's research team found an average of 12 potatoes tucked in between sugarbeets in the plots planted with the fewest potatoes and 69 potatoes in the plots planted with the most,. The results were yield losses ranging from 25 percent to 61 percent.

Currently registered sugarbeet herbicides have little effect on volunteer potatoes, Morishita says. The volunteer potatoes produced tubers as large as 6 ounces--2 ounces more the U.S. No. 1 minimum requirements.

Morishita also found that the best time for hoeing out volunteer potatoes was when tubers were just beginning to form underground--about a month after plant emergence.

Hoe too soon, and you'll be returning, he says. The stored energy in the tuber will send up a new potato plant that's still capable of nipping sugarbeet yields.

Hoe too late, and the potatoes already will have begun to take an unacceptable toll on the beet crop.

Morishita will repeat the timing-of-removal portion of his ?interference? study this year.