High weed moistures can create errors in combine yield monitor readings.

One of the first things a farmer notices - when using a monitor - is that a patch of weeds seems to put a huge hit on yields. Fortunately, that yield loss probably is exaggerated.

"Harvesting patches of green weeds may temporarily coat the moisture sensor," points out University of Illinois agronomist George Czapar. "That can result in a very low yield reading."

Earlville, IL, grower Mark Atherton knows the feeling. He doesn't have a lot of weeds, but like most farmers, he gets breakthroughs.

"I was combining soybeans that were yielding 60 bu/acre, with moisture running about 13%," he reports. "Then I hit some green weeds and there was a dramatic change. The yield dropped to 40 bu and the moisture shot up to 30%."

Atherton stopped the combine and checked the moisture sensor.

"I could see that green weed material had built up on the sensor," he notes. "That obviously was affecting both the moisture and the yield."

Green weeds on the moisture sensor can make the grain seem wetter than it is, agrees Paul Jasa, University of Nebraska ag engineer. "Consequently, when grain is corrected for moisture, the yield reads lower than it should."

If a moisture sensor gets coated, clean it with a soft rag, recommends Chuck Studer of John Deere Precision Farming.

Nebraska's Jasa points out that the problem is most common when a moisture sensor is mounted in the grain tank. That's where early monitors were placed - in a nearly flat position where weed material stuck to them.

"When a yield monitor unit is installed today on a new or existing combine, the moisture sensor can be mounted on the outside of the clean grain elevator," Jasa points out. "It's straight up and down, and the green weed material flows through it better."

Farmers need to determine whether yield reductions are actually caused by weeds, says Illinois' Czapar.

"Sometimes, yield losses from the amount of weed competition are greater than would be predicted," he explains. "Low plant populations, reduced stands, poor drainage or other soil conditions also may be limiting yield in those areas. But they're not as obvious as weeds."

Because weeds are opportunistic, they tend to take over poor stands, says Jasa.

Ideally, say agronomists, farmers should scout fields to determine locations of weed infestations. Is there a good stand of corn or soybeans to compete with the weeds? Or did weeds move in to fill a void created by a poor stand?

That information is especially valuable when analyzing yield maps and trying to determine the reasons for yield variability.

Backpack GPS scouting systems can help, Czapar points out. Layers of information collected during the growing season can be linked with yield maps to help document crop loss due to weeds, as well as insects and diseases.