The entire state of North Dakota was surveyed in the spring and summer of 2000 to measure the population and distribution of weed species and document changes in the weed spectrum since 1979. The 2000 surveys were based upon surveys conducted in 1978 and 1979 and were completed in the spring before herbicides were applied and again in summer prior to harvest.

"These surveys are interesting because many herbicides we use today were registered after 1979,"

Zollinger says. "One might suppose we should have eradicated many of the weeds we contend with every year with newer and more effective products. However, these surveys tell a different story. It appears weeds are as resilient and tenacious as ever and they adapt to changes in our chemical and cultural weed control efforts." Zollinger is an associate professor in the NDSU department of plant sciences and extension service weed specialist.

The top ten weeds in 2000 are the same as in 1979 but their order of ranking is slightly different. The top ten are: green foxtail, wild oat, yellow foxtail, kochia, wild buckwheat, Canada thistle, redroot pigweed, common ragweed, field bindweed and common lambsquarters.

"We have weeds that were found in the 2000 survey but not back in 1978-79," Zollinger says. "Added to the list are wild-proso millet, eastern black, hairy, and cutleaf nightshade, biennial wormwood, tall waterhemp, lanceleaf sage, yellow nutsedge, Venice mallow and swamp smartweed. Weeds that were in the 1978-79 survey but not in the 2000 survey are nightflowering catchfly and prairie wild rose."

There is some good news in the survey. The number of weed-free areas sampled in fields increased from 27 percent in 1979 to 54 percent in 2000 and the weed index of most weeds decreased. "The weed index is a composite value comprising many measured parameters and can be likened to a ‘misery index’ of how bad that weed is in North Dakota," Zollinger says. "The weed index decreased for most weeds indicating that growers are decreasing the weed seed bank in their fields."

Less weed seed in the soil translates to fewer weed populations in fields. For example, the weed index of green foxtail decreased from 236 in 1978-79 to 103 in spring 2000 and 74 in summer 2000. Lower weed indexes and weed frequency and the higher number of weed-free fields in 2000 as compared to 1979 indicate lower weed problems even though the plant species complex across the state has remained similar.

Weeds that had a higher weed index in 2000 were yellow foxtail, kochia, Canada thistle, common ragweed, quackgrass, and common cocklebur. Growers may have seen a higher incidence of these weeds in their fields.

Some individual species comparisons:

Wild oat: The weed index value for wild oat was 69 in 1978, 55 in 1979, 35 in the spring of 2000 and 39 in the summer of 2000. The lower occurrence and index value may be from several years of good control using effective herbicides that have been registered since 1979.

Yellow foxtail: Reports going back to 1903 indicate that yellow foxtail was more abundant than green foxtail. In the 1978 and 1979 surveys, green foxtail occurred on 94 percent of fields and yellow foxtail occurred on 23 percent of fields but in the 2000 survey, green foxtail occurred on 60 percent of the fields and yellow foxtail occurred on 25 percent of the fields. "Even though green foxtail frequencies have decreased considerably, yellow foxtail occurrence has been stable for more than 20 years," Zollinger says. "It confirms grower observation that yellow foxtail has become more problematic."

Kochia: Kochia was more abundant and found at greater densities in 2000 than 1979. Acetolactate synthase (ALS) inhibiting herbicides were introduced in the mid 1980's and herbicides of this mode of action have been registered in most crops grown in North Dakota, including small grains, corn, soybean, dry bean, field pea and pulse crops, canola, sugarbeet, potato and alfalfa. Kochia resistance to ALS herbicides was documented within four years after introduction for field use. Most fields in North Dakota contain kochia that is ALS resistant which may explain the higher than anticipated occurrence.

Canada thistle: Densities of Canada thistle nearly tripled from 1979 to 2000. Surveys conducted yearly by the North Dakota Department of Agriculture show Canada thistle has continued to increase across the state and has now surpassed leafy spurge in the number of acres infested in 2001. Other surveys conducted in sunflower and dry bean also confirm the dramatic increase in Canada thistle populations. Above average precipitation beginning in 1993, increase in number of no-till acres, high cost of control, lack of winter snow and moderate winter temperatures may be factors contributing to the increase of Canada thistle infestations according to Zollinger.

A similar increase was seen with other perennial weeds. Density of field bindweed and perennial sowthistle doubled from 1979 to 2000 and tripled for common milkweed.