How Weeds Have Changed Over 20 Years

Jerry Doll

Farming systems are constantly changing. Mother nature responds with changes and one of the first signs of these changes are shifts in existing weed populations and the appearance of new species. The following information is based on perennial weed surveys (Doll, J. D. and D. J. Kleiber. 1994. 1994 Perennial Weed Survey for Wisconsin. NCWSS Proc. 49:82-83); personal observations (as well as those of crop consultants and my weed science colleagues); weed samples sent to the University of Wisconsin Weed Diagnostic Lab for identification; and questions we receive on how to manage specific weeds at field days, over the telephone and at the Weed Doctors’ booth at the Wisconsin Farm Progress Days. To these sources add a dose of intuition and the reality of being an extension weed scientist in Wisconsin for more than 20 years.

Most of the annual weeds that have increased have been in Wisconsin many years (Table 1). Those of somewhat recent introduction include wild proso millet, woolly cupgrass, and flixweed. Bur cucumber is located primarily in the western tier of counties along the Mississippi River while wild cucumber is an edge-of-the-field species that seems to increase in abundance every year. Eastern black nightshade is primarily found in soybeans and alfalfa and its increase is probably due to the growth in soybean acreage. Smooth hawksbeard is appearing in no-till corn and soybeans. The book ‘"Weeds of the North Central States" shows this weed as only in the northern counties but it is found regularly in the southern areas of the state as well. Giant ragweed had been found primarily along field edges and by waterways. Now it is common in entire fields as is common ragweed.

The widespread adoption of sulfonylurea herbicides as postemergence treatments in corn has increased the increased the abundance of both smooth and large crabgrass. Shepherd’s purse is most evident in winter wheat and older alfalfa fields. Flixweed is at the very early stages of appearance in Wisconsin. I saw the first specimens in crop land (primarily winter wheat) in 1989 but reports of this species as a weed are becoming more numerous in southern areas.

Canada thistle has become much more abundant in recent years. We used to see milkweed as an occasional plant in fields but recently whole fields with sizable infestations have surfaced. Leafy spurge has been a roadside plant in Wisconsin for many years. Now it has moved across the fence and has invaded pasture land. Dandelion is most evident in older hay fields. The more frequent cutting, higher rates of potassium, and absence of a forage grass contribute to its increased abundance in no-till soybeans. Few herbicides are used in established forages. Thus if weeds appear, they often go to seed and this increases future weed pressure. This probably accounts for the increase in blackseeded plantain. It was formerly a headland phenomenon. Frequent traffic of field equipment thinned the alfalfa in these areas and plantain became established. After years of going to seed, the infestation became more widespread and probably seeds harvested with the forage were spread to other fields in manure. Now it can be a serious problem throughout forage fields and in no-till corn as well.

Changes in tillage practices are always mentioned as a cause of perennial weed increases. This is probably a significant factor for the increases in Canada thistle, wirestem muhly, milkweed and hemp dogbane. Canada thistle is common in CRP sites and this may lead to infestations in adjacent areas.

Repeated use of the same herbicides has created resistant weed biotypes that are no longer controlled by herbicides with four modes of action (Table 3). Our first case of resistance was confirmed in Marathon County in 1979 and common lambsquarters is the most widespread resistance problem in the state. Biotypes of smooth pigweed resistant to these herbicides appeared next and this we also have many fields with this problem. The rest of the herbicide resistant weed have been found in only a few locations but they illustrate the potential for the development of many additional problems of the these and other species developing herbicide resistant biotypes.

The decline in farm number is also a possible reason weeds are increasing in some areas. We have lost 20,000 farms in the past 20 years and farm size has increased from an average of 190 acres in 1997 to 220 acres in 1998. This often means that fences are removed as fields get larger and fewer farms have livestock. The fence row vegetation often includes perennials like wirestem muhly and hemp dogbane and biennials like burdock. Once the fence is removed, these species often become weeds of cropped land. Comfrey was a herbal garden plant from the days of Wisconsin’s settlement. As farmsteads have been removed or anytime a garden was converted into a field, an instant infestation of this difficult-to-control perennial occurred.

Another way new weeds appear in fields is via invasion from field edges, fence rows and roadsides. The Wisconsin State Dept. of Transportation greatly curtailed roadside spraying during the controversy over Agent Orange. More recently this agency has greatly restricted mowing along the state, county and town highways in deference to environmental groups who believe that these are favorable wildlife habitats. This gives a rich environment for seed production and weeds like Canada thistle, perennial sowthistle, hemp dogbane, and milkweed are well equipped to send seeds long distances from where they are formed. The abundance of wild carrot, wild parsnip and spotted knapweed along roadsides clearly reflects the lack of mowing that has allowed these biennial species to become dominant in many areas.

Not all weeds have increased in abundance. It appears that annual smartweeds, shattercane and field bindweed are less common now than previously. Yellow nutsedge seems to cycle: after wet years like 1993 it becomes more abundant and then effective control for several years reduces its seriousness. I know of no biennial weeds that are becoming less abundant.

The phrase "change is constant" is certainly reflected in weed population dynamics. We can rest assured that more changes will happen over the next 20 years as well. The continual change in farming practices, the advent of herbicide resistant crops, and the resilience of Mother Nature assure that changes will happen.

1998 Proc. Wisconsin Fertilizer, AgLime, and Pest Management Conf. Vol. 37: 144-147.