Managing winter weed with herbicides

WOOSTER, Ohio – Populations of winter annual weeds seem to have been at an all-time high over the past several years.

Weeds such as common chickweed, henbit, purple deadnettle, and marestail (horseweed) have increased to the point that they require changes in herbicide management in some fields.

Several factors may have caused the increase in winter annual weeds. Most winter annuals emerge in the fall, and the warm weather in late fall during the past several years has resulted in higher populations.

No-tillage tends to promote winter annual populations, since there is no tillage in fall to disrupt their emergence.

However, winter annuals can emerge after an early fall tillage, and have been a problem in tilled as well as no-till fields.

Soybean harvest. Early soybean harvest in 1999 and 2000 allowed earlier than typical fall tillage in some fields, providing a window after tillage for winter annual emergence.

Another factor may be the switch from preplant/preemergence herbicide programs (Squadron, Canopy etc.) to Roundup Ready and other postemergence programs, since we have observed winter annual weed problems showing up more often following postemergence programs.

If so, this may indicate that the preplant herbicides are either: preventing seed production by these weeds in the spring, or persisting into the fall at rates that are high enough to reduce winter annual emergence.

Continued problems with winter annuals may warrant reconsideration of the utility of total postemergence programs.

Winter annual weeds. Chickweed, deadnettle, and most other winter annual weeds emerge primarily in the late summer or fall, although some emergence can occur in the spring.

Marestail and common chickweed can follow a winter annual or summer annual life cycle, and has the potential to emerge over most of the year depending upon environmental conditions.

Winter annuals survive the winter with little or no further growth, resume growth in late-winter or early spring, and flower and go to seed in late-spring or early summer.

Negative consequences. Consequently, the major negative effect of most winter annuals occurs at the time of crop establishment.

Marestail does not flower until late summer, and directly competes with crop growth along with foxtail, ragweeds, and other summer annuals.

We have occasionally observed completion of the winter annual life cycle within a shorter period of time.

For example, when chickweed gets an early start in late summer, it may set seed by late fall or early winter.

Characteristics. One characteristic of winter annuals that allows populations to increase over a short time period is the lack of after-ripening needed for seed viability.

While seed from many weed species requires a year or more of after-ripening in the soil in order to germinate, seed from winter annuals is typically ready to germinate as it leaves the plant.

Some of the problems caused by dense populations of winter annual weeds include prevent soil from drying in spring, prevent soil from warming up quickly in spring, interfere with spring tillage and crop establishment, harbor insects that cause problems with crop establishment and early season growth, some serve as host for soybean cyst nematode, longer-lived winter annuals such as marestail interfere with crop growth through much of the growing season and interfere with harvest.

Cyst nematode. OSU research has shown that some winter annuals can serve as hosts for soybean cyst nematode. In Ohio, the primary potential host appears to be purple deadnettle.

OSU research indicates a possibility that deadnettle emerging in late summer could serve as an alternate host for cyst nematode when the soybeans senesce, allowing completion of another nematode life cycle and increasing their populations.

A suggestion. We are therefore suggesting that in fields where cyst nematode is a known problem, deadnettle should be controlled by late September or within 30 days of deadnettle emergence to interrupt the nematode life cycle.

Have fields tested for the amount of soybean cyst nematode if you are unsure whether it is a problem.

b> Strategies.< Since the major problems with chickweed and deadnettle often are the slow soil drying and interference with crop planting and tillage, the primary goal of winter annual management should be to allow maximum time for weed death and dessication.

This may be accomplished with fall or early spring herbicide applications or tillage. Applications too soon before planting may not kill plants rapidly enough, especially during periods of cold weather.

Control can be achieved with tillage or herbicides, although herbicides will be the method of control in no-till fields.

Pros and cons. Some advantages and disadvantages of the various management strategies:

* Tillage in fall or early spring controls most winter annuals, but soil may be too wet for tillage at these times.

* Herbicide treatment between mid-October and early December provided the most consistent control of emerged plants, but can miss spring emerging weeds unless herbicide with residual activity is used.

* Herbicide treatment in March gave more erratic control of emerged plants than fall treatment. Larger plants and cold weather may prevent rapid plant death and dessication needed to prevent problems with planting.

Likely candidates. Winter annual populations vary greatly among no-till fields, and some fields have few winter annuals at this point. We do not see a great advantage to fall applications in these fields.

Any type of program that has been consistently effective in prior years, including application of herbicides at planting for burndown, can still be used.

Since there appears to be a general trend for increasing winter annual populations, however, these fields should be scouted each fall to ensure that populations are not increasing.

Preventing seed production in low populations of winter annuals will minimize the risk of future problems.