With recent rains and cooling temperatures, winter annual weeds are emerging and actively growing in row crops across Nebraska. Scouting during harvest can give no-till producers a head start on weed control in 2014.
In many no-till fields, winter annual weeds have become prolific. This increase has been associated with increased adoption of conservation tillage practices, widespread adoption of glyphosate-resistant crops and the subsequent increased reliance on total postemergence weed control programs, say Rodrigo Werle, UNL weed science graduate student, and Lowell Sandell, UNL weed science Extension educator. Another factor, they say, is the reduced use of soil residual herbicides in corn and soybean production.
The impact of winter annual weeds in cropping systems is sometimes overlooked because these weeds typically complete most of their lifecycle prior to or shortly after corn and soybean planting. However, dense mats of winter annual weeds may result in delayed soil warming in spring, direct and indirect competition for water and nutrients during initial establishment of the summer crop, and difficulties at planting, the UNL experts point out. Moreover, winter annual weed species can act as alternative hosts for pests. For example, henbit is known to be an alternative host for soybean cyst nematode (SCN), a major soybean pathogen.
Recent research on emergence timing of winter annual weeds across Nebraska showed that downy brome, tansymustard, henbit, Carolina foxtail, and field pansy emerged mostly in the fall (more than 90% of total emergence). Virginia pepperweed and purslane speedwell had the majority of the seedlings emerging in the fall (approximately 70% of total emergence), but some seedlings also emerged during late winter and early spring. Shepherdspurse and field pennycress had some seedlings emerging during fall (approximately 30% of total emergence), but the majority of it occurred in late winter and early spring.
If in previous years farmers have noticed fields infested with fall-emerging species such as marestail, henbit, downy brome, tansymustard, Carolina foxtail, field pansy, Virginia pepperweed, and purslane speedwell, these fields should be scouted and perhaps managed after crop harvest in the fall, Werle and Sandell say. Scouting and management of fields infested with mostly spring-emerging species such as shepherdspurse and field pennycress is more appropriate in spring.
Marestail is a troublesome weed in Nebraska no-till fields and glyphosate-resistant populations are prevalent throughout much of the state. Research has shown that marestail tends to germinate mostly in the fall in Nebraska, but in other areas of the Corn Belt it germinates through the spring and summer.
According to UNL observations and conversations with growers, henbit and marestail seem to be the most prevalent winter annual weeds in much of eastern Nebraska.
The following are fall management tips:
•Numerous herbicide tank-mix options are available for control of most winter annual weeds. Consult pages 59 (corn) and 103 (soybeans) of the 2013 Guide for Weed Management in Nebraska (EC130) for effective options for your winter annual weed spectrum.
•In the Nebraska research, the majority of fall emergence was completed by early November, thus targeting herbicide applications from late October to mid-November would be prudent. Younger weeds tend to be more susceptible to herbicide treatments in the fall or early spring when they are small. Waiting until late spring may not result in the desired control if these weeds are at an advanced growth stage.
•If glyphosate-resistant marestail is one of the primary targets of control, the use or inclusion of a growth regulator herbicide (2,4-D and/or dicamba) is necessary for adequate control. UNL weed experts believe that 2013 was a relatively light year for marestail, in part due to extreme drought conditions during the fall 2012 germination period. With recent precipitation, marestail is germinating and could become more of a problem in 2014 if timely management is ignored.
•Field pansy has become more prevalent in southern Nebraska in recent years. In our trials, 2,4-D and dicamba have been relatively ineffective on this species. Products that contain chlorimuron (e.g., Classic, Authority XL and Valor XLT) have shown good results with fall applications. Applying a product with chloriumuron in the fall will require rotation to soybeans the following year.