The cool weather we experienced this spring has delayed weed emergence and growth, so the need for burndown herbicides in no-till may be questioned for fields currently being planted. In addition, some people question the need for a burndown treatment with Roundup Ready beans due to the ability of Roundup Ultra to control larger weeds than many postemergence herbicides.
Providing the crop with an even start with weeds is a critical component of successful weed management, thus if weeds are present at planting, it usually is best to control them shortly before or after planting. The composition of weeds present at planting is dependent on field history, planting date, and spring weather conditions. Even though we accumulated few growing-degree days during the past month, many of the early germinating weeds are already present. In the Ames area, kochia, giant ragweed, common ragweed, and Pennsylvania smartweed emerged during the week of April 21 or earlier, and there likely are other species present in many fields.
Although these weeds are small now, they will grow very quickly when temperatures increase. If they are not controlled at planting, this rapid growth will force growers to apply postemergence herbicides very early in the season to prevent these weeds from getting too large for successful control, or from causing early-season competition losses. Weeds may not be visible from the road, but they probably are present in most fields at this time. Unless the field has been walked to confirm the lack of weeds, a burndown herbicide should be a component of the weed management program.
What about fields planted to Roundup Ready beans? Some people have inquired about delaying the burn-down herbicide until one or two weeks after planting. The rationale for this approach is to allow the postemergence application to be delayed until later in the season (four to five weeks after planting) than if the burndown was applied prior to or shortly after planting. In some situations, this could eliminate the need for sequential postemergence treat-ments and, therefore, reduce the number of herbicide applications from three to two.
We believe this strategy should work in fields with moderate infestations of annual weeds. However, if the field has significant infestations of winter annuals or perennials such as dandelion, or heavy infestations of early germinating annuals such as giant ragweed, kochia, or woolly cupgrass, we believe a standard application timing for the burndown herbicide would be appropriate.
This article originally appeared on page 54 of the IC-478 (7) -- May 5, 1997 issue.