MSU Extension Field Crops

Early season weed control decisions should be front and center as Michigan farmers plant this spring. Winter annual and early summer annual weeds compete with emerging crops for water, nutrients and light leading to reductions in crop yield. These weeds can also interfere with planting and potentially serve as hosts for destructive insects.

In no-till soybean, yield losses of up to 7 bushels per acre have been observed if weeds are not controlled until soybean is at the unifoliate stage (VC) compared with controlling early season weeds prior to or at planting. Waiting to control these weeds when soybean is at the V1 stage (first trifoliate) can reduce yield approximately 0.5 bushel per acre per day. In today’s market, this could lead to approximately $38 per acre reduction in profits at the end of the season.

In addition to the potential losses of yield and profits, one challenge that we have if we don’t control weeds prior to planting is that as these weeds continue to grow, they become harder to control. This is especially a problem when controlling herbicide- (glyphosate and ALS) resistant horseweed (marestail). If resistant horseweed is not managed prior to planting, there are no postemergence herbicides for control in Roundup Ready or non-GMO soybean. Also, if not controlled early, this weed will also be more difficult to control in LibertyLink, LibertyLink GT27, Roundup Ready 2 Xtend, Enlist E3 and XtendFlex soybean.

There are several steps that need to be followed to manage herbicide-resistant horseweed that include the use of effective burndown applications and the use of good soil-applied residual herbicides. These steps are outlined in the Michigan State University Extension herbicide-resistant horseweed factsheet that can be found in the MSU Weed Control Guide for Field Crops or on MSUWeeds.com. Additionally, keep in mind many of the burndown herbicides and effective soil-applied residual herbicides that we use in soybean need to be applied prior to soybean emergence or severe crop injury can occur.

Tillage can also be an effective way to manage weeds prior to planting soybean. However, if weeds become too large, additional tillage passes or herbicide application several days prior to tillage many be needed to effectively control early season weeds. In addition to tillage, growers should also consider the use of soil-applied or preemergence herbicides in their weed control programs. These applications will help reduce the number of weeds that need to be controlled postemergence, provide help in controlling herbicide-resistant weeds (i.e., waterhemp, horseweed and Palmer amaranth), and provide some insurance to prevent early season weed competition that may emerge with the crop in postemergence if weed control practices are delayed.

For example, we have observed over a 20% reduction in yield in narrow soybean if weeds were not controlled prior to reaching 6 inches tall. If a farmer is planning on controlling weeds when they are 4 inches tall and that application is delayed, waiting just two additional days could result in yield losses from 1.5 to 3 bushels per acre per day and would cost anywhere from $38-86 per acre in profit due to reductions in yield.

A complete listing of burndown herbicide programs and their effectiveness ratings can be found in the no-till soybean section, Table 2P of the Weed Control Guide for Field Crops. Remember treatments that contain one part per acre of 2,4-D ester need to be applied a minimum of seven days before soybean planting.

Corn is also very susceptible to early season weed competition. For example, we have observed a 7 bushels per acre yield loss if weeds were not controlled prior to reaching 6 inches tall and 31 bushels per acre yield loss if weeds were not controlled prior to reaching 12 inches tall. Therefore, timely spring weed control is important in corn to minimize the impact of weed competition. Starting with a weed-free seedbed with tillage or an effective burndown herbicide program helps protects corn from yield loss later in the season.

Further, one way to minimize early season weed competition is applying preemergence residual corn herbicides. Preemergence residual herbicides are a good insurance policy by controlling early emerging weeds in corn and allowing for timely postemergence herbicide applications when weeds are small, thus preserving corn yield. However, sometimes corn planting operations can get ahead of the sprayer and there are several preemergence residual herbicide options that can be used once corn has emerged. A complete listing of these herbicides can be found in Table 1H of the Weed Control Guide for Field Crops.

If you are in the process of making vital herbicide decisions for your corn acres, check out the MSU Economics of Commercial Weed Control Programs. Over the past five years, these studies have found that the median economic return for one-pass preemergence programs is $540 per acre, $650 per acre for one-pass early-postemergence programs, and $720 per acre for two-pass pre followed by postemergence programs.

This article was originally published in Michigan Farmer.