MSU Extension Field Crops

For field crop growers throughout the state it has been a late, wet and challenging spring leading to planting delays. Due to this delay, many critical weed control decisions need to be made. These decisions fall under two scenarios: 1) planted acres or those that will be planted soon and 2) prevented planting acres.

Weed management on planted acres

Soybeans

They key to a successful weed control program is knowing your enemy. Many winter annual weeds that are often the focus of spring burndown herbicide applications are already setting seed or will soon and no longer need to be the focus of herbicide programs. Switching gears to focus on summer annuals weeds such as common lambsquarters, common ragweed, velvetleaf and several pigweed species should now be the priority. Additionally, it is extremely important to have a plan in place to control glyphosate and multiple-resistant horseweed (marestail), waterhemp and Palmer amaranth.

Currently, in some fields around the state, horseweed is 6 to 8 inches tall and waterhemp and Palmer amaranth are just starting to emerge. Roundup (glyphosate) alone will not control these weeds. Also, using 2,4-D ester in the burndown is no longer an option with the seven days prior to planting soybean restriction. Herbicides like Sharpen, Gramoxone + Metribuzin, Liberty or registered dicamba formulations (XtendiMax, FeXapan or Engenia) for use only in Roundup Ready Xtend soybeans are options that will provide control of herbicide-resistant horseweed. Many of these products will also provide control of emerged waterhemp and Palmer amaranth.

Remember, some of these products need to be applied prior to soybean emergence or can only be used in specially traited soybean varieties once soybeans start to emerge (i.e., LibertyLink, LibertyLink GT27 or Roundup Ready Xtend soybean). If applications are made prior to planting, consider adding a residual herbicide to keep these later emerging weeds controlled.

For more information, refer to the factsheets “Herbicide-Resistant Horseweed (marestail) in Michigan Keys to Management in No-till Soybean” and “Multiple-Resistant Palmer amaranth and Waterhemp: Management in soybean, corn and alfalfa” in the back of the 2019 Weed Control Guide for Field Crops from Michigan State University Extension. Additional information on burndown herbicide options can be found in Table 2P, “Weed Management in No-Till Soybean,” in the 2019 MSU Weed Control Guide for planting restrictions, length of residual activity and maximum weed height for burndown herbicide applications.

Corn

There is more flexibility when making delayed herbicide applications to corn than soybeans. There are a number of soil-applied herbicides that can be applied after planting up until corn and weeds reach a certain size or growth stage. Delayed applications of soil-applied herbicides in corn can be utilized under two scenarios:

• You planted but did not have time to apply herbicides prior to the weather turning wet.
• Applications could only be made after the rush to plant was over.

A complete list of soil-applied herbicides that can be applied after corn emergence are in Table 1H of the 2019 MSU Weed Control Guide. Note not all soil-applied herbicides can be applied after corn emergence, for example Princep, Sharpen and Verdict need to be applied prior to corn emergence.

Although there is flexibility, we recommend applying this category of herbicides immediately after planting if possible, as delayed application increases the risk of poor herbicide performance (especially for grass control) as many of these herbicides do not provide post-emergence activity. If weeds are present, applying glyphosate (Roundup) or glufosinate (Liberty) to glyphosate- or glufosinate-resistant corn is recommended. See specific herbicide labels for tank mix restrictions.

Final considerations

In general, when making postemergence herbicide applications later in the season, there is potential for herbicide carryover. Keeping in mind your plans for 2020, consult Table 12 - Herbicide Crop Rotation Restrictions in the 2019 MSU Weed Control Guide for rotation restrictions.

Finally, if thinking about switching to planting soybeans on acres originally planned to be planted with corn, knowing what herbicides have already been applied in preparation for corn will aid in this decision. Many corn herbicides have multiple month rotation restrictions to soybean. A very common corn herbicide that falls into this category is atrazine. Specifically, if 1 pound of active ingredient per acre of atrazine has been applied, the planting restriction to soybeans is 10 months.

Weed management on prevented planting acres

Make weed control a priority on prevented planting acres, fallow ground will be a hot spot for weeds to flourish and emerge all season. The primary goal for weed control on these acres is to prevent weeds from producing seed and increasing the soil seedbank. Weed control on prevented planting acres can be accomplished with multiple passes of tillage, mowing or herbicides. Plan for a least two passes of weed control during the season; without crop competition, weeds can rapidly grow.

Tillage  

Tillage is an option to control small weeds on fallow prevented planting acres. Tillage is not recommended to control large weeds, as large weeds are difficult to completely uproot. It is recommended to plan an herbicide pass first to clean up the field and then follow-up with tillage later in the season when a new flush of small size weeds emerge.

Mowing

Mowing is not recommended as the primary weed control tactic in fallow prevented planting acres. Mowing early in the season will only remove the primary growing point of the weed, often resulting in bushy weed growth (see picture). Mowing could potentially be used later in the season, when resulting weed growth will be killed by frost before producing seed.

Herbicides

Multiple passes of herbicides can be used to control weeds in fallow prevented planting acres. When choosing what herbicides to apply, consider the rotational crop planned for 2020, cost and herbicide resistant weeds. One cost-effective herbicide program is glyphosate (Roundup) plus 2,4-D. Gramoxone and glufosinate (Liberty) are also options that can be used for control, especially if glyphosate-resistant weeds are present, although weeds need to be small for these herbicides to be effective. Visit the MSU Plant & Pest Diagnostics website for a complete list of herbicide-resistant weeds in Michigan.

In general, plan to apply one herbicide pass soon followed by a second application later in the season after new weed emergence and growth.

Cover crops

One potential option for weed control on prevented planting acres is planting a cover crop. Consult your farm service agency and crop insurance agent to understand rules and regulations surrounding planting cover crops on prevented planting acres. In addition, the MSU cover crops team has published a factsheet outlining cover crops for prevented planting that outlines goals, potential advantages and potential problems. 

When choosing a cover crop to plant for weed control, the first step is to review field inputs. Previous herbicide applications can limit what cover crops you can plant. Refer to the “Managing Weeds in Cover Crops: What Herbicides Can I Use?” webinar discussing spring applied corn and soybean herbicides and their potential to injure fall-established cover crops. In addition, MSU weed scientists Christy Sprague, Karen Renner and Aaron Brooker conducted research evaluating the impact of soil-applied and postemergence corn herbicides on establishment of June interseeded cover crops. This research has direct implications if you applied a corn herbicide this spring and would like to establish a cover crop prior to fall.

The second step is to decide what species to plant. Grass species to consider are oat and sorghum-sudan grass. Advantages of planting these grasses are they are both competitive against weeds, they are summer annuals and can be established in hot summer conditions, and you can use plant growth regulator herbicides (example: 2,4-D) to control broadleaf weeds after planting. Disadvantages include cost and availability of sorghum-sudan grass seed.

Legume species to consider are red and berseem clover. Advantages of these species are that they fix nitrogen and can be plated in early summer. Disadvantages include weed control after planting, as you are no longer able to apply the cost-effective plant growth regulator herbicides.

Brassica species to consider are radishes and rapeseed. Advantages of these species are they produce a large taproot that breakup compaction. Disadvantages include late summer planting date, if planted too early in the summer brassicas can bolt and produce flowers without the desired root growth and plant cover needed for weed suppression.

Conclusions

Regardless of what your current planting scenario is, make weed control a priority during this wet difficult season. Good weed control will preserve yield this season and reduce new weed seed inputs into the seedbank to compete with next year’s crop.

MSU Extension offers additional educational resources and programs to help farmers as they deal with delayed planting seasons at https://www.canr.msu.edu/agriculture/delayed-planting-resources.