Weed Management

Agronomy News is a statewide newsletter for farmers, consultants, researchers, and educators interested in grain and row crop forage production systems. The topic for this edition is Preemergence Herbicides, Mowing Pastures: To Mow or Not to Mow, Checking Vegetative Growth Stages, Summer Nutrient Management Tips, Wheat Yields & Double-Crops—Time is Important to Maximize Yield, New Extension Agent, 2021-2022 Cover Crop Sign-Up, LEAD Maryland Fellowship Applications, MDA Pesticide Exams, MDA Pesticide Disposal Program, Weather Outlook, and Regional Crop Reports.

As planting gets underway, it is important to stay on top of our most troublesome weeds. In particular, marestail/horseweed, common ragweed, and Palmer amaranth. Below are two new charts to help determine the type of products to be included in the tank mix to best manage these weeds in corn and soybean.

Lately, I have seen several fields with Palmer amaranth flowers (Figure 1). It is imperative that these plants be managed before harvest (Palmer amaranth is listed as a noxious weed in Maryland and Delaware). At this point, mowing or herbicides will not improve yield nor prevent Palmer from germinating next year. Plants need to be physically removed from fields. Palmer amaranth has both male and female flowers on separate plants (Figure 2), and focus should be placed on removing the seed-producing females first. A single female can produce approximately 600,000 seeds per plant, and these seeds can remain viable for six years. Palmer seed matures within 2 weeks of flowering and if left unchecked, a single plant can infest an entire field within a few years (Figure 3). 

Commercially acceptable weed control is difficult to achieve in plasticulture vegetables even with pre- and post-emergent herbicide applications. These applications often do not provide season long weed control between rows. Moreover, several weed species have become resistant to commonly used postemergence herbicides, including common ragweed and pigweed species. Cover crops are an additional tool that could be successfully integrated into a weed management program.

In Maryland, populations of common ragweed have developed resistance to three herbicide groups, Group 9 (glyphosate), Group 2 (FirstRate, Synchrony), and Group 14 (Flexstar, Valor). These herbicide-resistant populations limit options for effective postemergence control. Even though alternative herbicides are available, control can be inconsistent when applied to common ragweed plants exceeding 4” tall (Figure 1). 

Herbicide resistant common ragweed (Ambrosia artemisiifolia L.) is prevalent on the Lower Eastern Shore of Maryland and other areas in the region. In 2019, common ragweed populations on the Eastern Shore of Maryland and Delaware were found to have two or three-way mode-of-action resistance to herbicides. They were resistant to glyphosate (group 9), cloransulam (Firstrate; group 2 “ALS inhibitors”), and/or fomesafen (Reflex; group 14 “PPO Inhibitors”). Early-season management of common ragweed is strongly dependent upon reducing seed emergence and controlling ragweed populations prior to soybean planting. However, due to the limited herbicides with efficacy against common ragweed, it is even more critical to utilize alternative and multiple means of control.

Mowing is a relatively inexpensive mechanical weed management option that imposes minimal disturbances to the soil. Several types of commercial mowers including rotary, flail, reciprocating sicklebar and reel can be used to suppress weed growth. Still, mowing is generally not thought of as a formable integrated weed management (IWM) tool as it is not congenial to most cropping systems or all land types. For instance, having a smooth soil surface free of rocks or other obstructions is a necessity for mowing operations, and if mowing close to the ground, the soil surface should be even. Some have declared that mowing is primarily used to limit seed production and restrict unsightly weed growth in un-tilled herbaceous and woody perennial crops.

Pros and cons of vinegar as an alternative to glyphosate

Poison Hemlock (Conium maculatum, Fig. 1), is a member of the plant family Apiaceae, which contains a few important crops such as carrots, celery, and parsnips. This weed is a tall, invasive, and highly poisonous weed that is sometimes mistaken for one of its crop relatives. It is also commonly mistaken for Queen Anne's Lace (Daucus carota).

In last month’s issue of Agronomy News, I discussed considerations for glyphosate-based burndown programs. This month I wanted to share some of my results using similar programs to manage a rye (Fig. 1) and hairy vetch (Fig. 2) cover crop. Treatments were applied on April 11, and consisted of glyphosate, glufosinate, paraquat, glyphosate + 2,4-D, glyphosate + dicamba, glyphosate + glufosinate, and glyphosate + paraquat. Currently, this is what I am seeing: