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Tillage often is performed as an option to destroy all visible weed growth, but at the same time burying or planting weed seeds that may cause issues, not only in the current growing season but also in the future. Any disturbance of the soil surface may initiate weed seed germination. No-till fields give ample opportunity to control weeds through fall application of residual herbicides or a spring burn-down. In a properly managed no-till field weed seeds stay on or near the ground surface and are open to predation and decay. Also, keeping weed seeds near the surface of the ground increases the effectiveness of soil applied herbicides.

It can be an effective option to consider planting those fields last that had high weed density in 2019. This will give more opportunity to control early emerging weeds with burn-down and pre-emergent herbicides. Making sure of proper control of these early weeds before planting is extremely critical. Use a multi-step approach to weed control through the use of pre-emergent herbicides with overlapping residuals, with a tank mix of herbicides with multiple sites of action. If possible use an herbicide with separate sites of action for the post-emergent application. Under lower weed pressure situations, one herbicide site of action may be sufficient all growing season. Cover crops in-between cash crops or on prevent plant ground could also help suppress weeds, decrease an overabundance of water and give a healthy soil structure to plant later in the spring. Always scout fields before and after application of herbicides to determine what herbicide to use and if there will be a need for additional control measures.

In 2019, South Dakota endured one of the wettest years on record. These conditions caused some fields to not be planted and had no weed control measures accomplished all season. This gave way to some prevent plant fields to be over taken by various types of weeds, such as Waterhemp, Kochia, Marestail and other annuals. As a result, producers need to plan in advance on how to deal with these fields that contain an overabundance of weeds. Weeds in these bare fields have deposited significant amount of seeds on the soil surface, which can easily germinate when adequate moisture and temperature are available. Managing the weed seed bank involves managing weed seed production and germination. Producers need to limit the production of weed seeds. This will in turn limit opportunity for weed seed germination in the field.

How to Manage Weedy Fields in 2020

As we look forward to the next growing season, it is very likely that there will be fields or areas within fields that will be inaccessible to field equipment. There are some burn-down herbicides that may be used by aerial application to help suppress weeds in those fields. Information on aerial application can be found on the herbicide label.

Weed communities in agronomic fields are dominated by annual species. Summer annuals initiate growth each spring from seeds found in the upper soil profile (Figure 1). In most fields, a small percentage of the emerging plants survive and contribute new seeds to the soil seedbank. Historically, most research of the annual weed life cycle has focused on seed dormancy and emergence (A), effect of control tactics on weed survival (B), and weed seed production (C). The fate of seeds between the time of maturation on the plant and entering the seedbank (D) has largely been ignored. However, current research at Iowa State University and other organizations has shown that significant seed losses routinely occur in agronomic fields, and these losses may influence the effectiveness of weed management programs. This article will provide a brief summary of some of the current research in this area and the potential importance of seed predation to weed management.



Field crickets on seed card.

ISU research cited in this article was partially funded by:

Plant seeds are storage organs for high energy compounds that supply plant embryos the resources needed to germinate and develop into seedlings. These energy reserves are an excellent food source for a variety of animals that live in or near agricultural fields, including ground beetles (carabid beetles), crickets, mice and others. Estimates of cumulative seed losses due to seed predators have ranged from 20% for barnyardgrass and lambsquarter in a chisel plow system (Cromar et al. 1999) to 88% for giant ragweed in no-tillage (Harrison et al. 2003).

The value of intercepting weed seed before they enter the seed bank is somewhat of a forgotten control tactic. In the 1930’s and 40’s, combines were commonly equipped with a weed seed collector that separated and collected weed seed from chaff as the crop was harvested. When modern herbicides were introduced in the 1950’s, it was considered less expensive and more convenient to control weeds with chemicals, and these accessories quickly disappeared from combines. In Australia, seed collectors are again being used on combines due to widespread herbicide resistance and the loss of effective herbicides. Rigid ryegrass infestations have been reduced by as much as 70% through use of weed seed collectors during harvest (Gill, 1995). The effectiveness of weed seed collectors varies among weed species depending on timing of seed shed. Weed species that drop the majority of their seed prior to crop harvest would not be impacted significantly by use of weed seed collectors.