Tillage and Weed Management

Tillage is the most important factor influencing weed management in row crops. Any tillage treatment can be considered as a weed management strategy: the primary function of tillage is to manage weeds. The effects of tillage on weed management can be direct or indirect. Examples of the direct effects of tillage on weed management would be the physical destruction of weeds by cultivation or the dilution of the soil weed seed reservoir. Indirect effects include the relative placement of herbicides in the soil and the impact on herbicide degradation. Another factor that must be considered is the affect of tillage on plant residue on the soil surface. Residue management impacts weed management, but is the critical component for soil erosion potential. These factors must be evaluated when developing a tillage system: the positive effects of tillage on weed management weighed against the negative impact on soil erosion potential

Weed management update for 2014

Weed management in 2014 will continue to be more challenging regardless of the weather. Factors such as prevented planting, poor weed control in 2013, increasing populations of weeds with evolved herbicide resistances and recently, the discovery of Palmer amaranth in Iowa will make weed control decisions interesting. However, other factors such as grower and dealer attitudes, promotional incentives, desire for simple and convenient tactics and perceived costs of more diverse alternative approaches to weed management programs are still primary considerations impacting weed control. Increasing the diversity of weed management tactics will improve the consistency of weed control, mitigate herbicide-resistant weeds and increase profitability. However, these tactics will require more planning, time, and possibly higher initial costs; to effectively diversify weed management requires that fields be considered individually and possibly unique strategies for each field. All possible “tools” should be considered and as many as possible included. There is a long history demonstrating that simple and convenient approaches to pest management, and in particular weed management will inevitably fail biologically and economically. The objectives of this paper are to provide an update of changes in the industry that may impact weed management decisions for 2014, review the state of herbicide resistant weeds in Iowa, provide some information about alternative weed management tactics and list some perspectives about weed management decisions

Herbicide Resistance, Weed Population Shifts, and Weed Management Stewardship: Is Anything New?

As glyphosate-resistant soybean and corn production systems are becoming the standard for Iowa and the Midwest, the selection pressure imposed on the agro-ecosystem by glyphosate is likely increasing at an increasing rate. World-wide, ll weed species have been reported as having evolved glyphosate-resistant biotypes (Heap 2006). Nine of the glyphosate-resistant biotypes were reported after 2000, and four glyphosate-resistant biotypes were reported in 2005. It is clear that the evolution of glyphosate-resistant weeds is increasing at an increasing rate. While ISU reported the existence of glyphosate-resistant common waterhemp in 1998, the relative economic importance of glyphosate-resistant weed biotypes in Iowa has to date, not been of major consequence. However, the lack of significant glyphosate resistance in Iowa weed communities can be anticipated to change based on reports and observations of glyphosate-resistant weed biotypes in neighboring states and anecdotal comments from the agricultural community Furthermore, the occurrence of Asiatic dayflower and other weed population shifts attributable to selection pressure from glyphosate use is more frequent. Thus, the need to consider the implications of not stewarding glyphosate use is more important than ever

2013 Herbicide Guide for Iowa Corn and Soybean Production

The success of weed management programs, more specifically herbicide programs, varied considerably during 2012 reflecting the importance of environmental conditions on herbicide performance. Variability of success was seen not only in the postemergence herbicide applications that continue to dominate herbicide use but also in the soil-applied residual herbicides; all herbicide applications were strongly influenced by tillage system, crop planting date, timing and amount of rainfall, and resulting weed emergence timing. While more soil-applied herbicides were used in Iowa during 2012, there are still too many acres of corn and soybean that are treated only with glyphosate. Importantly, the trend of no new herbicide sites of action continues and while new herbicides will be available in 2013, they have old sites of action, many of which have existing resistant weed populations. The new products and changes in herbicides will be described in this paper. The implications of the 2012 drought on herbicide degradation and the potential for herbicide carryover will be addressed. Furthermore, an update on the development of new herbicide resistant crops and the anticipated implications of these technologies when deregulated and available commercially will be discussed.https://lib.dr.iastate.edu/extension_ag_pubs/1180/thumbnail.jp

The evolution of herbicide resistant weeds in Iowa: description, implications, and solutions

There has been concern about the evolution of glyphosate resistance in some Iowa weeds for many years although the field-wide existence of any problems has not been previously reported. It is important to recognize that in reality, common waterhemp (Amaranthus rudis*) with resistance to glyphosate was reported in fields near Everly and Badger, Iowa as early as 1998 (Zelaya and Owen 2002). However, the likelihood of weeds evolving resistance to herbicides pre-dates glyphosate-resistant weed biotypes by five decades (Harper 1956). The first identification of herbicide resistant weed biotypes pre-dates glyphosate-resistance by four decades (Ryan 1970) and currently there 19 herbicide mechanisms of action to which weeds have evolved resistance which compromise 334 weed biotypes from 190 weed species and infest an area that is impossible to estimate (Heap 2009). To date, 16 weed species have evolved resistance to glyphosate. All of these incidents of evolved herbicide resistance have one thing in common; the mismanagement of herbicide use. This paper will describe the current herbicide resistant weed situation for Iowa in general terms, address the implications of herbicide resistant weeds and suggest solutions to the problems

Weed Management Update for the Next Millenium

In general, the 1999 growing season was excellent with regard to weed management. Weather patterns provided frequent rain thus enhancing soil-applied herbicide performance. While these rains also made timely postemergence applications a challenge, the efficacy of herbicides was quite good and larger weeds were consistently controlled. On the down side, these same weather conditions also reduced crop tolerance to herbicides, favored multiple and delayed weed emergence, and caused difficulties for mechanical weed management strategies. Regardless, 1999 was a successful cropping season

The Current State of Herbicidal Weed Control

The recent adoption of GM crops resulted in a dramatic impact on the use of herbicides for weed control in Iowa. Particularly, glyphosate-resistant soybeans dominate the varieties that are planted and anticipated increases in grower adoption of glyphosate-resistant corn have resulted in the unprecedented use of one herbicide being applied to most of the row crop acres in Iowa. There are a number of important benefits for the use of glyphosate-resistant crops and the concomitant applications of glyphosate. The benefits that can be objectively established include broad-spectrum weed control, the consistency of weed control, minimal concerns for crop injury, and the low cost of glyphosate. Another benefit that is difficult to substantiate is the favorable environmental profile of glyphosate-based cropping systems. However, growers also tout as important benefits for the glyphosate-based systems the simplicity of weed control, the flexibility of application timing, and the ability to control large weeds. Significant risks are also a consideration of the glyphosate-based systems, and in fact several of the aforementioned perceived benefits could be better described as risks. The risks include the alleged simplicity of the glyphosate-based system, the selection pressure imposed on weed communities, pollen introgression of the glyphosate resistant trait into non glyphosate resistant corn, and issues relating to application timing. One other consideration of the adoption of glyphosate-based technology has been the slowed development of new herbicides which may be needed to overcome the risks associated with the glyphosate-based systems. These benefits and risks, as well as the development of existing and new herbicides will be discussed in this paper. The corn and soybean herbicide effectiveness charts have been included as a reference

Weed management for 2017 and beyond

It has been approximately 30 years since the last new herbicide mechanism of action (MOA) was introduced and it is unlikely that a new MOA will be introduced in the near future. Furthermore, weed management issues continue to escalate, particularly the increasing number of herbicide-resistant weed populations and the increasing population densities in fields with herbicide-resistant weeds. For example, in Iowa, multiple resistance in waterhemp is the norm rather than the exception and the rate of spread is accelerating. The recent wide spread introduction of Palmer amaranth in Iowa further contributes to future weed problems. Regardless, farmers in Iowa remain “techno-optimistic” that new herbicide solutions to the weed management problems will be soon introduced (Dentzman et al. 2016). This “techno-optimism” is contrasted by the “techno-skepticism” of farmers in the south. Interestingly, Iowa farmers also express concerns that new resistances in weeds are inevitable with the anticipated new herbicides but that the future new herbicides are essentially the only option for effective weed control. A number of current and future issues will be considered and perspectives provided in this paper

Weed Management Update, 2001

While much of the discussion in agriculture has focused upon the use of genetically modified organisms (GMO), particularly BT com hybrids, there are other issues that should be considered as weed management plans are formulated for 2001. It is very likely that a major factor that will influence herbicide efficacy and degradation, and thus weed populations, is the lack of soil moisture. Dry conditions were prevalent throughout much of Iowa during the late summer and fall2000. How the 2001 season begins will be most influential on herbicide phytotoxicity to seedling crops, the potential for herbicide carryover from 2000, the interaction of residual herbicides and products applied in 2001, and the effectiveness of the herbicides. The lack of certainty and predictability of the weather dictates that several options for weed management be developed, and that growers maintain flexibility in weed management programs. Locking in a specific weed management plan early in the winter may be less than desirable. While convenient, and likely to provide some economic incentives, the lack of adaptability to changes in the environment places great risk on accepting a specific company weed management plan early in 2001

Weed Management Update

There have been a number of new herbicides registered, co-packs introduced, and prepackage mixtures developed. Also, there have been some new developments with some of the common weeds in Iowa. Finally, there has been a rapid adoption of the glyphosate resistant technologies. The purpose of this paper will be to assess these changes and provide some indication as to their significance to Iowa agriculture