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

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

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 Science Update 1990

There are a number of major considerations with regards to weed management in 1990. Several weed species have been identified as potentially serious statewide problems. Herbicide carryover has the potential to be an issue in 1990 and should be considered seriously as decisions are made for crop rotation, tillage, and herbicide selection. Finally, there are several herbicide candidates that demonstrate significant weed control and may impact overall weed management choices in 1990

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

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