If current climate model forecasts are accurate, controlling weeds in the future is going to become increasingly difficult. According to a new study by the National Center for Atmospheric Research (NCAR), warmer weather associated with climate change is expected to create increasingly dry conditions over a majority of the globe during the next 30 years.

Aiguo Dai conducted the study using 22 computer climate models, a comprehensive index of drought conditions and analyses of previously published studies. Dai said that most of the Western Hemisphere as well as parts of Eurasia, Africa and Australia will be at risk for extreme drought later this century. Dai warned that the findings were based on the most current projections of greenhouse gas emissions.

If what Dai predicts is true, the implications for weed control could be significant for U.S. producers. The problem is two-fold: herbicide effectiveness and impacts on certain weeds.

Research from North Dakota State University showed that "weeds growing under hot, dry conditions often become more tolerant to herbicides. Plants may develop a thicker wax layer on the leaf surface, which is a barrier to herbicide absorption into the plant. Herbicide movement within the plant will likely be reduced due to a slowed rate of translocation and metabolism. Consequently, application of herbicides under such conditions often results in reduced weed control."

As a result, systemic herbicides such as 2,4-D and others usually see reduced weed control during hot, dry conditions. This may increase the use of adjuvants, which can improve weed control under certain conditions. However, contact herbicides become more active with higher temperatures. The only downside is the risk of greater crop injury.

Soil-applied herbicides often are challenged during dry conditions because they need the moisture in order to activate, but volatile herbicides work better in dry soil conditions. Post-emergence herbicides may be a better alternative in dry weather. And researchers expect fall-applied herbicides will provide better weed control than spring-applied herbicides in dry years.

The other side of the drought problem is its impact on drought-tolerant weeds like kochia, Russian thistle and perennial weeds. These weeds could become more troublesome as climate changes.

On top of these weed control challenges, producers worldwide continue to face growing herbicide-resistant species. In mid-October, Australian scientists announced they had discovered new cases of herbicide resistance in annual ryegrass, which is the most serious and costly weed of Australian cropping. The annual ryegrass was discovered to be resistant to paraquat.

In the United States, glyphosate resistance remains the most watched and raises the most concern. As a result, it's particularly timely that Monsanto announced in late October that it was developing a new platform on weed management to provide a framework to manage weeds in the absence or presence of glyphosate-resistance. The Roundup Ready PLUS platform offers best management recommendations to control glyphosate-resistant weeds by placing an emphasis on residual control to manage tough weeds. Monsanto also announced collaborations with Makhteshim Agan Group and Sumitomo Chemical's subsidiary Valent U.S.A. Corp.

Although it's great to see companies partnering to provide more information to retailers and crop consultants, the weed control challenges farmers are expected to face in the future due to drier conditions will change the way weeds are fought. Herbicide effectiveness and hardier weeds and a changing weed spectrum are issues that will challenge production agriculture for years to come. Partnerships and education will be key in combating these problems.