Farmers have been battling with weeds since the advent of agriculture, about 6,000 years ago. Most of the weapons used against weeds were mechanical until the last century or so when herbicides became the primary weapon. At the same time that the weapon became more lethal, farming techniques changed to give weeds the advantage. Farms became huge monocultures and crop rotations were abandoned in favor of the most profitable crop.
When weeds evolved defenses against the herbicides, farmers responded by increasing doses and manufacturers created new products to which weeds hadn’t yet evolved resistance. Finally, the use of herbicides skyrocketed when crop seeds were invented that weren’t killed by the herbicides, so that huge amounts of herbicides can be used without killing the crop.
When Roundup with the active ingredient glyphosate went on the market in the 1970s, its manufacturer, Monsanto, claimed that weeds would not be able to evolve resistance to it. And apparently that was initially true until Roundup was used on a huge scale when herbicide-resistant seeds were put on the market in the 1980s. Norman Ellstrand of UC Riverside explains why: “He argues that the reason was that farmers applied glyphosate to relatively little farmland. As they applied it to more and more acreage, they raised the evolutionary reward for mutations that allowed weeds to resist glyphosate. ‘That ups the selection pressure tremendously,’ he said.” *
There are now 24 species of weeds that are resistant to glyphosate and they are rapidly expanding their range in agricultural areas all over the world. In 2012, an agricultural consulting firm reported that 34% of farms in the US had glyphosate-resistant weeds. In the first half of 2013, half of all farms in the US are reporting glyphosate-resistant weeds.
Let’s take another approach
Obviously, we are losing the war against weeds. So, let’s examine the strategy we have been using and try another approach. Weed ecologists are now studying the strategies that weeds have used to cope with the weapons we have been using against them. Here are a few of those strategies:
- Some weeds have changed color so that they are indistinguishable from the crop they are hiding in.
- Weeds that grew in dry ground, evolved to thrive in wet ground in rice fields that are flooded much of the crop season.
- Some weeds became shorter to escape the mowing and harvesting of the agricultural crop.
- Some weeds drop their seeds and go dormant before the crop is harvested and create seed banks that can sprout when conditions are more favorable for them.
- Parasitic weeds wrap around their host and steal nutrients from them.
Some weed ecologists believe that a better understanding of the mechanisms used by weeds to foil our attempts to control them will enable us to devise better weapons against them. They believe that developing new herbicides and/or using more of them will always be a short-term solution.
For example, David Mortensen of Penn State is “investigating controlling weeds by planting crops like winter rye that can kill weeds by blocking sunlight and releasing toxins. ‘You want to spread the selection pressure across a number of things that you’re doing so that the selection pressure is not riding on one tactic,’ he said.”*
Regardless of what method is used to control weeds in the future, let’s consider the toxicity of the method the most important criterion for judging their effectiveness. Even if it kills fewer weeds, the least toxic alternative is the best alternative in our opinion.
*Carl Zimmer, “Looking for Ways to Beat the Weeds,” New York Times, July 15, 2013.