Agriculture Backfire: Intensive Farming Practice can Turn a Native Plant into a Pervasive Weed 

We all know that native species is important for the local environment. But what happens when they become problematic and quite damaging, especially for agriculture? Well, unfortunately, that’s what’s happening to common waterhemp, a North American native plant. 

A team of researchers, led by the ones at the University of British Columbia (UBC), did the study on waterhemp. They compared 187 waterhemp samples from modern farms and neighboring wetlands with more than 100 historical samples dating as far back as 1820 which had been stored in museums. 

Studying the plant’s genetic makeup in the last two centuries has allowed the researchers to see how the plant evolve across changing environments. 

First author Dr. Julie Kreiner said, “The genetic variants that help the plant do well in modern agricultural settings have risen to high frequencies remarkably quickly since agricultural intensification in the 1960s.” 

During the study, the authors found hundreds of genes the weed’s genome that aid its success on farms. They also discovered mutations in genes related to drought tolerance, rapid growth and resistance to herbicides. 

Dr. Kreiner said, “The types of changes we’re imposing in agricultural environments are so strong that they have consequences in neighbouring habitats that we’d usually think were natural,” 

With these findings, hopefully the study can improve conservation efforts to preserve nature and environment within areas dominated by agriculture. Reducing gene flow out of agricultural sites and choosing more isolated natural populations for protection could help limit the evolutionary influence of farms. 

Being a native plant, common waterhemp wasn’t always a type of plant that farmers want to get rid of completely. However, due to the plant’s evolution to become more resistant to herbicide, it’s become very difficult to remove them from farms.  

Five out of seven herbicide-resistant mutations were found in the current waterhemp samples. And they’re absent in the historical samples.   

Encouraging new dislike for the native plant 

Coauthor Dr. Sarah Otto said, “While waterhemp typically grows near lakes and streams, the genetic shifts that we’re seeing allow the plant to survive on drier land and to grow quickly to outcompete crops. 

“Waterhemp has basically evolved to become more of a weed given how strongly it’s been selected to thrive alongside human agricultural activities.” 

What’s troubling is that waterhemps that carry any of the seven herbicide resistant mutations have produced an average of 1.2 times as many surviving offspring per year since 1960 compared to ones without the mutations. 

Now, mutations against herbicide can be found in nature. But, of course, at a lower frequency. Nonetheless, it raises questions about the costs of these adaptations for plant life in non-agricultural settings. 

“In the absence of herbicide applications, being resistant can actually be costly to a plant, so the changes happening on the farms are impacting the fitness of the plant in the wild,” Dr. Kreiner said. 

Moreover, the authors found that agricultural practices have reshaped where particular genetic variants are found across the landscape.  

Within 60 years, a weedy southwestern variety progressed eastward across North America, spreading their genes into local populations as a result of their competitive edge in agricultural contexts. 

Coauthor Dr. Stephen Wright said that expanding this research across species and scales will make it more understandable as to how farming and climate change could drive rapid plant evolution. 

“Understanding the fate of these variants and how they affect plants in non-farm, ‘wild’ populations is an important next step for our work,” said another coauthor Professor John Stinchcombe. 




Does agricultural practice need more improvement? 

Speaking of agriculture, cover crops has been encouraged and subsidized by the US Department of Agriculture in order to reap more benefits, like carbon storing, healthier soil, less fertilizer runoff, and the need for less chemicals in agricultural soils as of late. 

The purpose is to mitigate climate change, and it may look good in theory. However, according to Standford University-led research, the reality is more complicated than the promised benefits. 

Lead author Jillian Deines said, “Use of cover crops is rapidly spreading. We wanted to see how these new practices affect crop yields in the real world, outside of small-scale research plots.” 

David Lobell, the senior author, added, “Agriculture is a very tricky business to get right, and things typically don’t work out as planned. Our view is that constant monitoring, evaluation, and learning is a key part of making agriculture truly sustainable.” 

Planting cover crops during off-season can actually be productive. Because, it can result in large reductions in runoff and nitrogen leakage into streams and groundwater, reduced soil erosion, and reduced need for weed control chemicals. It’s also be a rather cost-efficient strategy to keep CO2 out of the air. 

That’s why the US government has subsidized this practice since 2016, incentivizing more than $100 million per year. Cover crops are used on only about 5% of fields in the primary corn-growing region of the U.S. 

Works well in the long run 

The researchers used satellite imagery to do a field-level analysis of yield impacts from cover cropping across the U.S. Corn Belt: looking over about 20 million acres of farmland in states like Ohio and Michigan.  

Then, they analyzed field which had grown cover crops for at least three years and compared them to similar fields with no cover crops. 

On average, the yield level of fields with cover crops declines about 5.5% for corn and 3.5% for soybeans. Corn has more loss due to the crop’s greater need for nitrogen fertilizer, a chemical that common cover crops also use. Maize crops also need water, which cover crops can deplete after dry growing seasons. 

When talking about money loss, the declining yield leads in a loss of about $40 per acre for corn and $20 per acre for soybeans. When farmers are faced with such loss, together with the cost of implementing cover crops (around $40/acre), long-term adoption of planting cover crops gets more challenging. 

However, the researchers wrote that the drawbacks are not permanent: the benefits will take a while to be in effect and farmers will likely become better at implementation. 

Implementation means having alternatives to the current most used cover crop in the US Corn Belt: rye. When farmers have more research and study guide, those alternatives may result in higher primary crop yields in some regions. 

Another implementation includes removing cover crop with enough lead time before planting primary crops so that it could reduce the possibility of significant yield penalties. 

Lobell concluded, “Learning by doing is really important, and adjustments are almost always needed both in the sense of farmer practice and government policy. Learning by doing is really important, and adjustments are almost always needed both in the sense of farmer practice and government policy” 



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