In my series 17 Questions about Glyphosate, question 14. deals with glyphosate resistant-weeds: whether they pose a problem, and why campaigners against glyphosate should be the last ones to worry about this particular issue. Question 15. looks at the soil ecosystems: what do we know about the effects of glyphosate on soil micro-organisms? Does it affect nutrient balance and mineral uptake? Plus comments on what one troubled study found out about earthworms. Question 16. delves into whether there is a relationship between glyphosate and the situation of Monarch butterflies or bees.
14. What about resistance and superweeds?
Firstly, by talking about superweeds we are getting farther away from understanding a problem. Let’s instead talk about something well defined, without the embellishments and vague impressions added by the term superweed.
A better question would be: is there a problem with glyphosate-resistant weeds? Certainly, weeds developing a widespread resistance against glyphosate would nullify all of its earlier outlined environmental benefits, and farmers would have to revert to tilling as a means of weed control, as well as older, more toxic herbicides. That would be a big blow to the farmers and the environment. Conversely, it would also make the campaigns for a glyphosate-ban redundant, as glyphosate would no longer be used, or its use would be greatly reduced. So… from the perspective of said campaigns against glyphosate, problem solved?
This is the extent of the resistance problem: if resistance becomes all too problematic, farmers stop using glyphosate. Conversely, most people who worry about glyphosate would not deem cessation of its use an issue at all. Isn’t it rather more problematic (from their perspective) that glyphosate use is not creating enough glyphosate-resistant weeds in order to undermine its usefulness for farmers?
But putting that seemingly solved dilemma aside, are glyphosate-resistant weeds a problem? Will farmers be able to continue its use? If you would like to read more about the world of herbicide resistance, its history, management tools, and future prospects, you can read several informative pieces on that from, you guessed it, the weed ecology professor Andrew Kniss. For instance: The Cost of Preventing Herbicide Resistance, or Herbicide Resistance Predates Herbicides by 80 Years. In The Large-scale Impacts of Herbicide Resistance, he writes:
Since the discovery of glyphosate-resistant Palmer amaranth, arguably the most damaging of all the so-called superweeds, cotton yield in the affected states continues to increase.
To be clear, though, I don’t want anyone to get the impression that glyphosate-resistant Palmer amaranth hasn’t had a major impact. On the contrary, there is ample evidence that farmers in the South have had to adapt to this particular problem by using other herbicides, re-introducing tillage, or using cover crops. These additional weed control practices cost money, and growers with glyphosate-resistant weeds may very well have seen a reduction in their net economic return. There are surely many examples of individual cotton fields where yield was significantly reduced by this weed. But I think it is important to note that these herbicide-resistant weeds have not had the devastating impact on crop yields, at least not on a macro-scale, that is often implied by the sensational coverage of this topic.
The overall rate of newly confirmed herbicide resistant weed species to all herbicide sites of action (SOAs) has slowed in the United States since 2005. Although the number of glyphosate-resistant weeds has increased since 1998, the evolution of new glyphosate-resistant weed species as a function of area sprayed has remained relatively low compared with several other commonly used herbicide SOAs.
The rate of weeds developing resistance to herbicides has slowed since 2005. There are other herbicides that are more vulnerable to resistance developing in the weeds they are meant to kill than glyphosate. There is a plausible case to be made that wider use of glyphosate took pressure off those herbicides and contributed to this lower rate.
15. Does glyphosate interfere with soil organisms or nutrient availability?
There are a couple of claims on glyphosate’s effects in the soil, basically along the lines of: glyphosate interferes with plant mineral uptake in soil, or causes other problems with soil dynamics. The short answer to that is, that there is no evidence of unwanted effects in soil characteristics with appropriate use of glyphosate.
One of the older reviews on the topic, from 1992, has characterised glyphosate in relation to the environment and other organisms as follows:
There is no residual soil activity, it does not leach into non-target areas, and it is non-volatile. It is practically nontoxic to mammals, birds and fish, showing no bioaccumulation in the food chain; it is biodegraded into natural products. When used correctly, glyphosate poses no threat to the environment and its inhabitants.
A 2012 review article published in the Agricultural and Food Chemistry Journal found no difference in plant diseases or mineral nutrient availability with or without glyphosate or RoundUp Ready (RR) crops. That’s not to say that when it’s sprayed, glyphosate could probably have an inhibiting effect on soil bacteria before it gets broken down. At least a temporary inhibition is what happens with sufficient millimolar concentrations in the lab (see here and here), although the dynamics might be different in soil: one study found that glyphosate antibacterial effect seems to be neutralised by the presence of humic acid, which is the major organic component of the soil. This could be part of the reason why an earlier study, from 2001, also found that glyphosate did not affect microbial communities in the soil, and noted that artificial media assays were of limited relevance to what actually happened in the farmed soils. You can also read more about the studies on glyphosate and soil at Biofortified: Does glyphosate restrcit crop mineral uptake?
Jonathan Eisen, the earlier cited microbiome researcher who presented on this topic before the US National Academy of Science, also made the following comment in the discussion following one his blog posts:
I am actually interested in how glyphosate might affect microbiomes. I think it is incredibly unlikely that ingestion of glyphosate in food has any significant effect on microbiomes in animals. But I do think it is possible that glyphosate affects soil and plant microbiomes and that in turn could affect what microbes come into animals (via eating the plants). So I think this is interesting and worth pursuing. This notion is not about glyphosate good or bad. Just about testing the possibility that it indirectly affects animals microbiomes (and if so, this could be a beneficial effect).
This is an interesting area of research, and it’s always good to keep an open mind and a critical eye out on new evidence accumulating on the topic. A recent seven year field-study, published in 2016, found no effect from glyphosate with RR crops or non-RR crops with regards to macronutrients or exoenzymes (which are signs of microbial activity) in the soil. In fact, no-till and conservation tillage, which glyphosate enables, has a documented positive effect on the soil physiochemical and microbiological properties – seeing how traditional tilling method is actually a very soil-disruptive type of weed management.
Most laboratory studies worry about glyphosate’s potential inhibory effect on bacteria, observed when glyphosate concentrations are high enough, but there is also a field study published in 2013, which found that glyphosate present in 50 millimolar concentrations increased bacterial activity and heterotrophic bacterial population:
The results show that in a soil with a long historical use of glyphosate (soil 1), the heterotrophic bacterial population was significantly (p<0.05) increased. Also, by increase in the bacterial population, the herbicide existence as the possible nutrient source is enhanced.
They conclude that said increased microbial activity may be beneficial or detrimental toward plant growth, soil microbial ecology, and soil quality. Perhaps glyphosate use could enrich microbial communities both thanks to no-till and directly through its presence in low concentrations. Perhaps in some situations the concentrations would get too high – like in areas where long cold winter conditions slow down natural breakdown processes – and there could be a noticeable negative effect on the microbial populations. But to determine that, we need good, tangible data of field conditions. The potential relationships at work here are far from simple, and proper conclusions can never be drawn from hypotheses alone.
The gist of this question is quite practical: if glyphosate did cause big issues for beneficial soil microbes or nutrient problems for plants, that would soon lead to problems for the farmers. Note that glyphosate has been used for 40 years, and to the best of my knowledge, no such shift has happened. Impacts such as reduced yields, poorer soils, more bacterial diseases, or abnormal development would be something that farmers would be very quick to get to the bottom of.
One of the latest studies on a different vein, but raising alarm about glyphosate in the soil, was one where the researchers claim to show its effect on earthworms. They also make a somewhat contrary finding about glyphosate use increasing soil nutrient levels. Andrew Kniss, again, makes an illuminating analysis of the study in his piece Dead Plants Are Probably Bad For Earthworms.
The short of it is: the amount of glyphosate was an order of magnitude higher than real life scenarios; glyphosate was used in combination with an organic herbicide and the effects of the two can’t be separated; basic understanding of nutrient balance was missing; and vital controls were overlooked. Earthworms did not do as well in soils where all plants had died. The study authors had not thought that this factor would need its appropriate control – a plot where all plants were hand weeded, for instance, or the soil would have been tilled to kill the plants, or another method of plant removal was used. When such crucial controls are lacking, the information content of a study unfortunately drops significantly. Andrew Kniss concludes:
All of the effects on earthworms and nutrients observed in this study could simply be due to killing the plants in the pots. It is perfectly plausible the exact same effects would be observed if the plants were clipped or pulled out of the pots.
In either way, even if this study would not have such blatantly obvious flaws, we could not draw too many conclusions based on its results alone. Science is really not about one study one way or another. To account for bias, misconduct, poor design, random chance, etc, a study needs to go through the scrutiny of the scientific community as well as stand the test of time – it must be confirmed by independent groups of scientists, through several robustly designed studies presenting multiple lines of evidence. Shoddy studies do get published – but they get buried under the larger context of pieces which demonstrate more solid methodology and which do get confirmed by yet other independent research groups. Science works by accumulation. To date, it has not accumulated a convincing body of evidence supporting the existence of big problems for the soil ecosystems from the glyphosate used in farming.
16. Does glyphosate harm Monarch butterflies or bees?
Glyphosate is a herbicide, in other words, it is toxic to plants. Its target enzyme is not found in insects or animals, so it is generally not very harmful to them – and as confirmed by a recent study, even direct sprays are not lethal to bees. Update: a more in-depth look at some persistent efforts to try to link glyphosate and bee health here: No, Glyphosate Is Not a Threat to Bees.
Glyphosate could have an indirect effect on Monarchs, however, because it has enabled great advances in weed management, and as a result, while other farming impacts such as use of resources and nutrient leeching are reduced, the number of weeds such as milkweed, which the Monarchs thrives on, have decreased. In the keynote address which Ted Nordhaus from the Breakthrough Institute gave at the first annual Institute for Food and Agricultural Literacy Symposium, he makes an eloquent point on this issue, should it be so that efficient weed control were behind Monarch decline:
There is a correlation between glyphosate use and butterfly decline. But it’s not that glyphosate is killing the butterfly. It is an herbicide that targets plants, not insects. Rather, glyphosate is killing milkweed, a weed in which monarchs lay their eggs. While the decline of monarch butterflies is an unintended consequence of glyphosate use, the elimination of milkweed is not. It is one of the weeds that the herbicide is supposed to get rid of.
The trade-off here is straightforward and zero sum. You can either have more milkweed in cornfields or higher yields, but you can’t have both. If you choose more milkweed, then you are choosing lower yields, and, all else being equal, that means putting more land under cultivation to achieve the same level of agricultural output. With that comes attendant losses of habitat and biodiversity elsewhere.
Ultimately, the only way to have more monarch butterflies without reducing agricultural output or saving monarchs at the expense of other species is to create more monarch habitat outside of cornfields. This is an effort that a lot of people more concerned about monarch preservation as opposed to scoring ideological points about the food system have begun to focus on.
Another hypotheses put forward in Impacts of genetically engineered crops on non-target herbivores, Bt-corn and monarch butterflies as a case study, is that Monarch populations first boomed a few hundred years ago thanks to farming land area increase, as Monarchs may actually thrive better on cultivated land. Either way, the concern about the Monarchs does not stop at providing them large enough areas for breeding habitats in the US. The piece Monarch Butterfly Studies Tell a Perplexing Tale over Science magazine brings up other crucial aspects:
“But if the problem is that the monarchs are dying during the migration,” Davis says, “I’m not sure just trying to produce more at the start of the [fall] migration is the answer.” Other steps, such as protecting migratory pathways, may also be needed.
The overwintering habitat loss indeed seems to be the bigger factor in Monarch butterflies decline, as the conclusion of a recent study confirms:
…there are statistically significant negative temporal trends at the overwintering grounds in Mexico, suggesting that monarch success during the fall migration and re-establishment strongly contributes to the butterfly decline. Lack of milkweed, the only host plant for monarch butterfly caterpillars, is unlikely to be driving the monarch’s population decline. Conservation efforts therefore require additional focus on the later phases in the monarch’s annual migratory cycle. We hypothesize that lack of nectar sources, habitat fragmentation, continued degradation at the overwintering sites, or other threats to successful fall migration are critical limiting factors for declining monarchs.
If we truly are concerned about the fates of endangered species, we should be meticulous in evaluating the evidence for what really are the biggest factors influencing their place in the ecosystem. When it comes to farming, it is important that we use pesticides in a way to make sure we target the harmful insects and reduce the harm on others. The situation of many insects is complex – like in the case of bees, who, while essentially not affected by glyphosate, can be harmed by pesticides, but also do have several larger problems than the widely publicised neonicotinoid pesticides. More often than not, it is farming itself that brings about this difficult balancing act of providing for us while reducing the impacts on the environment. It’s the fact that we are so many, that what we do and the space we take up does greatly affect the environment. It is not an easy problem to solve, but efficient farming methods that leave as much nature outside of cultivated areas as possible are an important part of that puzzle.
Next up, the last question in my series, which could also have been a good starting point: 17. Can Glyphosate Research Be Trusted? What About Conflicts of Interest?
If you are interested in other environmental or health topics, you can find my other pieces on glyphosate over at 17 Questions about Glyphosate, and further resources under Farming and GMOs, The Environment, and Vaccines and Health. If you would like to have a discussion in the comments below, please take note of my Commenting policy. In a nutshell:
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