Long-term trends in the intensity and relative toxicity of herbicide use

Abstract

Herbicide use is among the most criticized aspects of modern farming, especially as it relates to genetically engineered (GE) crops. Many previous analyses have used flawed metrics to evaluate herbicide intensity and toxicity trends. Here, I show that herbicide use intensity increased over the last 25 years in maize, cotton, rice and wheat. Although GE crops have been previously implicated in increasing herbicide use, herbicide increases were more rapid in non-GE crops. Even as herbicide use increased, chronic toxicity associated with herbicide use decreased in two out of six crops, while acute toxicity decreased in four out of six crops. In the final year for which data were available (2014 or 2015), glyphosate accounted for 26% of maize, 43% of soybean and 45% of cotton herbicide applications. However, due to relatively low chronic toxicity, glyphosate contributed only 0.1, 0.3 and 3.5% of the chronic toxicity hazard in those crops, respectively.

Agricultural GMOs—What We Know and Where Scientists Disagree

“…From the perspective of pesticide applicators, the individuals more directly affected by these compounds, chronic toxicity is likely the more important of those two measures. Judged by its chronic toxicity to mammals, glyphosate is a relatively safe compound with lower levels of chronic toxicity than 90% of commonly used US herbicides [28]. The same analysis concluded that a reduction in glyphosate use or its discontinuation (as was recently proposed in Europe) would result in higher overall toxicity because it would likely be replaced by herbicides with higher toxicity.…”

“…First-generation GMOs have also served to shift the types of pesticide used, with the largest change being to increase the relative share of crop area produced using glyphosate. Evidence suggests that the switch from other herbicides to glyphosate has benefits for the health of growers even apart from any overall reduction in total amounts used [28].…”

“…The EIQ is a commonly-used measure of that compares pesticides across multiple attributes in order to generate a single value to reflect overall toxicity [28,29]. Pesticides vary greatly in their toxicity, and health impacts are much better understood by aggregate toxicity measures that reflect those differences, rather than by simply measuring volumes of pesticides applied.…”

Are herbicides a once in a century method of weed control?

“…US pesticide productivity has declined significantly (by more than 50%) over the past 30 years. In a study weighting herbicides by toxicity, herbicide use intensity was shown to increase for maize, cotton, rice and wheat over the last 25 years. Analyzing data from a large plot‐level, representative sample of US maize and soybean farmers from 1998 to 2011 provided ‘clear evidence of increasing herbicide use by GT variety adopters over time for both soybeans and maize, a finding that we attribute in part to the emergence of glyphosate weed resistance.’…”

Re-registration Challenges of Glyphosate in the European Union

“…Considering oral rat LD 50 or 24-month oral rat no observable adverse effect level (NOAEL) values for acute or chronic toxicity for all herbicide active ingredients used, glyphosate was estimated in a study (Kniss, 2017) to contribute only 0.1, 0.3, and 3.5% of the chronic toxicity hazard in those crops, respectively, on the basis of the hazard quotient approach weighting the hazard (toxicity) with the areas and dosages applied. Nonetheless, this estimation considered a factor termed "area-treatment" (instead of the absolute amounts applied), solely the average exposure (vs. exposure dynamics), and only of the active ingredients.…”