2A: Six Risk Drivers

Healthy babies are the goal, and understanding six key risk drivers is a critical step in mitigating impacts from pesticides.

Six primary factors determine the nature and level of reproductive and other health risks stemming from herbicide use:

  1. Length of the herbicide spray season.
  2. Number of distinct products applied.
  3. Volume applied of each product, and all products, plus their adjuvants and other “inert” ingredients.
  4. Cumulative exposures via the air, dust, and residues in drinking water and food.
  5. Number of applications and timing of exposures relative to the stage of pregnancy, and over the course of prenatal and infant development.
  6. Biological mechanism(s) through which adverse effects are triggered.

It is likely that most, if not all of these six risk drivers are on the rise across the rural Midwest. This section explains why.

1. Extended Time Period for Herbicide Use and Exposures

All genetically engineered, herbicide-resistant (GE-HR) crops offer farmers one important advantage – the ability to spray an herbicide over the top of a growing crop, hopefully killing weeds without harming the crop. This is an example of a post-emergence herbicide application.

Absent the new genetic trait incorporated in GE-HR crops, a post-emergence application of a broad-spectrum herbicide like glyphosate would kill both the weeds and the crop growing in a field. So, one outcome of GE-HR technology is extension of the time period during which certain herbicides are typically sprayed, and this in turn extends the time period rural residents are likely to be exposed to herbicides.

As this graphic illustrates, the yearly period where herbicides are applied is greater for GE-HR crops, resulting in a much longer window where harmful exposures can occur.

With today’s GE-HR crops, the duration of the herbicide spray season will more than double compared to when GE-HR crops were introduced (1996). For example, the 2,4-D and dicamba herbicide product labels state that applications on GE-HR soybeans can be made through the plant’s R1 growth stage (beginning bloom, i.e. one open flower at any node on main stem, when plants are ~24” tall).

Before the introduction of GE-HR technology, most applications of dicamba and 2,4-D herbicides were made from April 1 through ~May 20 (the typical spray season starts and ends earlier in southern states). But now, applications will be made over a much longer period.

Spraying will start up to 10 days before planting (as early as March 15). Planting will occur from late March through ~May 31. Plant emergence occurs 10-20 days after planting (from early April through early June). Between 54-68 days pass in most areas between emergence and the end of the R1 stage of growth (~early June through early August) (Casteel, 2017). So, 2,4-D applications will occur in soybean producing areas from mid-March through early August, or over a nearly five-month period.

The typical 2,4-D spray season on GE-HR corn will also about double.

2. Number of Herbicides Applied

Traditional corn hybrids have been replaced by genetically engineered, herbicide- resistant varieties on most farms throughout the Midwest.

In 1971 corn farmers brought a crop to harvest with an average of 1.09 herbicide applications per acre. This means most acres were sprayed with one herbicide and 9% were treated with a second one.

Two decades later in 1991, an average of 2 applications were made, in most cases one application of an herbicide targeting grassy weed species, and a different active ingredient targeting broad-leaf weeds.

The average number of herbicide applications rose to 2.3 in 1995, the year before the first GE-HR corn varieties were planted. Adoption of GE-HR corn hybrids, the vast majority of which were engineered to tolerate applications of glyphosate-based herbicides (GBHs), was much slower than in the case of GE-HR soybeans.

GE-HR corn was planted on 20% of national acres in 2004 and 70% in 2010. The corresponding level of adoption of GE-HR soybeans was 85% and 93% in 2004 and 2010 respectively.

Pesticide use data issued by USDA, and reviewed in detail in the Herbicide Use section, shows that an average of 3.2 herbicides were applied on national corn acres in 2016, a 39% increase since 1995. Trends in the number of herbicides applied on the national corn and soybean crops from 1971 through 2016 are presented in Figure 2A.2.


In the case of soybeans, farmers managed weeds in 1971 with an average of 0.7 herbicide applications. By 1990, reliance had risen to 2.1 different herbicides applied to the average acre, one targeting grassy weeds, and the other broadleaf weeds. The number of different products applied peaked right about when GE-HR soybeans were brought to the market in 1996, at about 2.68 per acre.

As Roundup Ready soybeans and GBHs gained market share, other herbicides were displaced, and the average number of herbicides applied per acre fell to a low of just 1.37 in 2005. In that year, not even one-third of soybean acres were sprayed with a second herbicide other than glyphosate. The mid-2000’s was the pinnacle of the Golden Era of weed management for corn, soybean, and cotton farmers, a period when one of the trickiest and most costly annual management challenges facing all farmers worldwide became one of the easiest and most reliable, come rain or shine.

But the heavy, annual reliance on glyphosate triggered incremental shifts in weed communities toward species that germinated over a longer period of time, sometimes avoiding a farmer’s last application of glyphosate.

Heavy reliance on GBHs also led to the emergence of weed phenotypes that had evolved ways to survive applications of glyphosate that previously proved lethal. The spread of glyphosate-resistant weeds, often referred to as “super weeds,” shifted the about decade-long, downward trend in number of herbicides applied onto an upward trajectory that has become progressively steep, with no end in sight.

By 2010, an average of 2.01 herbicides were sprayed on soybeans. The rising trend has continued, with 2.74 different herbicides applied in 2016. From the height of efficacy in Roundup Ready soybean weed control in 2005, the average number of different herbicides needed to bring a crop to harvest rose from 1.37 to 2.74, or a 100% increase over just an 11-year time span.

3. Volume of Herbicides Applied

Herbicide use has risen sharply in soybeans since the introduction in 1996 of GE-HR soybeans, rising from 1.10 pounds per acre in 1995 to 1.90 pounds in 2016, a 73% increase.

In the case of corn, overall herbicide use fell from 2.63 pounds per acre in 1995 to 1.83 pounds in 2002, but then rose the next year to 2.05 pounds. By 2016, it had risen to 2.20 pounds. These trends appear in Figure 2A.3.

The approval and rising reliance on 2,4-D and dicamba-resistant corn and soybeans will push average herbicide use per acre upward in the next few years. It remains unclear whether farmers will continue to make a second or third application with a GBH, on fields infested with multiple glyphosate-resistant weeds.

If farmers back off glyphosate as most university weed scientists are urging them to do, the trend in overall pounds applied will likely begin to turn downward. The herbicides that farmers add to their control programs as they reduce reliance on GBHs will, for the most part, be applied at a much lower rate of application.

Reliance on low-dose chemistry is among the key factors covered in the section on pesticide use indicators and in tables presented in the Herbicide Use section.

4. Cumulative Exposures

Rising use will inevitably lead to new routes of exposure, as well as higher levels of exposure under some circumstances. It is impossible to predict the correlation between the pounds of herbicides applied in an area, and exposure levels among individuals living in the area. Existing studies, however, suggest that rising use will lead to incrementally higher frequencies and mean levels of exposure, as well as a wider range of exposure levels.

5. Number and Timing of Applications

Soybeans are now sprayed over longer periods of time, and with additional herbicide active ingredients, compared to before the introduction of GE-HR seed varieties.

While the number of different herbicide active ingredients applied on corn and soybean fields have risen in the last decade, the increase in the average number of acre-treatments has been even greater.

An “acre-treatment” occurs when a single herbicide active ingredient is sprayed on a field. One application of a tank-mix product that contains three active ingredients accounts for three acre-treatments.

Acre-treatments have trended upward more swiftly than the average number of different herbicides applied because glyphosate is often applied twice, three, or even four times in a given production season.

The trend toward more acre-treatments on the average corn and soybean acre is likely to accelerate for three reasons. First, both 2,4-D and dicamba will be applied more than once on most acres planted to new, 2,4-D and dicamba GE-HR crops.

Second, farmers will need a more diverse mix of active ingredients, some of which will have to be applied twice or more, to deal with the proliferation of hard-to-kill resistant weeds.

And third, the industry is moving toward the manufacture and sale of special pre-mixed products containing multiple active ingredients.

Complex technical challenges often arise when mixing pesticide active ingredients together, since some active ingredients perform optimally with one type of surfactant or adjuvant, while other herbicides work best with others. Compared to farmers mixing together two or more herbicide active ingredients when they fill their sprayers, herbicide manufacturers have deeper technical skills and experience to manage compatibility issues that can arise with herbicide tank mixes. This is why herbicide manufacturers are introducing such a wide array of multi-active ingredient products for use on fields with glyphosate-resistant weeds.

The herbicide spray season is growing incrementally longer, and a greater diversity of active ingredients will be applied more frequently. This will inevitably increase the time periods when individuals living in rural areas may be exposed to herbicides. As a result, more women in their first trimester of pregnancy will be exposed to heightened levels of herbicide.

6. Herbicide Mechanisms of Toxicity

Herbicides pose risks for humans through a wide variety of mechanisms. Some risks are transitory and usually not serious, such as skin or eye irritation, or temporary nausea. Other impacts can lead to serious, life-long consequences including cancer, Alzheimer’s disease, liver or kidney disease, or metabolic problems.

Several of the most widely used herbicides in corn and soybean production are known to increase the risk of a variety of reproductive and developmental problems. The next sections provide an overview of some of the most important, published studies on the linkages between herbicide exposures and adverse birth outcomes. In most cases, readers can access the full abstract of cited studies by clicking on the author citation. A link is provided to several open-access papers (“open access” papers are available for viewing and/or downloading at no cost).

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