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The Nuances of Analyzing Yield Data

By Eric Sachs, Monsanto Scientific Affairs

On May 17, the U.S. National Academies of Sciences, Engineering, and Medicine (NAS) released a new report, “Genetically Engineered Crops: Past Experience and Future Prospects.” The committee’s two-year analysis considered a range of questions and viewpoints about the economic, agronomic, health, safety and other impacts of genetically engineered (GE) crops and food.  The committee’s extensive review employed a transparent and inclusive process to examine all concerns, uncertainties and public knowledge gaps in addition to studying a robust body of scientific evidence to arrive at objective conclusions on important topics related to GE crops.  These issues have never been more important – and Monsanto applauds the NAS on their commitment to encouraging science-based dialogue around agriculture technologies.

Already initial media reports have triggered a discussion around one aspect of the analysis – the question of how GE crops’ contributions to yield have compared to yield results achieved by conventional breeding alone. We welcome that discussion, and would like to share our thoughts on this important subject.

Yield Terminology

First, I’m going to take a moment to simplify what we mean by “yield.”

Potential yield: A plant has a genetic makeup that gives it the ability to produce a certain amount of a crop, usually under ideal weather and environmental conditions. For example, let’s take a pepper plant: for one variety of pepper, its potential yield may be 25 peppers per plant; for another, the potential yield may be 20 peppers. But, conditions will impact the plants over the course of its growing and crop-producing stages. These conditions include poor weather conditions (think drought, unusual stretches of cold weather, too much rain, etc.) and environmental conditions (such as bugs trying to munch on the plants or weeds competing for resources like sun and water).

Actual Yield: Actual yield is the final amount of a crop that a farmer harvested. So while the potential yield could have been 25 peppers, maybe you only harvested 17 peppers.

Genetic improvements can increase the potential yield ceiling under ideal growing conditions, or increase the actual yield by mitigating losses from pests, weed competition, diseases, or other stresses caused by extremes of climate.  All of the GE traits introduced to date in maize, soybean and cotton are intended to increase actual yield by limiting losses due to pests and weeds.  Increases in potential yield in the current crops have only occurred via conventional breeding.

One of the benefits of GE traits is their ability to protect the potential yield of crops. As mentioned above, weeds and bugs can compete with crops for nutrients and sunlight. Competition can impact the potential yield of a crop. So, if a farmer is able to manage weeds better, via a herbicide-tolerant GE trait, he reduces that competition for nutrients and gives the crop a better chance to realize its potential yield.

NAS Report Findings on Yield

The report provides the following information to help the reader appreciate the nuances of this topic:

  • As of 2015, most GE crop traits were intended to reduce yield losses from pest damage and/or weed competition.
  • GE traits have an indirect effect on yield by reducing yield losses.
  • GE traits can help to close the yield gap between actual and potential yield but do not increase the yield potential of the crop.
  • Research around GE traits aimed at increasing potential yields is underway, but it is too early to predict its success.

To illustrate the impact of GE crops on actual yield, the report cites yield data from small plots, large-scale and small-scale farms that demonstrate:

  • When attacked by target pests, Bt crops have increased yields compared to conventional crops treated with synthetic chemicals.
  • GE crops with herbicide resistance can provide higher yields due to the associated herbicide’s effectiveness for controlling a broad spectrum of weeds.

Importantly, the report acknowledges that “There is disagreement among researchers about the how much GE traits can increase yields compared to conventional breeding” but adds that “the sum of experimental evidence indicates that GE traits are contributing to actual yield increases.”

Despite all of the complex considerations when examining yield data, several media reports have focused only on the overall average yield trend lines from 1980-2011, as reported by USDA-NASS. The report states, “The nation-wide data on maize, cotton, or soybean in the United States do not show a significant signature of genetic-engineering technology on the rate of yield increase.”  The absence of an obvious change in the rate of increase in actual crop yields after the adoption of GE crops is taken as evidence that GE traits so far have not significantly contributed to increases in yield.  That conclusion is both confusing and inaccurate.

In practice, it is often difficult to measure the contributions of multiple factors to yield.  The impacts of the relevant factors are variable from field to field, and from year to year.  The rate of yield gain in any given year is influenced by the potential yield of the germplasm, the choice of agronomic practices, the type of crop protection methods employed, the use of GE traits and the influence of climate conditions.  Given that multiple factors play a roll, it is expected that gains caused by some factors will be offset or masked by declines caused by others.

After reviewing the report, our take-away from all of the information provided is that GE maize, soybean and cotton expressing pest and weed resistance traits have increased actual crop yields but that the average rate of yield gain in these crops has not increased or decreased after the introduction of GE technology.

Looking at the data another way

Since the evidence shows that GE crops do increase actual yields in field trials designed to measure the yield impacts of GE traits, my colleagues and I used another approach to see if the rate of yield gain had in fact changed after GE crops were widely planted.  We charted the USDA-NASS yield data for maize, soybean, cotton and sugar beet over a 10-year period before the GE crop was introduced, and after the GE crop had reached 50% penetration on U.S. farmland.  We also plotted a best-fit linear regression and tested for statistical significance.  There is clear evidence of an increase in the rate of yield gain after 50% penetration of GE corn, GE cotton and GE sugar beet.  These differences are not statistically significant given the small number of data points and associated variation caused by multiple factors impacting yield.  There is minimal impact in GE soybean, which is expected since the principal agronomic impacts of herbicide tolerance in GE soybean are improved crop management efficiency and expanded adoption of reduced tillage systems.

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It is not possible to identify the individual contributions of the various genetic and environmental factors that affected increased rate of yield gain in each of these cases, but the evidence is strong that GE traits are contributing to increasing or protecting actual yields.

Our hope is that the ongoing discussion of GE crops will fully consider the existing comprehensive data and the important nuances discussed above.  This closer examination of the yield impacts of GE crops illustrates how important it is to avoid overly broad and generalized statements when dealing with complex and multidimensional scientific questions.

As always, dialogue is important to our company, and we look forward to a broader conversation about the application of biotechnology and other agricultural sciences to benefit our growing and changing world.

1 Responses to "The Nuances of Analyzing Yield Data"

  1. Pingback: NYT Misses the Point on Yields, Pesticides and What Farmers Need | BIOtechNow

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