Response to PimentelResponse to Pimentel response which was not published in Science
Dr. David Pimentel’s response to our letter in Science(2005, vol. 307:1410-1411) misconstrues some of our points and is simply wrong in other areas.
First, Pimentel incorrectly claims that we equated herbicide tolerant crops (HT) with no-till cultivation systems. We clearly stated that HT crops facilitate both low- and no-till cropping systems, not just no-till. Low-tillage cropping encompasses a large spectrum of conservation tillage cropping, all of which significantly lower soil erosion compared to organic agriculture’s inherent heavy reliance on tillage for weed control.
The inaccurate focus on only no-till cropping results in a major underestimate of the beneficial impacts of HT crops, as demonstrated when Pimentel incorrectly states that “75% of U.S. soybean plantings include HT, but only 30% of them are planted with no-till.”
According to Dan Towery, just-retired director of the Conservation Technology Information Center located at Purdue University and funded by the USDA’s Natural Resources Conservation Service, fully 85% of U.S. soybean acres were planted to HT crops in 2004 and 61% of U.S. soybean acres were in low- or no-till cropping systems—more than double the percentage claimed by Pimentel.
Pimentel states that “herbicides are the most serious pesticide pollutants in streams and groundwater in the United States.” More accurately, herbicides are the most common pesticide pollutant. This is mostly in the form of seasonal triazine herbicide contamination of surface waters. Characterizing this contamination as “serious” is debatable, given the relatively benign risk profile of triazine herbicides. Nor does this have much relevance to the spectrum of herbicides commonly used in biotech HT crops, such as glyphosateGlyphosate is rarely a significant contaminant of ground or surface waters owing to its rapid breakdown in the environment. As noted by Fernandez-Cornejo and McBride (1), the “substitution caused by the use of herbicide-tolerant soybeans results in glyphosate replacing other synthetic herbicides that are at least three times as toxic and that persist in the environment nearly twice as long.”
They further note that “Glyphosate binds to the soil rapidly, preventing leaching, and is biodegraded by soil bacteria. In fact, glyphosate has a half-life in the environment of 47 days, compared with 60-90 days for the herbicides it commonly replaces. In addition, glyphosate has extremely low toxicity to mammals, birds, and fish. The herbicides that glyphosate replaces are 3.4 to 16.8 times more toxic, according to a chronic risk indicator based on EPA reference dose for humans.” (1)
The World Health Organization, in its comprehensive study of pesticides and chemical contaminants in water (2), places glyphosate in a category where “it is unnecessary to recommend a health-based guideline value for these compounds because they are not hazardous to human health at concentrations normally found in drinking water.”
Pimentel cites a completely outdated statistic from 1994—prior to the introduction of biotech HT crops—when he claims that “95% of corn production acreage in Iowa receives herbicides, and 70% of this land is also cultivated for weed control.” This statistic is no longer relevant, given the significantly increased spectrum of herbicides and HT corn combinations available to Iowa farmers today, nearly ten years after the introduction of HT biotech crops. As Iowa State University weed scientist Dr. Mike Owen states, “tillage practices in Iowa corn production have changed considerably since 1994.” (Owen, personal communication, 2005)
Pimentel claims that “soil erosion is a serious problem in the United States” and states that “agricultural soil is being lost at about 10 times faster than soil reformation and sustainability.” While Pimentel may or may not have accurately cited what is claimed in the 2003 National Academy of Sciences publication, a nearly identical claim by Dr. Pimentel was extensively debated in the pages of Science in 1999 between Dr. Pimentel and soil geomorphologist and erosion specialist Dr. Stanley Trimble of the University of California, Los Angeles. (Science, vol. 286:1477) http://www.sciencemag.org/cgi/content/full/286/5444/1477c
In their 1999 exchange in Science, Pimentel and Skidmore cited a USDA report (3) in which U.S. soil erosion rates were estimated at 13 Mg per hectare per year (13 tons per hectare per year), as well as another paper (4) where erosion rates were estimated at slightly less than 12 Mg per hectare per year. Pimentel and Skidmore then cited Troeh et al. (5) when claiming that “this erosion rate is a factor of 12 higher than soil sustainability, on the basis of the average rate of soil formation.”
Dr. Trimble responded to these claims first by noting that, in fact, the 13 tons per hectare per year figure is not an actual measurement of soil loss, but is an estimate “from models, and they do not predict movement of sediment to streams. If U.S. soils have indeed been eroding at such rates over the last two or so decades, where are the detritus and efflux?”
Trimble further noted that “Troeh et al., on the basis of USDA information, state that the soil-loss tolerances for U.S. soils range from 2.2 to 11.0 Mg ha-1 year-1 (2, p. 115). U.S. agriculture is mostly on soils with a soil-loss tolerance of 11 Mg ha-1 year-1 or more (3, p. 678). Hence, there appears to be little disparity between soil-loss tolerance and what Pimentel and Skidmore say is the rate of erosion. Even according to the USDA study cited by Pimentel and Skidmore, only one-third of U.S. agricultural land is eroding faster than the sustainable rate—a statement that remains to be proven. Although erosion rates may be periodically high in some regions, U.S. soil erosion remains a problem but does not seem to be a crisis.”
In other words, Pimentel’s past claims that agricultural soil is being lost 10+ times faster than soil reformation and sustainability is not supported by the papers he himself cites. It is important to note that this exchange came in response to an extensive, 20+ year physical analysis of actual soil loss for one entire highly-erodible basin in Wisconsin (Coon Creek) conducted by Dr. Trimble and published in Science. (Science, vol. 285:1244-1246, 1999) This exhaustive study found rates of soil loss to be far lower than those estimated by the USDA models cited by Dr. Pimentel and Skidmore. As such, U.S. soil losses are likely well below tolerable soil loss rates and are sustainable.
Moreover, there is simply no denying that genetic engineering has and will make possible even further reductions in soil loss from cropland, far below those possible through the tillage-dependent organic farming propagandized by Dr. Pimentel.
Finally, Dr. Pimentel mistakes our statement that “Humanity already farms more than one-third of the Earth’s total land area” as referring only to cropland, which Pimentel correctly notes is 11% of the earth’s total land area. Farmed land is both cropland and land in pasture and rangeland (26%), making the total estimated farmed area 37% of the total global land area. Accounting for pasture and rangeland is clearly relevant when the primary organic fertilizer is animal manure.
Alex Avery
Tom DeGregori
References:
1. Fernandez-Cornejo, Jorge and William D. McBride. 2004. ‘Adoption and Pesticide Use’, pp. 26-29 in Adoption of Bioengineered Crops By Jorge Fernandez-Cornejo and William D. McBride. ERS/USDA (Economic Research Service, United States Department of Agriculture) Agricultural Economic Report No. AER810. 67 pp, May 2002. http://ers.usda.gov/publications/aer810/aer810h.pdf.
2. WHO (World Health Organization). 1998. Guidelines for Drinking-Water Quality, 2nd edition, Volume 1 – Recommendations – Addendum – Health Criteria and Other Supporting Information, Annex 2. Tables of Guideline Values – Table A 2.2 – Chemicals Not of Health Significance at Concentrations Normally Found in Drinking Water. Geneva: World Health Organization.
3. Summary Report: 1992 National Resource Inventory (USDA, Soil Conservation Service, Washington, DC, 1994).
4. N. D. Uri and J. A. Lewis, J. Sustainable Agric. 14, 63 (1999)
5. F. R. Troeh, J. A. Hobbs, R. L. Donahue, Soil and Water Conservation (Prentice Hall, Upper Saddle, NJ, 1999)
