Roundup Ready Soya and Yield-Drag 

Doug Powell 12nov99

Benbrook (1999) claims that Roundup Ready (RR) soybean yield drag was evident in numerous 1998 field trials as summarized by Oplinger et al. (1999). Oplinger and co-workers collected yield data from 8,200 soybean performance trials conducted at universities in eight of the northern United States. The data indicated that average yields of RR soybean varieties ranged from 14 per cent less to 13 per cent more than conventional varieties. Averages of all tests indicated a 4 per cent decrease in yield of RR soybeans compared to conventional varieties.

Given the magnitude of the study, the overall trend may seem significant. However, the data would be more meaningful if Oplinger et al. had included statistical analyses indicating whether RR soybean yields were significantly different from their conventional counterparts. In addition, the soybean varieties evaluated were grown under varied environmental and production conditions. Benbrook (1999) cites interactions of weather, tillage methods, soil conditions, herbicides, insecticides, and surfactants as affecting soybean yield. Weeds are unpredictable and species that limit crop yields can change from year to year. Rose (1998) writes that weed species, row spacing, and the timing and application of herbicides can affect weed competition, and therefore yield, in soybeans. These different production conditions necessarily affect soybean yields. Furthermore, the data presented by Oplinger et al. (1999) is for a single growing season, which may not have been representative of the norm. Until multiple season data is collected and analysed, it remains difficult to accurately determine the significance, if any, of yield drag for RR soybeans.

There are numerous advantages to using RR soybeans. Glyphosate, the active ingredient in Roundup, is known for its low toxicity and has been used for nearly 30 years without any harm to beneficial insect populations. Honeybees are not harmed by Roundup, as show in studies mandated by the U.S. Environmental Protection Agency and the World Health Organization. The International Organization for Biological Control deemed Roundup less harmful than various other herbicides. Additional studies also indicate glyphosate does not harm earthworms, beetles and other insects. Glyphosate also biodegrades easily, breaking down into naturally occurring substances relatively quickly compared to other herbicides. Contamination of groundwater and field run-off is unlikely as the herbicide binds to soil particles.

Various media accounts contain allegations that more herbicide is required to treat RR soybeans than that required for conventional crops. Comparisons between herbicides should consider the amount of active ingredient used per acre, not the total amount of herbicide per acre, as well as toxicity and persistence in the environment. While newer, low-dose materials or the use of STS soybeans (soybeans resistant to sufonylurea) can reduce herbicide use to less than one-tenth of a pound of active ingredient per acre, in contrast to 0.75 or 1.5 pounds per acre of Roundup (Benbrook, 1999), these other herbicides, including sulfonylurea, can persist in the environment with the potential for deleterious consequences. Decisions to use herbicides should not be based solely on quantity of herbicide used, but should also address potential environmental effects and impacts on insect and animal populations.

Benbrook (1999) indicates that RR soybeans may out-perform conventional soybeans. This may be explained by more effective weed control or less crop injury from herbicides and weed management practices. Other herbicides, such as sulfonylurea and imidazolinone, can stunt soybean growth, cause plant damage, and decrease yields by impairing plants' immune processes and lessening the availability of minerals such as phosphorus.

In Canada, the first RR soybeans grown were known to be lower-yielding than conventional varieties. Nevertheless, the soybeans were registered because the RR trait was seen as advantageous for various other agronomic and environmental reasons.

Backcrossing the RR trait with better yielding varieties should be addressed. Oplinger et al. (1999) predict "soybean growers will continue to increase acres planted to RR varieties and will sacrifice maximum yield for ease of weed control." Cost-benefit analyses may indicate that using RR soybeans is still advantageous to farmers, regardless of the potential for yield drag.

• References Benbrook, C. 1999. Evidence of the magnitude and consequences of the Roundup Ready soybean yield drag from university-based varietal trials in 1998. Benbrook Consulting Services, July 13, 1999. RR-Soya-Yield-Drag.htm
• Oplinger, E. S., Martinka, M.J., and K. A. Schmitz. 1999.Performance of Transgenic Soybeans in the Northern U.S. Accessible in Adobe Acrobat format at http://www.biotech-info.net/herbicide-tolerance.html#soy .
• Rose, F. 1998. One herbicide application in wet year doesn't eliminate weeds in soybeans. Extension and Ag Information, University of Missouri-Columbia, September 10.