What About Yield Drag on Roundup Ready Soybean?

The adoption of glyphosate resistant (Roundup Ready) soybeans in Ontario has been swift. Approximately 45% of all soybeans grown in Ontario during 2002 were Roundup Ready (RR), with about 80% of US soybeans being RR. Although many growers are pleased with the RR production system, questions remain concerning the yield potential of some RR varieties.

US Research

A number of research trials conducted in the US continue to show a yield reduction of between 5 and 10% when comparing RR varieties to similar non-RR varieties. This is not the case in all regions, but appears to be more significant in the northern US states. For example, during 2001 in Minnesota the average yield of all non-RR varieties tested was 6 bushels per acre (bu/ac) greater than the average of RR varieties. But there was only a 1 bu/ac advantage in Iowa to non-RR varieties and in Illinois RR varieties actually yielded 1 bu/ac more than conventional non-RR varieties. (Gaska & Boerboom, UW, Madison)

What About Ontario RR Soybeans?

Small plot public variety trials conducted each year in Ontario continue to show that non-RR varieties often yield better than their RR counterparts. This is particularly evident for shorter day soybeans. The trend becomes less apparent as you move into the longer day soybean areas (3300-3500 CHU) where more top yielding RR varieties are available. (See Figure 1).

Field scale trials have also shown that non-RR varieties may outperform RR varieties in Ontario. Eleven field scale trials conducted by BASF in 2002 showed that non-RR varieties out-yielded RR varieties. Producers compared the RR and non-RR varieties that best suited their fields and found a 4 bu/ac advantage to non-RR varieties.

Having said that, many producers enjoy good success with the RR production system, especially under no-till conditions. Despite the apparent yield drag associated with many RR varieties, actual bu/ac yields may still be higher with RR under certain field conditions. This could be due to a number of factors, including the improved weed control some producers report with the use of glyphosate. But be aware of the yield potential of the variety you are choosing before assuming that RR is the way to go.

Figure 1. Ontario Oil and Protein Seed Crop Committee Variety Trials (2000-2002). RR varieties are circled

Possible Reasons For The Apparent Yield Drag?

There is disagreement among experts in explaining this apparent yield drag. Part of the answer lies in that the genetic source material used in breeding RR varieties comes from a Group V soybean line which is not well adapted for Ontario. In other words RR cultivars may "lag" behind non-RR cultivars simply because the RR gene was originally inserted into lower yielding and older genetic lines. Plant breeders use these "older lines" to breed the RR gene into newer varieties. When they do this they must overcome some of the negative aspects associated with the genetics of those "older lines". As plant breeding continues these poorer genetics will slowly be removed. This would explain why more longer day soybeans are higher yielding than the shorter day soybeans since more breeding efforts have been put into the longer day varieties to date. Plant breeders feel that they are "catching up" in their breeding programs and that newer RR varieties will continue to improve in yield potential.

Some researchers have suggested however that the yield suppression associated with RR varieties cannot be entirely explained by the differences in cultivar genetics. Some theories have been proposed, but considerable additional research is required before they can be thoroughly validated. A University of Nebraska study demonstrated that when taking sister lines of non-RR and RR lines, the non-RR sister lines yielded 5% more than their RR sisters. (Agronomy Journal. 93:408-412, 2001) This study also showed an overall 10% yield advantage when comparing the highest non-RR varieties over RR varieties. Others have suggested that after the application of glyphosate a short-term depression in aromatic amino acid levels occur in RR plants, which leads to a delayed plant immune response. This would leave the plants more susceptible to common root pathogens. One study has also shown that glyphosate applications on RR soybeans may depress soybean root nodulation and therefore nitrogen fixation. (King, et al., 2001, Agronomy Journal) While RR soybean plants are tolerant to glyphosate, the microorganism that fixes nitrogen in the soybean root, B. japonicum seems to be very sensitive to glyphosate. Having said that, much more research is required before any affects of glyphosate on the plant are understood well enough to assess the possible impacts on soybean yields in the field, if indeed there are any.

Points To Consider

Producers should consider the following points when considering which production system to use:

1. The potential yield of each variety
2. Previous weed pressure and weed control problems in the field
3. The cost of seed, herbicides, and tillage and IP premiums.

More field scale trials are required in Ontario to better understand the economics and yield potential of the two production systems.

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