Roundup Ready (RR) soybeans have turned in some whopper yield performances, like topping 90 bu/acre in one Ohio State University variety performance test in '98.

But extensive research data now strongly suggest there is, on average, a small yield lag with this new technology.

Data from more than 3,000 side-by-side comparisons from 40 university performance tests conducted across eight states in 1998 were summarized by Ed Oplinger, University of Wisconsin extension agronomist.

Here are the results:

* Average yields of RR varieties ranged from 14% less to 13% more than conventional varieties.

* When averaged across all tests, RR varieties were 4% lower in yield than conventional varieties. That would be a 2-bu/acre difference on a 50-bu/acre yield.

* When averaged across all locations, the top five RR varieties yielded 5% less than the top five conventional varieties in 200 comparisons.

Harry Minor, a University of Missouri extension agronomist, made similar comparisons in his state and came up with about the same yield lag - 1.6 bu/acre. In some comparisons he made of Group V beans from Arkansas and Mississippi, there was about a 5% yield lag with RR varieties.

Oplinger adds that another University of Wisconsin study used so-called sister lines - very similar genetics with the RR gene in one of the matched pairs. The yield difference in favor of the conventional varieties was 4-6%.

This yield lag factor for RR soybeans has been mentioned by many scientists and top-end growers who made side-by-side comparisons since 1996, when these transgenic varieties were first commercialized. And it was mentioned in each of Soybean Digest's Special Reports on Herbicide-Tolerant Crops since 1996.

Oplinger and Minor think this potential yield difference is fully realized by scientists and most progressive growers with yield monitors, especially if they have side-by-side comparisons.

"In the industry, breeders know that they don't have yield parity with Roundup Ready varieties yet," Minor declares.

The most heard explanation for the yield lag is that the RR gene often has not been put into the most elite lines that some companies offer. A more plausible explanation, say some soybean breeders, is that in the rush to get RR lines on the market, many companies have not made enough backcrosses to capture all of the yield potential in the parent lines.

"The breeders have been caught with trying to turn things out as quickly as reasonably possible to stay competitive," says Oplinger.

In either case, the conventional wisdom says that will be corrected in a reasonably short time, and yield lags will be eliminated.

"I don't see any reason not to have yield parity in the future with this technology," says Minor.

Oplinger and Minor don't think average soybean growers realize or believe that there is a yield lag, on average, for RR varieties. Or if the growers do, they do not care because of the ease and effectiveness of the weed control they've gotten.

Oplinger points out that about a million acres of RR beans were grown in 1996. That increased to 9 million in '97 and over 25 million acres in '98.

"Therefore, if performance is to be measured by grower acceptance, then RR soybean varieties have certainly performed well," says Oplinger. "Also, the use of these varieties has solved some major weed problems for some growers."

Veteran weed scientists can't recall any herbicide product launch that created the excitement and acreage explosion that RR soybeans created. And grower surveys by Monsanto continue to show extremely high grower satisfaction. Postharvest surveys of RR soybean growers showed that nearly nine of 10 plan to grow some RR soybeans in 1999. With hundreds of new RR varieties available, Monsanto officials expect RR acreage to increase again this year.

In university variety performance trials, weed control has to be virtually perfect in order to measure true genetic yield potential of varieties, Minor points out. For average growers, weed control on larger acreages is not always that good, especially if weather problems occur.

"A farmer may lose the potential yield advantage of good conventional varieties if he doesn't have real good weed control. And if he has weeds he has not been able to effectively control with conventional herbicide programs, then he should give the Roundup Ready system serious consideration. That's because he is likely to lose more due to poor weed control than to the possible yield lag with the Roundup Ready system."

Oplinger urges growers to take a pencil to all the factors, including the yield lag, herbicide costs, technology fee, etc. Then figure out any bottom-line differences for their operations.

So, what's the take-home message of all of this?

If you are going to grow RR soybeans, study the variety performance tests from your state university, private companies or farms to try to pick the highest-yielding RR varieties with the defensive packages needed for your farm. If you don't, you could be sacrificing maximum yield for ease of weed control.

"We have identified some Roundup Ready varieties, both from a yield and defensive package stand point, that growers definitely ought to consider," Oplinger concludes. "There is information out there to pick those varieties that best fit your needs - and your bottom line."