Roundup Ready soybeans make it easier to control weeds and keep fields clean, but there's a yield trade-off.

Roundup Ready soybeans contain a gene that prevents damage from Roundup Ultra, the most popular non-selective, glyphosate-based herbicide. Roundup kills conventional soybeans. With Roundup Ready soybeans, farmers simply spray Roundup Ultra over beans and weeds alike to kill everything but the soybeans. This strategy is particularly appealing for soybeans where weed management can be tricky because most broadleaf herbicides wipe out the soybeans along with weeds.

A two-year University of Nebraska study found that Roundup Ready soybeans yield 6 percent less than their closest conventional relatives and 11 percent less than high-yielding conventional varieties. This averages to three fewer bushels per acre or about 480 fewer bushels on a 160-acre field.

Agronomist Roger Elmore, who headed this study, said the research was initiated after producers began asking yield-related questions about the new soybeans in 1997, about the same time early test results from Nebraska and other state universities were released.

"Preliminary studies indicated something was going on," Elmore recalled.

Going into this research, Institute of Agriculture and Natural Resources scientists knew there were two possible causes for the Roundup Ready yield penalty: either spraying with Roundup or the gene insertion process. Their studies showed the herbicide itself had no effect.

Elmore and IANR colleagues then focused on the effects of the gene insertion process. They conducted dryland and irrigated field studies in weed-free situations at North Platte, Clay Center, Lincoln and Concord. They compared five Roundup Ready cultivars, their closest conventional relatives, called sister lines, and high-yielding conventional cultivars. In this study, researchers controlled weeds with conventional herbicides and by hand. Roundup wasn't used so scientists could compare yields without the variable of Roundup application complicating results, Elmore said.

The high-yielding conventional soybean lines yielded 58 bushels per acre; non-Roundup Ready sister lines, 55 bushels per acre; and the Roundup Ready sisters, 52 bushels per acre. This research showed that Roundup Ready soybeans' lower yields stem in part from the gene insertion process used to create the glyphosate-resistant seed, a scenario called yield drag. The types of soybeans into which the gene is inserted account for the rest of the yield penalty, or yield lag.

Despite lower yields and more expensive seed, Elmore predicts producers will continue planting Roundup Ready as well as conventional soybeans. That's partly because weed management can be challenging. The Roundup Ready system simplifies weed control, and Roundup Ultra is cheaper than most conventional herbicides.

"Farmers are willing to pay some penalty for the better weed control," he said.

This research helps producers decide whether to plant conventional or Roundup Ready soybeans and to weigh the trade-offs, Elmore said. Roundup Ready seed costs more and yields less, but fields can be practically weed-free.

Roundup Ready soybeans have become increasingly popular since their introduction in 1996. That year, 7 percent of soybeans planted in the United States were Roundup Ready, compared to 57 percent in 1999.

From an economic standpoint, Elmore said, "if you can control weeds with a conventional herbicide, you're probably better off to go with conventional soybeans. However, if weed control is a problem, Roundup Ready soybeans are perhaps the better option."

The Nebraska research provided scientific answers relatively quickly to questions by producers.

"Two years is fast for this kind of work, Elmore said. This project demonstrates the importance of a land-grant university responding to a pressing local need for research-based information.

The Nebraska Soybean Board helped fund this research. Other investigators were Fred Roeth, Bob Klein, Stevan Knezevic, Alex Martin, Lenis Nelson and Charles Shapiro.

– Cheryl Alberts

Agronomist Roger Elmore holds a handful of Roundup Ready soybean seed as a soybean test plot is planted at NU's South Central Research and Extension Center near Clay Center. A two-year study by Elmore and other IANR scientists found that Roundup Ready soybeans yield less than their closest conventional relatives and high-yielding conventional varieties.

Soybean pods beginning to fill out.

Ralph Klein, ag research technician, drives the tractor as Jenna Fitzke (left), a student worker, and Jim Pavelka, ag research technician, feed soybean seeds into the planter. Seed packets are stacked in front of them. They open individual packets and empty them into the planter. The special attention is necessary to assure plots are properly planted.