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Details Available on Roundup Ready Cotton

Crop and Soil Environmental News, March 1997

Charles Hagedorn
Extension Biotechnology Specialist

As reported in the February 1997 issue, Roundup Ready (RR) cotton will be available this summer. Monsanto owns this technology (called Bollgard), and has licensed it to several cotton seed companies. RR-cotton is resistant to vegetative injury from Roundup (glyphosate) herbicide, but subtle effects on reproductive development may occur if Roundup is applied beyond the four-leaf stage. Seed of RR-cotton will be in limited supply, and there will probably be sufficient seed for only about 10,000 acres. Growers who do not already have RR-cotton contracts by now will be hard pressed to find any seed. There will also be a per-bag technology fee of $32 per acre (that goes to Monsanto) plus a few additional dollars per bag of seed that goes to the company that produced the seed.

The following material was taken mainly from RR-cotton experiments at Jackson, TN, in 1995 and 1996, that were conducted to determine the effect of Roundup applied over-the-top of RR-cotton.

In 1995, Roundup was applied at three pints per acre to Coker 312 RR, Paymaster lines PM 1215RR, PM 1220RR, PM 1244 RR, PM 1330RR and PM 1380RR at four, five, and seven leaf stages. No visual injury to any of these lines was observed from any application. The percent of first harvest was similar from all cultivars and application timings except Coker 312RR, which was 14 percent lower with roundup application at the seven leaf stage compared to the untreated hand-weeded control. Coker 312RR and PM 13330RR had 21 and 7 percent lower total lint yields, respectively, with applications at the seven-leaf stage, PM 1380 had 11 percent lower yield after application at the five-leaf stage, but not at seven-leaf when compared with the untreated hand-weeded control.

In 1996, a single application of Roundup Ultra, also a glyphosate, at 1.5 pints per acre was made to weed-free Coker 312RR cotton at four, six, eight, 10 and 12 leaf stages. Sequential applications were also made at the six, eight and 10 leaf stages following treatment at the four leaf stage. No visual injury was noted. The percent of first harvest was reduced four percent by Roundup Ultra applied at the eight or 10 leaf stage. Total lint yield from any treatment did not differ from the untreated hand- weeded control. In a similar study with at least 10 common cocklebur plants per square foot, Roundup Ultra applied at the six leaf stage, or sequential applications at the six, eight or 10 leaf stage following application at the four leaf stage produced lint yields equal to the untreated hand-weeded check.

Treatment only at the four leaf stage permitted cockleburs to emerge after treatment and reduce lint yield by 87 percent. Cotton was unable to compensate later in the season for cocklebur competition when treatment was delayed beyond the six leaf stage. Roundup Ultra controlled cocklebur plants that emerged at the time of application. Based on these studies under optimum conditions for herbicide activity and good cotton production, the post over-top application window for Roundup Ultra on Roundup Ready may be wider than reported, especially in situations where the loss from weeds may exceed the loss from Roundup effects on reproductive development. Research to better define the injury:benefit ratio for Roundup Ultra applied to Roundup Ready later than the four leaf stage is planned for 1997.

These results are indicative of the management questions that still need to be answered with many of the new transgenic crops where rapid commercialization has allowed the technology to emerge ahead of research to determine the best ways to use the technology. For example, one Paymaster line that contains both RR and Bt genes will be available (in very limited quantities) this year - a full year ahead of the schedule originally predicted by the seed company. Little university-sponsored field research has yet been conducted on these varieties where different transgenic traits have been ństackedî or added together. Growers who hope to use RR-cotton, or any of the other transgenic varieties this year, should carefully evaluate their plans and be realistic about their actual needs for these materials.

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