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Corn+Soybean Digest

Cost-Effective Weed Control

Let's face it, the projected price picture for soybeans this year is downright gloomy. That's why it's extremely important you look at every input dollar in producing this year's soybean crop.

And since weed control is the major pest management expense for Southern soybeans, you need to assess the situation for each field and attempt to use the most cost-effective control system available.

As we near planting, soybean farmers are making up their minds about which varieties to plant. Last year, 20 million acres were planted to Roundup Ready (RR) soybean varieties across the U.S. The prediction for '99 is for about another 15 million acres. Besides RR, sulfonylurea-tolerant soybean (STS) varieties will be available in the Southern region, and Liberty Link varieties may also be marketed for the first time in the Midwest.

The question remains: How can herbicide resistance best be utilized for maximum economic benefit in '99?

When selecting the best varieties, there are several key points to consider.

1 Don't Underemphasize Disease Resistance. First, consider nematode and disease resistance. In the Southeastern Coastal Plain, for example, root-knot, cyst and lance nematodes are all prevalent and can cause problems in many fields. In the Midsouth, phytophthora is a significant threat, while throughout the South resistance to stem canker should be addressed.

Most herbicide-resistant varieties have resistance to race 3 of soybean cyst nematode, while a lesser number have resistance to both races 3 and 14. The problem for Southern farmers is that few herbicide-resistant varieties are resistant to root-knot or lance nematodes. They are potentially very dangerous, especially in the major cotton growing areas of the Southeast.

A farmer in these areas who does not sample and assumes that root-knot is not going to be a problem could be courting disaster.

For this reason, it's suggested that soil assay samples in each field be taken around harvesttime. That way, you can assess the nematode situation when selecting varieties in winter or spring - whether herbicide-resistant or conventional.

What if nematode samples were not taken in the fall of '98? That's where a knowledge of field history - including previous crop rotation schemes, pests and other problems, and overall yield records - are valuable.

Chances are slim that root-knot and/or lance will be a problem if good cotton and corn yields have been obtained previous to considering a field for soybeans in '99. If you're in doubt, get varieties with the best disease package available.

2 Know Your Weeds. The second most important point to consider when buying herbicide-resistant varieties is the weed spectrum for each field.

In the South, RR varieties have been a lifesaver for farmers with weed problems such as sicklepod, pigweed and cocklebur. Some farmers, for example, have been very satisfied with only one application of Roundup Ultra on drilled RR beans.

For other broadleaf weeds, like morningglories, farmers have found the RR system only a partial answer. For success with morningglories, a tankmix of Roundup Ultra and a herbicide like either Classic or Blazer has proved effective.

In addition, STS varieties, which possess tolerance to Synchrony or higher-than-labelled rates of Classic and Pinnacle, seem to be gaining favor with many farmers. They've observed outstanding control of many broadleaf weed species at fewer overall costs than the RR system.

Some have reported that using an STS variety, along with a good pre-emergence herbicide for annual grass and small-seeded broadleaf control, then one application of a sulfonylurea post herbicide, does the trick. And it's all done at less than $20/acre. The key is to know what weeds are present and decide if one of the herbicide-resistant systems or a conventional system would be most cost-effective.

3 Don't Forget Yield. Yield potential is the third consideration when selecting varieties for the South, herbicide-resistant or not. Varieties are truly different genetically for yield, and every state has a testing program in which unbiased estimates of yield potential and other traits are measured.

Almost every Southern state has instituted a separate RR test for each maturity group tested. These results are available from county extension offices, as well as on the Web. Check out these yield results.

4 Evaluate Costs. Finally, the cost factor for buying herbicide-resistance technology must be evaluated, but only after the above three points have been considered.

For example, if there are no significant disease or nematode problems, or weeds for which Roundup may have some weakness, then the RR system may be the most cost-effective option. If root-knot or lance nematode is a pest, and if weeds like morningglory or hemp sesbania are prevalent, then an STS or conventional variety/herbicide system may be more profitable.

Farmers who make the best yields and profits in '99 will be the ones who thoroughly analyze the needs of each field and then select varieties and weed management systems that meet those needs.

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