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Buy Into Weed Biology

Is spraying herbicide "A" and "B" your best attempt to arrest weeds in corn, with perhaps a cultivation pass if time allows? Or have you honed an integrated strategy that matches weed control programs - featuring cultural, mechanical and chemical tactics - with such critical factors as weed spectrum, soil characteristics, hybrid selection, tillage system, crop rotation, application technique, previous herbicide use and more?

Think about the $2,000 you spend per 100 acres, minimum, to stop many but not all weeds in corn. Are you or your dealer controlling these input dollars? By taking an integrated strategic approach you can make those input dollars work harder, not only for this year's crop but in the future, too. Oh sure, managing weeds like a crop takes work, but you don't have to tackle it all yourself. An astute co-op or dealer's agronomist, or extension weed specialist, or paid crop consultant/farm manager can help you establish a game plan. If you become a student of weed biology, it can pay dividends besides saving input dollars.

Chemical dependent. In a new Iowa State University/Leopold Center survey, 1,000 Iowa farmers reported that they do not use alternative weed management strategies frequently, and, when they do use them, they are not as effective as herbicides. Crop-protection chemicals are the primary strategy in growing both corn and soybeans: 76% of respondents cited that herbicides were "very important" for corn, while the value was 84% for soybeans. But when asked about effectiveness, the farmers assessed herbicides as "very effective" 51% of the time in corn and 60% in soybeans.

Mike Owen, Iowa State professor of agronomy and weed science and team member involved with this study, believes the above number differences likely reflect differences in planting time, herbicides available and the high use of post applications in soybeans.

When asked how steel ranks as a weed-control tool, 36% of the growers rated cultivation as a "very important" weed management strategy in producing corn (30% for soybeans), but then rated it "very effective" only 28% of the time in corn (24% for soybeans).

Owen says these effectiveness ratings are much lower than current research suggests. "In fact, cultivation is highly effective and consistent, when done in a timely fashion, and can play an important part of an alternative weed management strategy," he says. "Same goes for a rotary hoe, one of the least-used weed control tools out there. When used early with consideration given toward emergence, it can work extremely well."

"Marketing has created high expectations for herbicides, and performance guarantees have resulted in diminished management by growers," Owen says. "That's why the survey showed that unplanned post re-sprays were used on 16% of corn acres and 19% of soybean acres. It's mostly due to poor planning by growers, or by the source who supplies them with information or application. Hence the need for weed knowledge."

Weed biology tool. Farmers should know which weed species populate their fields, but scientists are taking this a step further by stating that growers should learn when and for how long specific weeds emerge, and what percentage of weed seeds actually germinate.

One tool that takes weed biology and forecasts seedling emergence and growth of some common annual weeds is called WeedCast. Frank Forcella, weed researcher at the USDA/ARS North Central Soil Conservation Research Lab in Morris, MN, developed this Windows-based software with Cheryl Reese and help from many Midwest weed scientists.

"The easy-to-use software was developed using actual Midwest growing conditions (soil temperature and moisture) and actual weed germination results," Forcella says.

For each field, you simply enter soil type, water capacity, weed species, and the previous crop and tillage practice. Then each day you enter the date, high and low temperature and rainfall. "This information will give you fairly accurate predictions on weed seedling emergence timing, weed growth and weed seed emergence potential by species," Forcella says.

"Then by examining weed growth predictions, a grower can properly time his rotary hoe or cultivation operation to control the most weeds, determine when to make a preemergence or postemergence application, and match the appropriate herbicide to the existing weed problems," he says.

If you have Internet access, you can download WeedCast from the USDA/ARS Web site at Once there, click on "user products," then on "weed ecology and management," then on "download WeedCast version." For more information, contact Frank Forcella at 320/589-3411.

The Leopold Center at Iowa State also is sponsoring research to develop a tool that will improve weed management decision making.

Bob Hartzler, extension weed specialist at Iowa State, working with Doug Buhler, a USDA/ARS researcher, initiated their weed emergence research in 1995 with four problem weeds (giant foxtail, woolly cupgrass, velvetleaf and waterhemp) and have since expanded the study to include 26 weed species.

While the study is several years from completion, results to date show broad differences among species in emergence timing and scope (see chart), which is a huge reason why weed management is such a difficult task. It doesn't take more than a quick glance to understand why waterhemp (aside from control of ALS resistant species) is defying many control efforts, considering its late period of maximum emergence. "That's why single-pass weed management is a myth and alternative strategies are a must," Owen says.

Chemical components. Keeping weed biology and alternative weed management strategies in mind, we shift gears to examine what's new on the chemical side of the weed-control equation.

New corn products for 1998 (some pending EPA approval) include Accent Gold, Aim and Spirit in the postemergence category; Balance, Field Master, Python and Axiom in the soil-applied segment; and the new transgenic Roundup Ready (RR) corn.

DuPont adds Accent Gold to its lineup "in response to farmers' desire for a post grass and broadleaf product that offers contact and residual control in one shot," says Ron Stohler, communications manager for the company. It contains the same ingredients as Basis Gold, but the atrazine is replaced with the active ingredient also found in Hornet.

Aim, from FMC, is a narrow-spectrum, contact broadleaf product from the Authority family that can boost control of velvetleaf, lambsquarter and some pigweed and morningglory species when used as a tank-mix partner.

Novartis offers a post-broadleaf cousin to Exceed called Spirit. Spirit contains the same ingredients as Exceed, only in different ratios to reduce the carryover concerns that has plagued Exceed, while increasing grass control. The company now has strict use restrictions to combat follow-crop injury (see chart).

The only other post product news is that Valent's Resource will have new tankmix options such as Roundup for RR corn, Clarity, Buctril, Exceed, Laddock and Stinger. And growers should watch summer trials for an experimental product called Distinct from BASF, which contains dicamba and a synergist said to be unique, the company claims.

Soil-applied chemistry. Balance from Rhone-Poulenc is a new, low-dose soil-applied chemistry that has a good fit on tough weeds like velvetleaf, waterhemp, woolly cupgrass and wild proso millet, but may need help on certain grasses. It also has a unique aspect - its herbicidal activity can be recharged after a rain, the company claims.

Monsanto has formulated a new three-way premix containing the active ingredients in Harness, Roundup and atrazine called Field Master. It's claimed to be a replacement for Extrazine.

Python from Dow AgroSciences (was DowElanco) is actually Broadstrike, but now it's being marketed alone as a narrow-spectrum broadleaf tankmix component, not premixed with Dual or Treflan.

Bayer's new Axiom, which contains some new chemistry and some of Sencor's active ingredient, is a soil-applied grass herbicide with some control of small-seeded broadleaves. But it may need tankmix help on certain tough grasses and broadleaves.

The other news with soil-applied chemistry is that Zeneca's Surpass EC, 20G and TopNotch can now be fall applied in Iowa (north of Hwy. 30), Minnesota and North and South Dakota.

Herbicide-resistant seed. Big news here is the launch of Roundup Ready corn and the reduced price of Liberty herbicide.

Dekalb will be the only supplier of RR corn this year, offering six hybrids in limited supply, all carrying a $18/bag technology fee. The grower agreement will be similar to RR soybeans, with one big exception. Since both the European Union and Japan have yet to approve the import of RR corn grain, growers may be responsible for finding their own domestic market for the half to one million acres of this new crop.

As for controlling weeds, Monsanto recommends a two-pass program of putting a preemergence herbicide down followed by Roundup, due to the need for early residual weed control. Monsanto is giving a $6.50/acre rebate to growers who use Harness, Harness Xtra or other qualified products on RR corn.

AgrEvo, in obvious response to this new competitor, cut the price of Liberty herbicide by $2.50 to $4/acre (depending on rate used), which is applied to Liberty Link or other warranted hybrids (such as all Novartis Bt hybrids with YieldGard).

Adding complexity to the use of Liberty is Dekalb, which produces glufosinate-resistant (GR) corn that also tolerates Liberty applications. However, AgrEvo won't support Liberty's use on GR corn. "If growers have a complaint with Liberty use on GR corn, then they'll have to contact Dekalb because we can't stand behind our product without a contractual agreement with DeKalb," says Jeff Springsteen, AgrEvo's market manager for corn. "And we don't agree with Dekalb charging a $12 per bag technology."

Dekalb's Larry Mix, marketing manager for agronomic trait products, says it will stand behind the use of Liberty on GR corn from both a crop safety and weed control efficacy standpoint. "We are confident in our technology," Mix says.

The accompanying four-page chart (see printed article) has more details on these and other products. Note that we've added "site of action" for each herbicide. This is important when determining a weed-resistance strategy. Many experts say to avoid using a product with the same site of action more than two years in a row. This is critically important for ALS herbicides, but not for a herbicide like Roundup. More on this in an upcoming issue.

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