ISSUE 5   June 7, 2007

BROMOXYNIL ON ESTABLISHED ALFALFA

Examination of the ND weed guide pages 44-45 shows bromoxynil can be used on establishing (seeding) alfalfa but not on established alfalfa. This may be surprising since one might think established alfalfa might be more tolerant. The reasons bromoxynil is registered on seedling alfalfa rather than established alfalfa is as follows:

  1. The risk of detecting residues on established alfalfa is too high and the manufacturer only supports seedling alfalfa.
  2. Occasionally established alfalfa tends to show more crop response which may be a result of established plants having more foliage to express the crop response.

 

2007 HERBICIDE/ADJUVANT PRICES

Because the ND Weed Guide is printed in December before the new herbicide prices are released for the coming season, the prices in the 2007 weed guide still contain the prices for the 2006 year and do not reflect 2007 prices. The herbicide price list has been updated for 2007 and can be found on the web version of the weed guide found at the NDSU Weed Science web site: www.ag.ndsu.edu/weeds. The Herbicide Compendium and price list is part of the Weed Guide. This web address is also listed on the cover of the weed guide.

Herbicide prices do not include cost of such additives as surfactants, oils, fertilizer or application costs. Prices may vary depending on area of the state, wholesaler, bulk discounts, seasonal changes, quantities purchased and particular programs the manufacturing company offers. Prices are averages based on statewide dealer survey for small quantities. Producers should consult local agricultural product suppliers for exact price of each product in their area.

For 2007 summary, most herbicide prices increased the normal 5-15% increase but some did not change much. It seemed that the herbicides that are less used had a slight price decrease and those that are more popular had a slight price increase.

 

EARLY SEASON WEEDS IN CORN

With the increase in corn acres, all corn growers, especially first time or new corn growers should ask this question, "When does weed competition start to affect corn yield potential?" A few bushels lost to weed competition can easily result in a $10/a or $15/a loss or more which is enough money to be concerned about. The answer to the question varies depending on the weed species, their density, and their size but based on years of field trials in the NC region, the rule of thumb is that weeds need to be controlled before they exceed 4 inches in height or by the time corn is about the V3 (3-collar) stage to prevent significant yield loss.

There is some debate on what factor is primarily responsible for cornís yield loss from this early-season competition. Is it competition for water, nutrients, or light? Competition for water as a primary factor certainly seems reasonable in dry springs or on coarse soil, but competition for water seems less important on medium soils in seasons with average rainfall. Competition for nutrients most likely would relate to nitrogen. Do 4-inch tall weeds remove a significant amount of nitrogen to affect corn growth? Probably and certainly from a dense mat of foxtail grass weeds. Scientists at northcentral universities are also investigating this question. The third option is competition for light. Early in the season, it seems unlikely that corn is truly in competition for light because the corn is typically taller than most weeds.

However, another mechanism might be affecting corn growth and competition with weeds. Plants can detect if other plants are growing nearby because the spectrum of light changes. Light reflecting off plants has more far-red light and less red light, so the ratio of red to far-red (R:FR) light decreases. One hypothesis is that corn detects the presence of weeds when the light spectrum changes and then corn growth shifts to more shoot growth at the expense of root growth. Over time this would limit cornís yield potential if true.

Scientists have field tested this idea during the past two summers by measuring corn growth and yield when grown with "normal" light conditions (weed-free corn) and low R:FR light (simulated weed competition). By using extra corn plants to simulate weed competition the light spectrum changed but corn plants in the low R:FR conditions (the simulated weed competition treatment) were taller, had longer leaves, and had less tillers than corn plants in normal light conditions. Hand-harvested corn grain yield was also similar.

Is light quality a critical factor affecting corn growth and a significant component of early-season weedís interaction with corn? Perhaps not. A simple answer would be nice to explain and predict weed competition. However, the interactions between the weeds and the corn are probably more complex and may be driven by a mixture of water, nutrient, and light factors.

 

NDSU CORN MICRO-RATE

Page 20 in the NDSU Weed Guide shows reduced rate options for POST herbicides used in corn. The treatments are called "Micro-rate" in the guide out of convenience and connotation but is not meant to be used as series of sequential application similar to the sugarbeet micro-rate.

Stout is listed as the herbicide of choice but Steadfast, Accent, or Option could be used in the place of Stout. As listed, Stout is Accent plus a small amount of Harmony GT. Stout does not contain rimsulfuron which is in Steadfast. Stout costs $14-$15/A at the full 3/4 oz/A rate, which averages lower than the $24/A cost of Accent at the same equivalent active ingredient rate of 0.5 oz ai/A. Depending on rates of herbicides in the microrate the cost of the mixture could be as low as $16 to $18/A.

It is very important to understand when the corn micro-rate can and cannot be used. One must substitute other management practices to compensate for reduced herbicide rates. (This was a quote from a great weed scientist - Dr. Alan Dexter). Paragraph C4 on page 84 of the weed guide gives some of this information. Reduce rates of Stout, Steadfast, Option should ONLY be used on small green foxtail and wild oat grasses (less than 2 to 3 inches). Do not reduce the rate if the grasses are large, stressed by environment, or if your target grasses are yellow foxtail, wild proso millet, volunteer cereals, field sandbur, or quackgrass. These other grasses require the full labeled rate and you still may not get adequate control. Adjuvant choice is very important. Only use a basic blend at 1% v/v or MSO type adjuvant at 1 to 1.5 pt/A.

Of coarse, rain events may cause other flushes to grass and broadleaf weeds to emerge, which may require re-treatment with registered products.

Richard Zollinger
NDSU Extension Weed Specialist
r.zollinger@ndsu.edu


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