ISSUE 5 May 30, 2002
WILD OAT - SPRAY NOW OR LATER?
Cool weather has caused heavy infestations of wild oat not only in small grains but most other crops. Wheat growers know that wild oat is one of the most competitive weeds and can quickly cause significant yield reduction if not controlled soon after emergence. The same is true with wild oat in other crops including soybean and especially corn. If you trying to decide whether to spray early to control the wild oat or to wait and spray when the weed flushes occur it is much wiser to spray early and then spray later with a good broadleaf herbicide. Corn maybe just emerging and wild oat size can be anywhere from 1-lf to 4 inches tall. Wild oat at this advanced stage compared to the early stage of corn requires removal to avoid yield loss. In broadleaf crops, many POST grass products are available and some herbicides are available in corn like Accent, Steadfast, and Option. Basis contains Harmony GT and rimsulfuron (Matrix). The Matrix component has good grass activity and can control wild oat. Basis at 0.33 oz/A provides economic weed control at $5.50. Apply Basis with oil petroleum oil adjuvant at 1% v/v + 28% N at to corn only from spike to 4-leaf (2 collar). Do not tankmix with any product containing dicamba and be aware of injury concerns with corn less than 88 day maturity. If wild oat are larger than 3-lf then consider using Steadfast or Accent + MSO + nitrogen source. The recent hot weather may initiate foxtail emergence. By the time foxtail is 1 to 2 inches tall the wild oat may be 4 inches tall or more. The foxtail and wild oat can be controlled with one application of Accent, Steadfast, or Option but should be made soon to avoid yield loss.
COLD WEATHER ON CORN
Corn injury due to the extremely wet and cool weather we have had over the last several weeks have been noted. Frost and hail injury, insect feeding, diseased coleoptile and root systems and herbicide injury may have occurred. It is difficult to assign blame on any one of the individual stresses as the cause of stunted and dead plants. Rather, it is a combination of all stresses at once.
When corn is under stress from factors other than herbicides, it is growing slowly and is not able to adequately metabolize (degrade) the herbicides to non_toxic levels. Since herbicide uptake is by diffusion, stressed plants continue to absorb herbicide, but are not able to metabolize it since their growth is slowed. As a result, buggy_whipping and onion_leafing which can be caused by chloroacetamide herbicides (Dual, Harness, Surpass, Degree, Outlook, Define, Axiom, Lasso, Microtech). Although many would like to
solely blame the soil-applied herbicide as the cause of the unhealthy corn, it is likely that the herbicide injury would not be present if the other stresses were absent. In addition, other stresses are distinctive and symptoms could be differentiated as well.
Below are typical symptoms noticed with various corn herbicides: Corn unfurling underground (and buggy_whipping and onion_leafing) can be caused by cool, wet soils, soil compaction, chloroacetamide (specific products mentioned above), and 2,4_D herbicide injury. In most cases it is likely that the cool, wet weather slowed plant growth and the plant’s ability to metabolize (detoxify) the herbicide. Since most soil_applied herbicide uptake by plants is by diffusion, it is likely that the corn plants continued to absorb herbicide, but were not able to metabolize it and, thus, show injury symptoms. This effect occurs with all chloroacetamide herbicides labeled in corn, and it doesn’t appear that any one product is worse than another. In certain years, more injury may occur with a certain product, but it has not resulted in consistent yield reductions. Variety sensitivity can vary as well. Some seed companies have charts that indicate which hybrids are more sensitive to chloroacetamides. If you routinely use the upper end of the label rate of a chloroacetamide and have a poorly drained soil, it would be beneficial to do visit with your seed dealer determine varieties with more tolerance to chloroacetamides.
EPA APPROVES HERBICIDE TO CONTROL THISTLES IN FLAX FIELDS
EPA approved Section 18 emergency exemption for Curtail® M in flax for control of Canada thistle and perennial sowthistle. The exemption allows a single ground or aerial application of Curtail® M at a rate of 1.33 to 1.75 pints per acre. Applicators must observe all instructions, precautions and warnings on the product label and have a copy of the exemption use directions in their possession during application. A 72-hour pre-harvest interval is mandatory. The exemption, which allows treatment of 50,000 acres of flax, expires July 31.
CORN HERBICIDE UPDATE
Acetochlor 75 (Dow)
Mode of action: Unknown + photosynthetic inhibitor
a.i.: acetochlor + atrazine
Comments: A pre-mix of 5.5 lb/gal acetochlor + 1.5 lb/gal atrazine with use rates from 1.8 to 3 qt/A depending on soil type. Rate of 2 qt/A = 0.75 lb/A of atrazine. Use similar to other atrazine premixes. Atrazine is a restricted use herbicide. Follow rotational crop restrictions.
Mode of action: PPO inhibitor
Crops: Small grains, barley, oat , corn.
Comments: New liquid 2EW formulation available for 2002. Labeled for preplant burndown and fallow prior to planting field corn, soybean and wheat with no planting interval restrictions since these crops are labeled and have established tolerances.
Mode of action: Carotenoid inhibitor - same as Balance
Comments: Excellent corn safety, controls many broadleaf weeds including large common lambsquarters, does not control wild buckwheat, increases yellow foxtail control, and has a crop rotation restriction to mustard crops, flax, drybeans and edible legumes including field pea, potato, and sugarbeet. Add atrazine at 0.25 to 0.38 lb ai/A for improved broadleaf control, including wild buckwheat and ragweed. Maybe tankmixed with Accent, Basis, Basis Gold,and Steadfast for grass and broadleaf weed control. Adjuvants are restricted to a petroleum (COC) oil type. Use of MSO adjuvants are currently prohibited but field research will be conducted in 2002 to evaluate corn safety from various treatments applied with MSO for the possibility to allow use in 2003. High soil pH increases rate of breakdown.
Guardsman Max (BASF)
Mode of action: Unknown + photosynthetic inhibitor
a.i.: active isomer of dimethenamid + atrazine
Comments: Similar to Guardsman but contains Outlook instead of Frontier.
Hornet WDG (Dow)
Mode of action: ALS inhibitor + growth regulator
a.i.: flumetulam + clopyralid-K+
Comments: New K salt of clopyralid formulation contains 25% less ai than SP formulation but use rates will 25% more resulting in no change in ai/A (2 oz WDG = 1.6 oz SP. New formulation changes signal word from Danger to Warning. Several tankmix options allowed.
Mode of action: ALS inhibitor
a.i.: foramsulfuron + safener (35+35 DF)
Comments: An SU herbicide applied POST will control many grass and broadleaf weeds. Short residual with no cropping rotation restriction the following year. Weaknesses in weed control include Polygonum species like wild buckwheat and smartweed. Controls many broadleaf species including nightshade species which is different from most other sulfonylurea herbicides. May be sold in future with iodosulfuron in premix to improve weed control. Safener may reduce or eliminate injury from dicamba/Distinct when applied in tank-mixture. Apply ONLY with an MSO adjuvant at 1.5 pt/A + 28% nitrogen at 1.5 qt/A.
Mode of action: Acetamide (Mode of action unknown)
a.i.: dimethenamid-P (active isomer). 6 lb ai/A
Crops: Same crops as Frontier.
Comments: Use rates are 60% of Frontier rates. Replaces Frontier. Pending federal registration on potato. 2002 ND Section 18 approved for sugarbeet.
Mode of action: ALS inhibitor
a.i.: nicosulfuron + rimsulfuron
Comments: Use on corn varieties of 77 to 88 CRM has not been allowed. However, DuPont now allows use of Steadfast, Accent Gold, and Basis Gold on corn that is no more than 12 inches tall, or less than or equal to 5-collar corn, which ever is most restrictive. Use caution with these products on short season corn varieties. Follow crop rotation restrictions.
Mode of action: ALS inhibitor + plant growth regulator
a.i.: halosulfuron 12.5% + dicamba 50%
Comments: Use rate of 4 oz/A provided equivalent rate of 0.67 oz/A of Permit + 4 fl oz/A of Banvel. Apply POST from emergence through 36 inch tall corn. Add NIS at 0.5% v/v or petroleum oil adjuvants at 1% v/v. Nitrogen fertilizer can be added if required. Rainfast within 4 hours. Permit component controls some large-seeded broadleaf weeds AND is very effective on yellow nutsedge.
NDSU Extension Weed Specialist