NDSU Crop and Pest Report
Weeds


ISSUE 4  May 22, 2003

 

2003 PULSE CROP HERBICIDE OPTIONS

Field pea, lentil, and chickpea generally are poor competitors with weeds. Weeds should be controlled early (by two to three weeks after crop emergence) to minimize risk of reduced pulse crop seed yield and quality. Use integrated weed management strategies including cultural, mechanical, and chemical options.

The following list displays the currently-labeled (May 2003) herbicides available for use in pulse crops.

Herbicides labeled for field pea:

Herbicides labeled for lentil:

Herbicides labeled for chickpea:

Significant herbicide label changes for field pea:

C Pursuit – labeled for soil and POST application. Apply at the rate 0.72 oz/A of Pursuit DG for annual grass and broadleaf weed control. Apply PPI within one week before planting or PRE up to three days after planting. Pursuit can be tank-mixed with registered soil-applied grass herbicides to increase weed control. Crop rotation restrictions exist.

Refer to herbicide labels and the 2003 ND Weed Control Guide for details on herbicide use and crop rotation restrictions for the numerous herbicides that can be applied either before or after pulses. Get answers to your specific pulse weed management questions by contacting NDSU crop specialists or extension agents.

Gregory Endres
NDSU Extension Area Agronomist, Carrington
Brian Jenks, NDSU Weed Scientist, Minot

 

SPARTAN IN SUNFLOWER

Spartan (sulfentrazone) is registered in sunflower through a 2003 Section 18 emergency registration. Spartan applied PRE at 2.67 to 5.33 oz WDG/A controls most annual small-seeded broadleaf weeds, such as kochia, pigweed species, lambsquarters, nightshade, smartweed, Russian thistle and biennial wormwood and may suppress buckwheat, mustard, ragweed, Russian thistle, and marshelder. Spartan may provide some grass but no perennial weed control. Rate should be adjusted for soil type, organic matter, and especially soil pH. Use higher rates if applied up to 30 days prior to planting. Sunflower has good tolerance to Spartan on medium to fine textured soils with organic matter above 3%. Crop injury may occur on soils with low organic matter and soil pH greater than 7.5, especially on calcareous outcropping. Do not use on coarse textured soils with less than 1% organic matter. Close furrow at planting to avoid injury. Poor growing conditions at and following sunflower emergence, cold temperatures, soil compaction, or rate too high based on soil type and organic matter may result in sunflower injury.

NDSU research has shown excellent weed control in many different environments throughout the Great Plains region. However, consistent control of sensitive broadleaf weeds and control of grass and marginally susceptible broadleaf weeds greatly depends on at least 0.75 inch rainfall shortly after application and before weeds emerge. Dry conditions across ND in 2002 was a good example of the rain requirement necessary to activate Spartan. Spartan is a PPO inhibitor mode of action herbicide, and except for a biotype of waterhemp in Kansas, no weed resistance has been documented.

 

DICAMBA APPLIED PRE FOR BROADLEAF WEED CONTROL IN CORN

In an attempt to kill early emerging broadleaf weeds in corn without high cost, some have considered dicamba applied preemergence (PRE) to the corn. People have asked why dicamba is not listed in the ND Weed Guide for PRE application in corn when herbicide labels containing dicamba allow such use. Dicamba applied PRE in corn combined with any condition, including cold and wet, that slows or reduces corn emergence has the potential to injury corn, including stand loss. Depending on length of stress, the corn may not totally recover. Once corn has emerged (spike leaf) corn tolerance increases and risk of injury is greatly reduced. The ND Weed Guide carries recommendation from the NDSU Extension Service and we recommend dicamba be used postemergence in corn.

 

WILD OAT CONTROL IN CORN

Reports indicate that the cool, wet conditions has resulted in an enormous crop of wild oat. Wild oat is a very competitive weed in crops, including corn, and must be removed early to avoid yield loss. Competition studies in MN and other states show weeds must be removed before 4 inches tall to avoid yield loss in corn. This is assuming that weeds were controlled at or prior to planting. Of coarse, heavy weed densities may require earlier removal. The 4 inch weed stage would correspond to V-3 to V-4 stage corn. Why are corn fields not sprayed at this 4 inch stage? When most of the weeds are 4 inches tall many are still emerging and growers may tend to delay application until most weeds have emerged. Also, with the large farm in ND, can all fields be treated in this narrow application range and what about delays from rain and wind?

What are some postemergence control options?

Obviously, an early application of glyphosate in Roundup Ready corn is a no-brainer but what about conventional corn?

Basis at 1/3 oz DF/A (~$5.50/A) can be used on corn up to 2-collar and weeds that are 1 to 2 inches. However, Basis cannot be used on corn varieties less than 88 days maturity.

Though not recommended on the label but NOT illegal, Accent at a reduced rate of 1/4 to 1/3 oz DF/A (~$8 to $10.50/A) has controlled wild oat in NDSU trials. Drought stress, and cool or cold conditions at application can substantially reduce control, but with the rain and warm temperatures expected control should be adequate. One may ask the question, "With Accent at $21.00 at the labeled rate of 2/3 oz DF/A and Steadfast at $14.00 at the labeled rate of 3/4 oz DF/A, why not use a reduced rate of Steadfast? Steadfast label allows only ONE application in corn. If Steadfast is used for an early application the label restricts a later application. Accent has no variety maturity restrictions and multiple applications may be made not exceed 1.33 oz DF/A. NDSU research has shown that greatest weed control occurs when Accent is applied with a methylated seed oil (MSO type) adjuvant which significantly enhances control over adjuvant types when herbicides are applied reduced rates.

Wild oat is very susceptible to Option and reduced rates may be used, though not recommended but not prohibited on the label. NDSU has no research on wild oat control from Option at reduced rates but if wild oat is as susceptible to Option as Accent then rate reduction of 2/3 to ˝ rate may provide adequate control to small wild oat (1 to 2-leaf). The labeled rate of Option is 1.5 oz DF/. The reduced rate would 0.5 to 0.75 oz DF/A. Option has no variety maturity restrictions and multiple applications may be made not exceed 3.5 oz DF/A. Option must be applied with an MSO type adjuvant plus nitrogen fertilizer at 1 qt/A + 1 qt/A .

 

CRITICAL PERIODS OF WEED COMPETITION

Cool, wet conditions have delayed crop emergence and only slightly slowed weed emergence. A brief review of length of time weeds can compete with crops before yields are affected is in order. Weeds that emerge prior to crop emergence are the most competitive with crops followed by weeds that emerge with the crop. Weeds that emerge 2 to 3 weeks after crop emergence in many case do not compete and lower yield but do add to the seed rain and increase weed levels in the soil.

Yield loss associated with weeds is influenced by how long the weeds remain in the field. The critical period of weed interference is defined as the length of time weeds compete with crops before yields are affected. Weeds must be removed before the end of the critical period.

The decision of when to apply POST herbicides was often dictated by the herbicide’s effectiveness, rather than the critical period. The effectiveness of most herbicides available today allow application long after the critical period. The following table describes the effect of various factors on critical period for early season competition.

Factor

Relationship between factor and critical period (CP)

Weed density

As density increase, CP decreases

Relative time of emergence

As weed emergence is delayed, CP increases

Row spacing

As row spacing narrows, CP increases

Soil moisture

As soil moisture decreases, CP decreases

Crop stresses

As crop growth is reduced, CP decreases

Corn yield can be reduced 2 weeks after corn emergence with high weed densities (40 plants sq. ft.) but yield was not affected until 5 weeks after corn emergence under low weed densities (5 plants sq. ft.). Relatively small delays in weed emergence can result in a much longer critical period. Weather conditions and cultural practices also influence the critical period. Narrow-row spacing reduces the rate yield loss occurs.

Once the critical period is reached, yield loss can increase rapidly if weed control is delayed. A systematic scouting program can monitor the specific weed population encountered in each field to avoid these types of losses.

Richard Zollinger
NDSU Extension Weed Specialist
rzolling@ndsuext.nodak.edu


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