Considerations for Burndown and Early Season Weed Control

Importance of Early Weed Control

Crops are most vulnerable to weed competition at planting and as the new plants emerge. Significant yield is at risk if weeds are allowed to compete with crops during the first several weeks after planting.1 If not controlled, they can also decrease harvest efficiency and produce seed, which can impact future crops. The timing and intensity of weed emergence determines which species will be the most competitive with the crop, as not all weed species compete equally. The emergence patterns of different species remain relatively consistent, but weed emergence profiles may vary from year to year depending upon environmental conditions, crop planting dates, and other management practices. The unique emergence profile for each weed species is defined by the initial emergence date, the duration of emergence, and the distribution of emergence within this time period.

The presence of early emerging weeds generates several questions that are important for good weed management:

Figure 1  
  • What herbicide or tankmix do I include with my burndown application to provide an additional mode-of- action and residual control?
  • What is my planned follow up application to ensure I have effective residual control through crop canopy?
  • What herbicide sites of action and residual activity will be needed to control any weed escapes and late emerging weeds?

Timing

Ideally, weeds should be controlled at least a couple of weeks prior to planting, to allow for decomposition of the plant material. Planting into existing weeds, or heavy weed residue that has not had time to decay, can interfere with seed placement and reduce emergence due to poor seed-to-soil contact. If the burndown is delayed, planters should be adjusted to compensate for the increased plant residue.

The emergence profiles of weeds in a field can significantly affect the performance of weed management programs and should be a major consideration for planning. In addition to starting with a clean field, removing weeds after planting when weeds are less than 4 inches tall is necessary to preserve yield potential. For example, university research shows an average of 3 bushel per day of yield potential is at risk for every day 3- to 4-nch weeds are left uncontrolled after V3 to V4 stage corn.2

Burndown Applications

Starting with clean fields at planting is an essential step for proper weed management:

  • Preplant burndown tank mixtures remove early weed infestations and provide broad spectrum foliar and residual weed control.
  • Dicamba or 2,4-D may be required in burndown tank mixtures for tough-to-control weeds. Both products have planting interval restrictions of a week or more between application and the planting of corn or soybean.
  • Consult individual product labels for precise instructions.

A burndown plus residual herbicide tank mix, or tillage, may be required to remove early weed infestations:

  • Residual herbicides applied with a burndown application several weeks prior to planting may encounter environmental conditions that limit residual activity into the growing season.
  • An earlier than normal post herbicide application may be required to limit weed competition.
  • The early post application requires a residual component to control late emerging weeds that impact yield potential.
  • Residual herbicides have planting interval or crop rotation restrictions and precautions that need consideration if conditions warrant a change in planting intentions.
  • Consult individual product labels for precise instructions.

Tank mixed herbicides, with different sites of action, may cause antagonism that affects performance. For example, a fast acting contact herbicide can interfere with the uptake and translocation of a systemic herbicide by quickly shutting down weed growth. Higher herbicide use rates, ammonium sulfate (AMS), or adjuvants can help overcome antagonism. AMS has been shown to be an effective additive to condition hard water by deactivating antagonist salts (iron, zinc, calcium, magnesium, sodium, potassium4), prevent the binding of herbicides to soil particles on leaf surfaces, and improve foliar uptake. Some herbicides labels restrict the use of AMS. All labels should be checked for all additive rates and restrictions.

Fertilizer components and higher preplant spray volumes may also reduce herbicide activity, requiring higher herbicide use rates.

Environmental Factors

Environmental conditions affect the rate of weed growth, crop development, crop tolerance to herbicides, and herbicide performance. Fluctuating day and night temperatures are typical in the spring. The efficacy of a burndown herbicide application can be reduced by cold temperatures. It is recommended to wait on applying herbicides until nighttime temperatures are above 40° F and daytime temperatures are in the high 50’s to low 60’s.1 Weed control may be even more effective if there are several days of warmer weather prior to herbicide applications rather than applying on the first warm day of the season.

Low overnight temperatures and slow warming during the day can reduce the rate of weed development. Seedling weeds tend to be more susceptible to soil-applied herbicides under cool conditions because plant emergence is delayed and metabolism is slowed. Slower weed growth caused by heat, drought, or cold also affects herbicide uptake, translocation, and metabolism that may reduce performance of post applied herbicides. The best way to limit problems related to warm, dry, or adverse conditions is timely application to small weeds (less than 4” tall) rather than equipment adjustments.3 Usually, postponing herbicide application is risky because changing weather conditions may delay application until weeds exceed optimum size for good herbicide performance. Most herbicide labels contain statements regarding environmental influences on herbicide performance.

Treatment Recommendations

Scout fields and control weeds throughout the season. Proper application timing helps protect yield potential, ensures correct use rate for weed size, and considers the impact of environmental conditions on performance.

Weed Management Recommendations for herbicide resistant or tough-to-control weeds such as marestail, giant ragweed, kochia, lambsquarters, Amaranthus species, and others can be found at http://www.roundupreadyPLUS.com.

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