As air temperature and day length increase, early spring weeds respond aggressively and grow quickly. Sometimes they grow so fast that by the time crops are to be planted, the weeds can be difficult to control and then can cause problems in the crop. They steal soil moisture, tie up nutrients and can make planting difficult. Understandably, many farmers prefer to make as few trips across the field as possible for various reasons, namely economics. However, by waiting until near planting and including burndown and residual herbicides in the tank, optimal herbicide performance of both burndown and residual control can be hindered. Here are some general reasons to consider using early burndown (or early preplant aka EPP) applications and ways to optimize herbicide performance:

1. Improve weed control

Since weeds are bigger closer to planting time, the herbicides are not as effective and thus poorer weed control is obtained. Gramoxone (paraquat), for example, can be much more effective on <3" tall marestail as compared to larger ones and tends not to be impacted as much by cooler spring temperatures as is glyphosate. However, it’s best to wait until daytime temperatures are in the 50s F and nights in the 40s F. Herbicides work slower under cool conditions, so cold nights (<40°F) will reduce activity, particularly for glyphosate. 2,4-D is generally more active than glyphosate in cool weather; thus, tank mix them when possible. Also, after a cold spell, wait until a few days of warm, sunny weather have occurred before applying herbicides. In general, burndown herbicides provide the best control when annual weeds are actively growing, are 6 inches tall or less, and are still in the vegetative stage of growth (once they bolt, they are more difficult to control). Although spring is not the ideal time to control perennials, they should be at least 6 to 8 inches tall and preferably more when a systemic burndown herbicide is applied. Increasing the rate of the burndown herbicide may be necessary if weeds are stressed by cold conditions or are larger in size. Winter annuals that are flowering may require higher rates or different combinations of products. Using the correct adjuvant(s) is essential for optimizing burndown performance so check the herbicide label for details. MSO plus AMS must be used with Sharpen while AMS can improve glyphosate activity, especially under early season conditions.

2. Better spray coverage

Contact herbicides such as Sharpen and Gramoxone are much more effective when more spray droplets cover the many leaf surfaces. If weeds are too large the droplets are intercepted by the upper leaves and are not deposited on the lower leaves thus decreasing the overall injury/kill to that weed. Simply put, it is easier to kill smaller weeds. Furthermore, use at least 15 gallon/acre spray volume, however 20 GPA or more is generally better to optimize control.

Herbicide antagonism: Since many fields have an array of weed species and with the spread of multiple-resistant weeds across the region it is often necessary to tank-mix a variety of herbicides to improve control. However, tank mixing can bring complexities. Tank mixing atrazine or metribuzin with Gramoxone increases the activity; however, tank mixing these or other clay-based herbicides (WG, SC, DF, F, WP) reduces glyphosate activity. The antagonism can be overcome by increasing the glyphosate rate by 20 to 25%. Also, certain COC and MSO adjuvants can antagonize glyphosate.

3. Length of residual

Indeed, having many different herbicides in the tank can provide a clean seedbed, but the useful residual activity of those products is impacted significantly. If all the burndown and PRE herbicides are applied in one pass, say a couple weeks ahead of planting, that is two weeks of "wasted" herbicide residual activity. Once the crop is planted, there needs to be as much residual herbicide available, for as long as possible, to provide a weed-free environment to get established and increase its growth and development for optimal yield. Most residual herbicides provide effective control for about 4-6 weeks, if the herbicide is applied too early, then the crop may only experience 2 weeks or so of weed control before the herbicide decomposes and weeds start to emerge and compete with the crop. At that point, additional weed control tactics will need to be used. Also, if weeds such as Palmer amaranth, waterhemp and/or marestail are in your fields, the length of residual control once the crop is planted is even more critical. Residual products tend to provide more weed control value when applied at planting.

4. More burndown herbicide options and at higher use rates

One of my former colleagues had a saying: "Do you want to just pet the weeds or kill them?" Sometimes herbicide rates can be too low for effective control. Often, use rates of certain burndown herbicides are reduced to allow the crop to be planted sooner. For example, using 1 pint/A of 2,4-D ester and only waiting 1 week to plant the crop, instead of using 1 quart/A and waiting two weeks or more but getting much better burndown activity. Or deciding to use 1 fluid ounce per acre Sharpen and planting soybeans immediately or using 1.5 fluid ounce or 2 fluid ounce and waiting 15 or 30 days, respectively but getting better control in the long run. Furthermore, when tank mixing Sharpen and other Group 14/PPO herbicides that contain Valor (flumioxazin) or Authority (sulfentrazone), a 2-week minimum must pass before planting soybeans. Therefore, it would be better to use Sharpen earlier in a burndown-only application and then apply the other residual herbicides at planting for longer weed control in the crop. In addition, products like dicamba or Elevore can be used early to assist with burndown in certain soybean and corn settings. The ability to use higher rates or other herbicides can provide more effective control of weeds early season, especially for weeds like marestail.

In some cases, additional glyphosate or paraquat might need to be included with the PRE/residual herbicide application at planting if new weed flushes are present, but the weeds will be much smaller and less dense for an effective kill and a cleaner seedbed.

Flexible Burndown Herbicide Programs for Corn or Soybean

For various reasons, you may need to make "a game-time decision" as to whether you are planting corn or soybeans this spring. In that case, it would be best to apply a flexible herbicide program that allows planting of either crop. A limited number of corn and soybean herbicides allow you to switch crops if necessary. Here is a list of herbicides that can be used PREPLANT or PRE in both corn and soybean. Be cautious of application rates and wait periods that may differ for each crop. Also, keep in mind that some of these products and active ingredients are available in generic alternatives.

Burndown

• 2,4-D LVE — When applied at 1 pint/acre, there is a 7-day waiting period for soybean. This is also the safest use for corn. If applying 1 quart, wait at least 15 days to plant soybean.
• Basis Blend — Apply at 0.825-2.5 oz/acre in burndown programs before corn emergence; corn injury may occur under cool, wet conditions, so a 1.5 oz rate is a good compromise. In soybean, apply 0.825 oz at least 15 days before planting; tank mix to enhance control. It provides some weedy grass control and fair to good burndown activity on several key broadleaves.
• Elevore — Either corn or soybean can be planted 14 days after application when applied at 1 fl oz/acre. Provides control of glyphosate-resistant marestail.
• Glyphosate — This can be used in the burndown and POST in Roundup Ready crops. Increase the rate to 1.5 lb ae/acre for larger weeds.
• Gramoxone 3.0 SL/paraquat 3 lb/gal — Can increase the rate up to 2.7 pt/acre for larger weeds. Can tank-mix with metribuzin plus 2,4-D and still be able to plant corn or soybean. This mixture provides better burndown activity than paraquat alone.
• Harmony/Extra, Audit 1:1, and other thifensulfuron + tribenuron mixes — Can be added to the 2,4-D and/or glyphosate burndown at 0.45 to 0.90 oz/acre for both corn and soybeans. Must wait 7 days to plant soybean and 14 days for corn. Without glyphosate, it provides average (70-80%) control on many key broadleaves and no grass control; with glyphosate, it improves control of curly/broadleaf dock, wild garlic, common groundsel, field pansy, and Canada thistle.
• Liberty 280 — labeled up to 43 fl oz/acre in the burndown application. Liberty is most effective on small actively growing weeds under warm temperatures. Use sufficient carrier volume (minimum 15 GPA, 20 GPA is better) and spray nozzles to get thorough droplet coverage of the weeds.
• Reviton — Can be used in a burndown program before field corn or soybean. Corn can be planted immediately after application; wait 7 days to plant soybean if using 2-3 fl oz (0 days at 1 fl oz). It has a low use rate (1 to 3 fl oz/A) and can be tankmixed with other herbicides esp. glyphosate. MSO or COC plus AMS must be added to the spray solution. Reviton is similar to Sharpen and is less active on marestail but has better activity on field pansy/violet, primrose, and some grasses compared to Sharpen.
• Sharpen — Labeled for both corn and soybean. Use only the 1 fl oz/acre in soybean for burndown if planting soybean immediately. Higher rates can be used but be aware of planting restrictions for soybeans. Corn can be planted immediately at any labeled rate. Always add MSO and AMS as the adjuvants to the spray tank for optimal weed control.

Grass residual (primarily)

• Dual (metolachlor) products — Can be used in both corn and soybean for residual grass, yellow nutsedge, and some small-seeded broadleaf control.
• Outlook — Can be used in both corn and soybean for residual grass and certain small-seeded broadleaf control.
• Warrant — Can be used in both corn and soybean for residual grass, yellow nutsedge, and some small-seeded broadleaf control.

Broadleaf residual

• Lorox— Can be used in both corn and soybean. It primarily provides control of annual broadleaves but can suppress certain annual grasses.
• Metribuzin — This product was labeled preemergence at up to 5.3 oz per acre on medium textured soils with greater than 2% organic matter as part of the burndown program in either crop.
• Python — Python is effective on a number of annual broadleaf weeds and can be used in both corn and soybean. The use rate range is 0.8 to 1.14 oz/acre (up to 1.33 oz in soybean).
• Valor — Can be used in both corn and soybean. Corn may be planted 7 days after application at the 2 oz/acre rate (requires minimum of 25% surface residue and 0.25 inch of rainfall between application and planting). Do not tank-mix with common grass herbicides (Dual, Outlook, etc.) in soybean due to injury concerns.

Grass and broadleaf residual products

• Verdict — This is a mixture of the active ingredients found in Sharpen and Outlook. The most common application rate is 13 fl oz/acre for corn and 5 fl oz/acre for soybean.
• Zidua and Anthem Maxx/Flex – can be used in the burndown mix and either corn or soybean can be planted and will provide control of many annual weeds.
• Fierce – can be used in both soybean and no-till or minimum-till corn to control many annual grasses and broadleaves. In corn, you must wait at least 7 days to plant corn after application.
• Combinations that include a grass product (Dual, Outlook, and Warrant) plus Python would also provide residual grass and broadleaf control for both corn and soybean. 

In conclusion, knowing what weeds are causing problems and understanding that in some cases multiple trips across the field for better weed control during the growing season might be more economical in the long run, in order to ultimately protect and improve crop yields. However, one-pass burndown/residual herbicide programs may have a fit in certain situations.

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