As air temperature and daylength increases, 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 reasons to consider using early burndown applications:
Poor 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/horseweed as compared to larger ones and tends not to be impacted as much by cooler spring temperatures as is glyphosate.
Some herbicides (clay-based formulations like DF, WDG, FL) can antagonize glyphosate. For example, atrazine can reduce glyphosate effectiveness.
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, it needs to have 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 4-5 weeks, if the herbicide is applied too early, then the crop may only experience 2 weeks or so of weed control before the herbicide dissipates 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, 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.
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 fl oz/A Sharpen and planting soybeans immediately or using 1.5 fl oz or 2 fl oz 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 contains 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 applying 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.
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 crop yields.