- When wet and cool soybean seedlings readily pick up herbicides.
- Cold temperatures mean slow metabolization it very slowly, often resulting in injury.
- Other “wild card” in the scenario is the relative varietal tolerance to some herbicides.
It’s almost early June and soybean planting in Tennessee is in high gear!
Due to the very wet March through May we will be looking at a large soybean crop in our state this year. Moreover, it appears all the soybeans, whether they are first crop or double crop, will likely be planted in about a two-week window.
The main topic of calls I received last week was on some of the few May-planted soybeans being injured by pre-applied herbicides.
Soybean planting that began just before Memorial Day was abruptly cut short by three-plus inches of rain and yet another cold spell. These are ideal environmental conditions for pre-applied soybean injury.
Soybeans emerging through a layer of activated herbicide have to metabolize it to avoid injury. When it is very wet and cool the young soybean seedlings readily pick up the herbicide and then, due to cold temperatures, metabolize it very slowly, which often results in injury.
The other “wild card” to this injury is the relative varietal tolerance to some herbicides. Typically, under wet and cool conditions soybeans with poor tolerance to a herbicide will show more injury than another variety planted at the same time with good tolerance. This varietal tolerance can relate to the main three pre herbicides used: Valor, Sharpen and metribuzin. Of these three herbicides, metribuzin is of most concern as it can greatly reduce stands while Valor and Sharpen rarely, if ever, cause stand loss.
With Valor what happens is the herbicide attached to the soil is splashed up on the newly establishing soybeans during a heavy rain event. Everywhere the Valor comes in contact with the soybean seedling it leaves a burn. In extreme cases it can take out the apical meristem. Typically, even in extreme cases where the apical meristem has been damaged, the soybeans branch out and go on to yield well. We rarely see this splash injury in Tennessee as our no-till greatly reduces the splash of soil.
Sharpen on the other hand is more water soluble then Valor and does not splash but is taken up through the roots. Varieties sensitive to Sharpen will have their early season growth rate checked up to some extent but my experience is they shake it off and go on to yield well in most cases.
With metribuzin the story is different than with Valor or Sharpen as soybean injury from metribuzin will at times consist of very high seedling mortality. This is especially true for varieties sensitive to metribuzin.
I walked several soybean fields last week where metribuzin was part of the issue where soybean stand loss was excessive. In these cases the soybeans will have to be replanted. In all cases the soybean variety in the problem field was on the “Severe Injury or Ended in Death” categories on the University of Arkansas 2012 Metribuzin Screen.
Soybean fields planted to varieties with good metribuzin tolerance did not require replanting.
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I highly recommend that before metribuzin is applied to a soybean field the variety in the field needs to be checked against the University of Arkansas Metribuzin Screen. If your variety is in those last two categories -- Severe Injury or Ended in Death -- either change the variety or go with a non-metribuzin pre program.
I really like to thank the “Weeds Team” at the University of Arkansas for going to all the work to screen 300 plus varieties. This information can save replanting this year in a spring when we do not have time to replant.
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