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The 2015 growing season is not even into harvest yet, but there is already some considerations to be thinking about on your fields for crop planning in 2016. Herbicide applications get made without knowledge of what the rest of the seasons rainfall will look like; if a product with residual gets applied that generally is broken down it can still become an issue under dry situations. But what is the exact impact that moisture has on the breakdown? What other considerations need to be taken into account when considering a shift in crop plans due to herbicide carryover?

Moisture – This is the biggest factor in the breakdown of most actives and groups of herbicides. Moisture after application is what keeps the soil microbes active and actually feeding on the herbicides to break them down, or in some cases producing enzymes to help break them down further. Another key to watch is how often and how much comes. If you do not get any rain from application timing until mid-late September there is still a significant chance of carryover as there was not active break down occurring for a long enough period of time.  Virtually all herbicides need good moisture for breakdown, but in drier conditions even group 27 products like Infinity (pyrosulfatole component)) and Prestige (clopyralid component) can have carryover issues even where they generally aren’t a concern.

herbicide carryover diagram
image credit: Pioneer

pH – The pH of your soil is also a key consideration when it comes to herbicide carryover. The pH is a logarithmic scale showing how acidic or alkaline your soil is on a scale of 0-14 which actually has an effect on many different components of crop production. The pH  can impact the levels of  microbes in the soil that are present that work to break down the herbicide and also can change the rate of hydrolosis within the soil. Now, the pH can vary across the field which is one of the reasons you may see herbicides only cause issues in certain areas. This is where having properly soil tested and zoned fields can help give you insight into where to be checking for herbicide carryover first. Some of the families of herbicides impacted by pH include group 2 sulfonylurea’s which persist longer in high pH (>7) such as Ally (metsulfuron), group 2 imidozaline family herbicides which are in products like Ares (imazypyr component) or Odyssey Ultra (imazethapyr component).

Soil Texture and Organic Matter  (OM) – The higher the organic matter, the higher levels of microbial activity that tends to occur as well as the greater the ability of that soil to hold moisture and keep organisms actively breaking down the herbicide. OM also acts as a type of buffer and gives the herbicide more binding sites. If you have OM over 3%, then your risk tends to begin to decline. The texture of your soil can be important as well as the lighter the soil (eg: the lesser the clay content) the more prone to herbicide carryover your fields are. Again, both OM and soil texture can vary significantly across your field and this is where a strong soil testing and mapping program involved in your operation can have a positive contribution to making profitable decisions.

These considerations need to be taken seriously every year, but the margin for error is that much smaller when soil conditions trend toward the drier side. Always be taking all of these things into account and discussing them with your agronomist. If you are still on the fence, consider the option of a soil bioassay.