Large farms and erratic weather makes it difficult for producers to cover all their acres for nutrient and pest management. So, everyone is looking for ways to cut trips across the fields to save money and time. However, if we are not careful, we could be doing more harm than good and costing ourselves money in the short-term and/or long-term. The information below is targeted toward weed management, but some of the information will be true for insecticides and fungicides.
It all starts with the herbicide label. When tankmixing herbicides or herbicides with other products (fertilizer, insecticides, and fungicides) always read and follow the label for allowed tankmixtures and mixing order.
Tankmixtures can enhance activity (synergism), have no impact (neutral), or can decrease activity (antagonism). Antagonism can be due to chemical interactions in the spray tank, incompatibility in the tank due to poor mixing capabilities, or inhibition of herbicide once it is sprayed on the weeds.
Approved tankmix partners are clearly stated on the label and often included specific rates, if different than either of the individual herbicide labels: Examples: Liberty 280 (glufosinate) herbicide does not list Roundup (glyphosate) as a tankmix partner in cotton, because of some slight antagonism observed with this tankmixture. Another example of antagonism includes tank mixtures of Staple (pyrithiobac) with grass only herbicides (clethodim, fluazifop, quizalofop, or sethoxydim), which are not recommended because reduced grass control is often observed.
As crop budgets get tighter, the application of reduced rates (below label rates requirements) of herbicides becomes more appealing and more common. However, repeated applications of below label rates of herbicides substantially increases the likelihood for developing herbicide resistant weeds. This point has not been commonly emphasized for herbicides, but reduced rates have proven to promote insecticide and fungicide resistance multiple times over the past decades. Use of the full label rate also applies to herbicide tankmixtures, unless the label states otherwise.
As producers continue to struggle with glyphosate resistant weeds, some creative herbicide combinations have been used. Some with success and others not so much. However, I recently heard of some people that were applying reduced rates of Liberty (glufosinate) at 8-10 fl oz/ac tankmixed with Roundup (glyphosate) and claiming to obtain better control of weeds.
I have not found any research data that suggests any better weed kill from spiking glyphosate with Liberty; however, if additional efficacy is being observed it is likely due to Liberty’s high surfactant load increasing glyphosate uptake. If the benefit of spiking glyphosate with Liberty is the surfactant, then most surfactants are available at a lower price. More importantly, spiking glyphosate with Liberty equates to application(s) of sublethal doses of Liberty. This creates the perfect scenario for developing glufosinate (Liberty) tolerant weeds. Liberty herbicide is a critically important herbicide for controlling weeds in most of our current cotton varieties (LibertyLink and XtendFlex), and is one of the few postemergence options for effectively controlling glyphosate resistant weeds. It would be a huge loss of an effective herbicide and herbicide tolerant trait, if Liberty (glufosinate) resistant weeds were to develop.
Take home message: use the full label rates of the herbicides to help prevent further development of herbicide resistant weeds. See the publication titled Weed Management in Texas Cotton as a reference for identifying label herbicides and rates for cotton.
Authors: Gaylon Morgan, Texas A&M AgriLife Extension Service, College Station, TX;firstname.lastname@example.org; Paul Baumann, Texas A&N AgriLife Extension Service, College Station, TX; email@example.com; Josh McGinty, Texas A&M AgriLife Extension Service, Corpus Christi, TX, firstname.lastname@example.org