Page 2 - March Newsletter

Crop Production By: Jeanne Falk JonesCompare and Contrast Between Similar HerbicidesI recently asked farmers in northwest Kansas about their favor- ite irrigated corn herbicide program. I received lots of answers and it was clear that there are many paths to good weed con- trol in corn. One thing that showed up often was that decisions were made to include herbicides of certain families (same herbi- cide mode of action), but there was often some ‘wiggle room’ on which products they chose within a herbicide family that that was often dependent on specific characteristics of the herbicide. Because of this, I put together a bit of information about herbicides that are similar, but may differ in what fits best for your production system.I like to apply flumioxazin (Valor EZ/Panther SC/etc) in early spring to help with kochia control, but I have heard about sul- fentrazone (Spartan/Authority) also working. Would a sulfen- trazone-containing product be a better fit to prevent kochia? Flumioxazin and sulfentrazone are both in the same herbicide family of PPO inhibitors (Group 14). These herbicides work by causing damage to the cell membranes when the plant encoun- ter light, even before it emerges from the soil. The cell mem- brane damage makes the cells leaky, causes the cells to die and results in plant death. Both flumioxazin and sulfentrazone have activity on kochia and Palmer amaranth. Products that contain flumioxazin include Valor EZ, Panther SC, Zaltus SC, etc.There are a couple of things to keep in mind with these two herbicides. Sulfentrazone has a longer half-life (longer rotation restrictions) and because of this it is a better fit for fall applica- tions or on fields where the corn is planted later.The rotation restrictions are:Spartan Charge (sulfentrazone+carfentrazone (Aim))4 months to field corn Authority MTZ (sulfentrazone+metribuzin) –4 months to corn (if 14 oz or less) Flumioxazin has a shorter half-life and shorter residual. It is abetter fit for early spring applications because of shorter residu- al activity.The rotation restrictions by rate are:2 oz or less – 7 days to corn Up to 3 oz – 14 days to corn Up to 4 oz – 4 months to cornI can find metolachor and s-metolachlor at different prices. What is the difference between metolachlor and s-metolachlor and are they applied at the same rate?Both metolachlor and s-metolachlor are Group 15 Long Chain Fatty Acid Inhibitors and are the same herbicide molecule. There is R-isomers and S-isomers of metolachlor that are just slightly different in their structure of the molecule. Metolachlor contains both R-isomers and S-isomers. Chemists have found ways to change the manufacturing process, so that a greater percentage of the active isomer (S-isomer) of the herbicide is produced (like for s-metolachlor). By marketing a product that contains only the active isomer, a company can reduce the amount of product required for weed control.Table 1. Comparison of rates for Dual II (metolachlor) and Dual II Magnum (s-metolachlor).In addition, you might be wondering about the ‘II’ as part of its name (i.e. Dual II Magnum). The ‘II’ shows that there was the addition of a safener, to help limit corn injury. Therefore, Dual Magnum = no safener and Dual II Mangum = safener added.How do I pick between s-metolachlor and acetochlor?Both s-metolachlor and acetochlor are Group 15 herbicides. Acetochlor binds less tightly to soil colloids than s-metolachlor, and thus would be expected to require less rain for activation. Research on giant foxtail control at the University of Illinois looked at how the amount of rainfall affected control with s- metolachlor and acetochlor. It showed only statistically differ- ent levels of control at 0.25” with acetochlor having significant- ly higher level of control than s-metolachlor on giant foxtail.Table 2. Influence of rainfall on activation of two Group 15 herbicides. Simmons U Il. 1967.Dual II rate (pt/A)1.52.02.53.0metolachlor (lb/A)1.461.952.442.92Dual II Magnum (pt/A)Herbicide Inches of rain0 0.1 0.25 0.5 1.0- - - - - - - - - - % giant foxtail control - - - - - - - - - - S-metolachlor 55 50 57* 85 100 acetochlor 52 55 75 92 100* Indicates significance between the two herbicides within a rainfall amount.We often use atrazine, but would metribuzin be a better fit?Both atrazine and metribuzin are Group 5 (photosynthesis in- hibiting/photosystem II inhibiting) herbicides. These herbicides negatively affect processes and products necessary for the transport of chemical energy in the plant. Metribuzin is more water soluble than atrazine. In addition, the half-life of metribuzin is 7 to 28 days, whereas the half-life of atrazine is 1 year or more.What is herbicide half-life?A half-life is the time it takes for a certain amount of a pesticide to be reduced by half. This occurs as it dissipates or breaks down in the environment. The half-life does not actually mean the length of residual weed control, but can be utilized to com- pare between similar herbicides. The half-life is typically report- ed in a range of time.Many things play a role in how long a pesticide remains in the environment. These include things like sunlight, temperature, presence of oxygen, soil type and acidity, and microbe activity.1.001.331.672.00S-metolachlor (lb/A)0.961.271.591.91K-State Research and Extension is an equal opportunity provider and employer.