A review of glyphosate use for preharvest weed control

From Vince M. Davis – Extension Weed Scientist

Late-season weed escapes are very prevalent in corn and soybean fields this year. Poor residual herbicide activation and poor postemergence herbicide efficacy is part of the reason. Additionally, and perhaps more importantly, poor crop canopy development and late-season rainfall encouraged large weed flushes much later this year. In some cases, weed densities are quite high and a preharvest herbicide application may help limit seed production of some annual weed species and improve the efficiency of harvest operations. This will be particularly true where thick grassy weeds or lots of large broadleaf weeds like giant ragweed, common lambsquarters, and velvetleaf are in soybean fields.

We’ve already received a couple questions regarding preharvest applications of glyphosate in soybean. First, there are a couple other herbicides besides glyphosate, like paraquat and carfentrazone, which could also be used in this manner. Paraquat (Gramoxone Inteon) and carfentrazone  (Aim) are both contact herbicides. The advantages of these products versus glyphosate will be faster desiccation of weeds. However, proper nozzle selection to deliver appropriate droplet sizes and higher carrier volumes will be very important. Additionally, carfentrazone has a much narrower weed spectrum than glyphosate or paraquat. For more details regarding the preharvest use of carfentrazone or gramoxone products, please read and follow the directions accordingly. However, we get many more questions about how to use glyphosate as a preharvest aid so I will try to elaborate a little further.

There are too many glyphosate formulations for me to know them all, so the most important message is to read and follow the herbicide label for the specific product you are using. But, I’ll give you some ‘cliff notes’ that are important to know. Unlike paraquat and carfentrazone mentioned above, glyphosate is a systemic herbicide. A systemic herbicide translocates to the growing point once it is inside the plant. This movement typically happens with the phloem of the plant (i.e. with the ‘food’ for the plant), and we call this movement ‘source to sink’. In many ways, this movement is an advantage over contact herbicides because it is the new growth of the plant you most want to terminate. I went through that very brief lesson on herbicide action to make a very important point when it comes to using glyphosate as a preharvest aid, and that is; appropriate timing is very important.

Preharvest applications can be made in corn, but I know it is more common in soybean so I will focus on that crop. It is illegal to spray glyphosate after full bloom (R2) until soybean pods have lost all their green color. Why, because between R2 and R8 the soybean plant is developing seed. As the seeds develop, they are a ‘sink’.  Technically, the R8 growth stage is the final growth stage call ‘full maturity’.  Full maturity is defined as 95% of the pods having lost green color and is usually 5 to 10 days before the field is ready to harvest. However, the glyphosate label reads (as I’ve already stated) that preharvest applications should be made after ALL pods have lost green color. Also note, you should not make preharvest applications to beans used for seed because a reduction in germination or vigor may occur.  Applications made too early in pod maturity run greater risk of glyphosate being translocated into the seed tissue resulting in illegal residues in the seed.

On the flip side, there is a required preharvest interval for grain of 14 days between a glyphosate application and harvest of glyphosate-resistant (Roundup Ready and Roundup Ready 2 Yield) soybean.  Moreover, because glyphosate often takes some time to ‘work’ it may take near those 14 days before complete activity will be achieved.  So, in order to have time for the weeds to desiccate to the maximize amount, and to avoid excessive shattering from delayed harvest, it is imperative to scout fields closely in the final days of maturity to time the application correctly. This will be an even greater challenge in fields that reach maturity very un-uniformly this year due to the variable field moisture conditions.

Now for some good news, because of the translocation effect of glyphosate is from source to sink, in annual weed species that are setting seed glyphosate may help to significantly reduce the number of viable seeds following the preharvest application, versus no application at all. At this point in the season the application can’t reduce biomass, or save the production of yield, but could limit the size of future weed problems. Moreover, a glyphosate application will have much greater efficacy on any biennial and perennial weed species like thistles, quackgrass and common dandelion because their active ‘sink’ in the fall is the root system. So, a preharvest application will also reduce populations of those weeds more than an application of a contact herbicide.

How much glyphosate and adjuvant to use at a preharvest application? Rates and surfactants should be added according to the label for each specific glyphosate product. In the case of Roundup PowerMax, no additional surfactant is needed. The rate for a preharvest application on Roundup Ready and Roundup Ready 2 Yield soybean is 22 fluid oz/a.

Hopefully this helps with making properly timed and legal preharvest glyphosate applications where deemed beneficial.