Glyphosate resistant weed control

Hairy Fleabane (Conyza bonariensis)

After the meeting last week, there were quite a few growers staying after to talk with Brad Hanson (UC Davis/AES Weed Specialist) about glyphosate resistant weeds, especially fleabane.
Here are a few of their questions with answers:
How do I control these weeds that are missed by glyphosate?
Post-emergent materials including Rely (Glufosinate), 2-4,D, and Treevix (Saflufenacil) work well on glyphosate resistant fleabane. These tend to work a lot better on small plants but not so well on large established plants that are flowering. Paraquat works pretty well usually but there are some populations of fleabane that are resistant to paraquat in the San Joaquin Valley.

Growers with serious infestations should consider making a pre-emergent herbicide application this coming winter. These residual type herbicides control emerging seedlings, killing the weed before it becomes established. Depending upon your requirements for ground water protection, consider a winter application of rimsulfuron (Matrix), simazine (Princep), indazaflam (Alion), norflurazon (Solicam), or penoxsulam and oxyfluorfen (Pindar GT).

Kurt Hembree's (Farm Advisor, Fresno County) weed susceptibility charts provide more information on controlling these weeds.

There seems to be major growth flushes of fleabane, is this correct?
Fleabane has two major flushes – on in late fall and another in early spring. I’d estimate that 40% emerge in the fall, 40% in the spring, and the other 20% is scattered throughout the year.

What about glyphosate resistant grasses - in particular ryegrass?
Post-emergent options include sethoxydim (Poast), clethodim (Select Max), and glufosinate (Rely). Pre-emergents include eptc (Eptam), napropomide (Devrinol), oryzalin (Surflan), pendimethilin (Prowl H2O). More information can be found on Kurt Hembree's weed charts for almonds.

What else should we consider when attempting to manage this weed?
Treat weeds before they go to seed! This helps reduce the population within the orchard.
Rotate your herbicide post-emergent chemistries and consider rotating in a pre-emergent chemistry. Relying only on one herbicide to control a weed will lead to needed rate increases and further resistance development.

Treat weeds small - the smaller the better. Prevent stressing the weeds through mowing or excessive driving as this makes the weeds more tolerant to herbicide applications.

Remember that every weed is different and may need a different chemistry for control - there are no herbicides that control all weeds well. Tank mixing chemistries provides an effective means of covering the weed spectrum with one pass through the orchard.

Make sure to routinely change your spray nozzles to ensure adequate coverage and rate application. Misses often occur due to the natural degradation/abrasion of nozzles from the various products. This can lead to over- and uneven applications.

What if we suspect a glyphosate resistant weed?
If misses of a particular weed occur after applications of glyphosate, contact your local farm advisor. They will direct the weed sample to the proper channels for identification of possible glyphosate resistance. Eradicate these weeds by using any means available. Prevent them from going to seed.

For more information on fleabane and marestail, check out this free UC ANR publication.