Among many other things to do in the orchard, now is also the time to start our pre-emergent herbicide programs for the spring. With the price of glyphosate and glufosinate increase in 2021, I began working with pre-emergent herbicides for pecan at the Vidalia Onion Research Center. Though post emergent prices have come down some, the benefits of using pre-emergent herbicides remain economical. Additionally, the low-input pecan orchard planted at the Vidalia Onion Research Center serves as a great place to observe these herbicides. With small trees and lots of sunlight, these herbicides can be put to the test.

Once pecans are three years of age, most growers begin using Alion. Once you get a pre-emergent program established, it is benefited by having different herbicide groups in rotation. This is long-term resistance management. Whatever your base herbicide program is, I encourage you to rotate. I used the farm last year to apply numerous pre-emergent herbicides to compare, and I have noted a few things for you.

2023 Pre-Emergent Herbicide Ratings – Vidalia Onion Research Center

These herbicides were sprayed April 5th, 2023 with full soil contact and rain the following day. Each of these treatments were applied with13 gallon of water per acre, replicated three times and mixed with 2 quarts of glyphosate and 24 oz of glufosinate per acre on a sandy-loam soil. Let me draw your attention to a few things:

  1. The full season rate of Brake On and Matrix were used in the first application (above rating). All other herbicides listed were applied as half the seasonal rate with the assumption of a second application mid-season.
  2. Indaziflam (Alion and Centrus) had never been applied on this soil. The recommendation for using indaziflam is to apply the full rate (5oz per acre, two times) in the first year and a half rate the next time it is used. I applied a half rate and a full rate of indaziflam to compare on first-time soil. Both rates were statistically the same in their control.

I pushed these herbicides to 90 days even though most are going to give us between 60 – 80 days control. (With Alion, the label requires us to wait 90 days for the next application.) In 2023, we were dry through April and early May. We then received significant rain at the end of May which continued through June. All of the herbicides made it to 60 days after treatment. You need to do your second application as the weeds are coming back up, but you cannot wait until you get significant weed coverage.

60 Days After Treatment – VORC

Pre-Emergent Herbicide Tips

  • Weeds not yet germinated are controlled by pre-emergent herbicides. Once weeds have emerged (1st or 2nd true leaf stage), they will not be controlled. There are a number of pre’s that have post-emergent control, but most need to be applied before germination.
  • Good soil contact is required for these herbicides. If other weeds or organic matter is present, the herbicide will not contact the soil and therefore be ineffective.
  • Rainfall or an irrigation event of 0.5 – 1 inch of water must occur for herbicide to be activated. Some herbicides must be activated within 7 days. Other pre’s can remain on the soil longer. Try to apply before a rain!

On March 26th, I was able to get all the 2024 plots just ahead of the incoming rain. My rows are listed backwards because you come into the orchard on row 1. There will also be a few post-emergent herbicides going in on row 5 later in the season. I have a few herbicides in ’24 behind herbicides from ’23 which to observe the rotation. I also have different rates of common herbicides to observe control. The full-rate Alion plots were moved to new soil, and the 3.5 Alion plots are at the same plots from ’23. The Pindar plots were also treated on Nov. 2 as Pindar is a dormant season herbicide.

Posted in: