Overlapping residual herbicides for Palmer amaranth control will be essential in 2022. There are two main reasons for this. First is the continued evolution of dicamba-resistant Palmer amaranth in Tennessee. Our most recent greenhouse screens clearly show that some populations have upped their game with respect to their dicamba resistance from what was seen in 2020 or 2021. Second, the lack of Liberty supply greatly limits the last best option to remove Palmer amaranth in-crop that is dicamba resistant. As such, there have been numerous questions recently on how best to utilize overlapping residuals for Palmer control. Herbicide supply varies across Tennessee so below are 5 options/strategies on overlaying herbicides in cotton:
Option 1. Pre-emergence Brake 16 oz + Cotoran 16 oz or Diuron 16 oz followed by a Group 15 herbicide (Dual Magnum, Outlook, Warrant, etc.) on 1 to 3 leaf cotton and then Zidua applied via impregnated fertilizer on 5 to 7 leaf cotton.
Advantages: Brake will provide good Palmer amaranth and grass control if activated with >1/2” of precipitation. Probability for cotton injury very low.
Considerations: Most expensive option. Brake requires >1/2” of precipitation to become activated.
Option 2. Pre-emergence Cotoran 16 oz + Caparol 16 oz followed by a Group 15 herbicide on 1 to 3 leaf cotton and then Zidua applied via impregnated fertilizer on 5 to 7 leaf cotton.
Advantages: Good Palmer control and less expensive than other options. Requires less precipitation than some other options to activate. Probability for cotton injury very low. The Cotoran + Caparol Pre application has a lengthy, mostly successful track record of providing up to three weeks of residual Palmer amaranth control in Tennessee.
Considerations: Less consistent grass control than the Brake pre-emerge option.
Option 3. Pre-emergence Cotoran 16 oz + Prowl H2O 32 oz followed by a Group 15 herbicide on 1 to 3 leaf cotton and then Zidua applied via impregnated fertilizer on 5 to 7 leaf cotton.
Advantages: Good Palmer and grass control and less expensive than some other options. Probability for cotton injury low in no-till.
Considerations: Less consistent Palmer control than the Brake pre-emerge option. Prowl H2O requires a 1/2” of precipitation to activate. Prowl splashing up on seedling cotton can cause injury in tilled soil.
I typically have not recommended a Group 15 herbicide (Dual Mag, Outlook, Warrant, etc.) PRE-emergence (PRE) in cotton due to seedling injury concerns. However, more recent research we have conducted on tactics to mitigate seedling cotton injury utilizing Group 15 herbicides PRE has shown that using 1/2x of the labeled rate PRE followed with the other 1/2x early POST has resulted in reduced risk of injury while still providing good pigweed control. Other research has shown that applying the Group 15 herbicide Pre-plant can also reduce probability of seedling cotton injury. Based on this research below are some strategies in using Group 15 herbicides.
Option 4. Pre-emergence Cotoran 16 oz + Warrant 24 oz or Dual Magnum 8 oz followed by Warrant 24 oz or Dual Magnum 16 oz on 1 to 3 leaf cotton and then Zidua applied via impregnated fertilizer on 5 to 7 leaf cotton.
Advantages: Good Palmer and grass control and less expensive than some other options. Probability for cotton injury low.
Considerations: Less consistent Palmer control than the Brake pre-emerge option. Warrant requires a 1/2” of precipitation to activate. Though a rare occurrence, my experience has been that the use of high rates (48 ozs) of Warrant POST in cotton can cause considerable leaf burn.
Option 5. Pre-plant up to 21 days Reflex 16 oz + Dual Mag 16 oz or Warrant 24 ozs followed by Pre-emergence Cotoran 16 oz + Caparol 16 oz followed by a Group 15 herbicide on 1 to 3 leaf cotton and then Zidua applied via impregnated fertilizer on 5 to 7 leaf cotton.
Advantages: Good Palmer and grass control. Good option if field may be later planted.
Considerations: More expensive than some other options. Reflex can have mixing difficulties with glyphosate or Gramoxone in higher pH water sources. Adding 1 to 2% of an AMS substitute (Class Act, Choice, Quest, etc.) can improve mixing. Warrant requires a 1/2” of precipitation to activate.
If the Zidua fertilizer application is not an option, then it is very important to apply layby herbicides with a post-direct rig. Moreover, because layby applications are slow there will likely be some emerged pigweed so a contact herbicide is also needed. More on layby options later.
