Historically, glyphosate has always been a no-no for use as a harvest-aid in soybeans grown for seed because of its potential negative effects on seed germination. But, what about paraquat?
Since I have never conducted any harvest-aid research, I would not really consider myself an “expert” on this subject. I can only go by what I read and who I talk to (usually my weed science colleagues who work more on soybeans). Also, hope that we can all agree that we are never too old to learn new stuff.
A few thoughts about this paraquat/soybean seed germination issue:
1. A quick review of the published literature is as follows:
- Whigham, D.K. and E.W. Stoller. 1979. Soybean desiccation by paraquat, glyphosate, and ametryn to accelerate harvest. Agronomy Journal 71(4):630-633.
“Paraquat applied 4, 3, or 2 weeks before harvest did not affect soybean seed germination.”
- Cerkauskas, R.F., O.D. Dhingra, J.B. Sinclair, and S.R. Foor. 1982. Effect of three desiccants on soybean (Glycine max) seed quality. Weed Science 30:484-490.
“Paraquat applied at R7 stage (50% defoliation) caused 16% to 29% reductions in soybean seed germination, depending upon cultivar and seed germination test (potato dextrose agar or rolled celluose towels).
- Ratnayake, S. and D.R. Shaw. 1992. Effects of harvest-aid herbicides on soybean (Glycine max) seed yield and quality. Weed Technology 6:339-344.
“Paraquat had no effect on soybean seed germination when applied at R5 (beginning seed), R6 (full seed), R7 (beginning maturity), or R8 (full maturity) stages of growth.”
2. I did not realize this but the Gramoxone SL 2.0 paraquat formulation (Syngenta) has a label for use on soybeans grown for research, field trials, and seed production. This label includes a 3 day PHI which is very different from the regular soybean harvest-aid PHI label of 15 days. Other commercial formulations of paraquat, including Firestorm (Arysta), Parazone (AMVAC) and Helmquat (HELM Agro), do not have this harvest-aid/soybean seed production use on their current labels.
Gramoxone SL 2.0 Label (check out page 41)
3. Dr. Wilson Faircloth, Syngenta’s local Agronomic Services Representative, provided the following information about this subject:
“The PHI for soybean grain is set to avoid paraquat residue in grain. The application for soybeans grown for research or seed does not allow seed to enter grain channel, these soybeans must not enter normal grain channel. So if the question is around concerns over impact on seed germination, neither application will cause a problem with germination when applied to mature seed.”