By Mark Loux OSU Extension
In our windshield scouting of soybeans this year we have seen a lot of weed-free fields. This makes sense given the shift toward Xtend, LibertyLink, LLGT27, and Enlist soybeans over the past several years, which provides us with effective POST options for our major weed problems – common and giant ragweed, marestail, and waterhemp (now if we could just get rid of the baggage some of these traits carry). We are however getting many reports of late-season waterhemp as it grows through the soybeans and becomes evident. This also makes sense given that statewide we are in the midst of an overall increase in waterhemp, and continue to move up the curve in terms of the number of fields infested and the size of the infestations. Prevention and management of waterhemp and Palmer amaranth has been one of the primary goals of our state and county educational programs for half a decade or more. And one of the most important points about waterhemp and Palmer that we try to get across is their capacity for prodigious seed production – 500,000 to upwards of a million seeds per plant – and what this means for their ability to rapidly ramp up populations, infest equipment, etc.
The most effective way to prevent seed is to cut off waterhemp or Palmer plants just below the soil line, remove plants from the field, and burn or compost or bury deep enough. Plants left in the field can reroot at multiple nodes and regrow. Another option to at least reduce seed production – use a weedeater to cut the tops of plants off. Once plants develop mature seed (hard brown or black), most effective strategy may be to cut off and bag up seedheads and remove from the field. The value of herbicides this late in the season is questionable. PPO herbicides are the only legal option at this point, with following restrictions (DBH = days before harvest; from Table 18 of Weed Control Guide): Cobra/Phoenix – 45 DBH; fomesafen – 45 DBH; Ultra Blazer – 45 DBH. Carryover and injury to corn from late-season applications of fomesafen is possible. None of these herbicides are likely to kill large waterhemp plants although they may reduce suppress smaller plants enough to reduce seed. Keep in mind that PPO inhibitors would be completely ineffective in waterhemp populations that are resistant to PPO inhibitors.
We suggest taking some time from now into September to scout fields for waterhemp and Palmer amaranth with the goal of preventing seed. If you are lucky enough to have avoided waterhemp, use scouting to maintain this status, and prevent new infestations. If you are currently managing waterhemp infestations, consider late-season removal of plants as an important component of that management plan, and critical to maintaining POST herbicide utility. Scouting should include local roadsides and waterways, and areas of fields subject to flooding or near migratory bird or deer paths. Since combines are an effective dispersal mechanism, check the part of fields first harvested where combines are started up. If you need to harvest fields with waterhemp or Palmer amaranth, harvest these last followed by thorough cleaning of combines, grain carts, semis, etc. These efforts can go a long way toward avoiding future headaches and increased production costs.