Resistant sprangletop may be next concern

I have been writing on the theme “the weeds are talking, is anybody listening?” I have been focused on barnyardgrass, but there is a different weed talking this week.

I stopped by the Altheimer Laboratory in Fayetteville, Ark., a couple of weeks ago to visit old colleagues. Jason Norsworthy informed me of a couple of sprangletop samples sent to him from Louisiana that had tested resistant to Clincher. In fact, at least one of the samples blew right through a 10X rate.

I discussed these and received permission from Eric Webster from LSU to write about them. He said the samples were from two different parishes in Louisiana. He believes they probably are resistant, but he feels more work is need to actually confirm the resistance.

Apparently the sprangletop had been blowing right through both Clincher and Ricestar HT, and that is the reason the samples were pulled. As I have said about barnyardgrass, it doesn’t really matter if you confirm resistance or not if you can not kill it.

The discussions with Jason and Eric started me thinking about sprangletop and a lot of calls I received this past growing season. There is no doubt that sprangletop, especially tighthead sprangletop, has been increasing in Arkansas. Tighthead sprangletop is more difficult to control with most herbicides than is loosehead sprangletop. This is likely causing a shift to more tighthead.

In addition, Newpath and Beyond are weak on both species, so the increase in Clearfield acres is likely causing a shift to more sprangletop as well.

It is hard to draw a lot of conclusions from a crazy year like this year. However, I looked at sprangletop problems I felt Command should have controlled. I also looked at fields and received calls about both loosehead and tighthead sprangletop that was not controlled with either Clincher or Ricestar HT.

At the time I was thinking, “How can you not kill loosehead sprangletop under good conditions with Ricestar HT or Clincher?” I attributed it to the crazy year and it may well have been just a fluke year.

However, these samples from Louisiana sure raise a red flag.

While I am concerned about barnyardgrass first, we have fewer herbicide options for sprangletop control. Command has excellent pre-emergence activity on both sprangletop species. If it escapes, however, Ricestar HT and Clincher have been the primary postemergence herbicides used for control. If we lose these to resistance, we have a big problem.

I mentioned RiceBeaux in a recent article. It is a mixture of propanil and thiobencarb, which you know as Bolero. Thiobencarb has pretty good sprangletop activity and RiceBeaux provides an alternative to Ricestar HT and Clincher for sprangletop control. For thiobencarb to be effective, however, the sprangletop must be tiny. Actually thiobencarb has better soil residual activity on sprangletop than it does postemergence activity.

Once you get past Command, Clincher, Ricestar HT and RiceBeaux, we essentially have no sprangletop herbicides. If the resistance to Clincher and Ricestar HT is confirmed, and if it spreads quickly like it could being a point mutation, we have a problem.

The weeds are talking. I am an optimist and I think we can do a lot with our present weed control programs in rice. The point I am trying to make in these articles is I believe we are living on borrowed time with our current herbicides. We can continue to juggle things around for a while, but weed control likely is going to get much more difficult without some new technology.

Without new technology, the end result could be a shift away from rice to crops where better weed control technology is available.

Next week I will contrast $30 and $100 per acre weed control programs.

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