Dr. Jim Brosnan, UT turf weed scientist, has just confirmed that annual bluegrass (Poa) he collected from a golf course back in 2010 in West Tennessee is glyphosate-resistant (GR). In his study, they took mature tillers of Poa from the golf course and established them in a greenhouse. They also took known glyphosate susceptible Poa from the East Tennessee Research Center in Knoxville and did the same. They allowed the Poa to become acclimated for 5 weeks in the greenhouse and then conducted the glyphosate tolerance test. The glyphosate rates tested were 0.25, 0.5, 1, 2, 4 and 8x the normal use rate (x rate = 0.75 lbs ae or 22 ozs Roundup Power). The 8x rate provided only 74% control of the Poa from the West Tennessee golf course 14 days after application. In contrast, the known susceptible Poa showed greater than 95% control with 0.5x rate. The level of resistance of the West Tennessee biotype was 12 fold. This is the second documented instance of GR Poa, with the other being up in central Missouri last year.
Antidotal evidence I have gathered would suggest that the GR Poa is not isolated to this one golf course. Over the last several years I have gotten more calls on this grass not being controlled by glyphosate in burndown applications. I had some suspicion that overall the Poa in West Tennessee was showing more tolerance to glyphosate. However, since glyphosate is always tankmixed with a dicamba, 2,4-D and/or a Valor application in the spring, it was hard to determine if the lack of control was due to tolerance or antagonism. In light of Dr. Brosnan’s research I would speculate that both tolerance and antagonism are working together to cause the poor Poa control in some of our row crop fields.
This is now the 5th confirmed GR weed in Tennessee and the 9th species documented in the Mid-South. Compared to GR Palmer, Poa is not a huge concern. In fact, the standard practice now of applying essentially a second burndown application of paraquat along with a residual for control of GR Palmer will control escaped Poa. Where Poa can be a problem is trying to maintain a good depth when planting cotton. Large tufts of it can bounce a planter unit causing shallow seed placement. Cotton seed is way too expensive to have planted on top of the soil. So in fields where there is heavy Poa pressure, consider applying paraquat with the early preplant burndown rather than just behind the planter. With an earlier application, it should be burned down enough to not cause so much planting issues.
