Not with Barley, but I did DEX treatments in several dicots. Now from memory..... I think used 5µM DEX, but be sure to add a surfactant so the solution (spray treatment) forms a film on the plant material. I used Sillwet. You can get this stuff relative as Silwett gold, used on large scale in Agriculture and its nearly the same purity as the very expensive Silwett L77. Two sprays per week work fine for long term treatments.