I know that it seems a little late to be talking about early post-emergent cotton herbicides, but with much of the cotton being late this year there is still a lot of cotton acreage that is getting these sprays now.
I came across a field earlier that had some large pigweeds that had been sprayed with a glufosinate herbicide (i.e. Ignite or Liberty). See picture below.
You can see that the herbicide did burn the pigweed back, but it has since started greening up and will come back. These pigweeds were 12-14 inches tall when the herbicide was applied which is too late. These products must be applied on time in order to achieve control of pigweed.
Here are some guidelines from UGA Weed Scientist Stanley Culpepper, on using (and protecting) glufosinate herbicides.
1) Do not make more than 2 applications of glufosinate per year.
2) Spray when the biggest pigweed in the field is 3 inches or smaller.
3) Never ever use a reduced rate!
4) To maximize activity, wait 1.5 hours after sunrise to begin spraying and stop spraying at least 1 hour before sunset.
5) Apply at a minimum of 15 GPA using a speed, spray tip, and pressure that delivers a medium spray droplet.
6) Integrate herbicide programs with hand weeding, tillage, and/or heavy rye cover crop residues.
If you have any questions about using these herbicides, please do not hesitate to give Brock, Jay, or myself a call.