First question I have is..... is your pigweed ALS resistant?

My impression is the pigweed is starting after 1st cutting is removed, when you have open soil.  You may not notice it until after 2nd cutting, but it may be getting started much earlier.  You need to have the Warrant or Prowl applied before the pigweed germinate. So, you need to know when it is getting started.

If you don't have ALS resistant pigweed, I would try the Pursuit - look for a generic version.  Warrant or Prowl can work, but as I said, you need to have them on before pigweed germinate, so I would be included to try application after 1st cutting, not 2nd cutting.

I don't think this pigweed is related to fertility, so I would expect changing fertility to fix this problem.

Are you seeing pigweed on all stands, or just older stands?  In other words, are the stands starting to age, thinning a bit, and these older stands are the ones with the pigweed issues?