Introduction

Herbicides that inhibit the production of the branched chain amino acids valine, leucine and isoleucine are used for total vegetation management and selective weed control in a wide variety of crops.  There are currently four different chemical families that share this MOA (mode of action).  Before the development of glyphosate-tolerance crop technology, branched chain amino acid inhibitors were the mainstay for several major row crops.  While this is still a very important herbicide MOA, the major increase in herbicide resistance weeds since 1980 has been the direct result of selection pressure from these herbicides.  There are currently more weed species resistant to branched chain amino acid inhibitors than any other herbicide MOA.

This elesson will help learners understand the following:

1.  Where branched chain amino acid production occurs in plants and why it is important for normal plant growth.

2.  What the basic chemical structures are for herbicides that inhibit branched chain amino acid production.

3.  How these herbicides interact with the target site, acetolactate synthase or (ALS).

4.  How resistance occurs and how gene flow contributes to the spread of herbicide resistance.