If you want to measure the microbial degradation, you can add different concentrations of the herbicide and to quantify by calorimetry the microbial degradation rates based on the heat rate of that reaction. Then, you can evaluate the role of the herbicide quantity on the microbial metabolism through Monod´s kinetics or Michaelis-Menten equation

If you are interested in non-biological processes this article will help.

L.D. Hansen, E.A. Lewis, D.J. Eatough, R.G. Bergstrom, and D. DeGraft-Johnson, "Kinetics of Drug Decomposition by Heat Conduction Calorimetry," Pharmaceutical Research, 6(1), 20-27 (1988).