Iowa farmers rely mainly on herbicides for weed management. Inconsistent and unsatisfactory weed control is being realized as the spread of herbicide-resistant weed populations has increased. Populations of waterhemp (Amaranthus tuberculatus (Moq.) J.D. Sauer) and giant ragweed (Ambrosia trifida L.) have evolved resistance to glyphosate and other herbicides commonly used in crop fields. This research focuses on the growth and development of multiple herbicide-resistant waterhemp and assessing the evolution of new herbicide-resistant waterhemp and giant ragweed populations.

Significant differences in growth, flowering, accumulated biomass, and seed production were detected when the multiple herbicide-resistant waterhemp populations were compared to a herbicide-susceptible waterhemp population. While statistically significant differences were detected, the small differences are not likely to select herbicide-susceptible waterhemp populations over MHR waterhemp populations. Thus, it can be concluded that plants with multiple herbicide resistances are not likely incurring a fitness penalty and may remain in the agroecosystem.

Currently, very long chain fatty acid- and 4-hydroxyphenylpyruvate dioxygenase (HPPD, EC 1.13.11.27)-inhibiting herbicides are still efficacious on many waterhemp and giant ragweed populations, respectively. Since herbicides impart such a large selection pressure on weed populations, the recurrent use of specific herbicides may decrease efficacy longevity as only herbicide-resistant individuals will remain.