Palmer amaranth (Amaranthus palmeri) and common ragweed (Ambrosia artemisiifolia) are two of the most troublesome weeds in soybean. Both weeds possess widespread resistance to glyphosate and acetolactate synthase (ALS) inhibiting herbicides resulting in the use of protoporphyrinogen oxidase- (PPO) inhibitors to control these biotypes, although PPO-resistant biotypes are increasing. New soybean herbicide-resistant trait technologies enable novel herbicide combinations. Combinations of two herbicide sites-of-action (SOA) improved control 19 to 25% and 14 to 19% of Palmer amaranth and common ragweed, respectively, versus using one SOA (mesotrione, dicamba, 2,4-D, or glufosinate alone). Seed production of 5 to 10 cm Palmer amaranth and common ragweed was reduced greater than 76% by fomesafen, auxin (dicamba and 2,4-D), or glufosinate containing treatments. Some weeds survived and set seed even when treated at the proper size. As weed size increased from 10 to 30 cm, control diminished and fecundity increased, underscoring the importance of proper herbicide application timing. Effective preemergence herbicides reduced the number of weeds present at the postemergence application compared to no treatment, reducing the likelihood of herbicide resistance development. Dicamba, 2,4-D, or glufosinate applied alone or auxin + glufosinate combinations reduced Palmer amaranth seed production greater than 95% when applied at first visible female inflorescence; this first report, in addition to previous reports on individual herbicides, indicates this application timing may be useful for soil seed bank management. This research informs mitigation of herbicide resistance spread and development. / Master of Science in Life Sciences / Over 30 million hectares of soybeans were harvested in 2019 in the United States, totaling over $31 billion in value. Two of the most troublesome weeds in soybean, Palmer amaranth (Amaranthus palmeri) and common ragweed (Ambrosia artemisiifolia) can cause even greater yield reductions in soybean, up to 79 to 95%, respectively. Frequent, exclusive, and repeated use of a single herbicide has led to multiple herbicide-resistance in both of these weeds. Co-applying two effective herbicides reduces the likelihood of resistance development. New soybean varieties have been genetically modified for resistance to herbicides that were previously unusable, allowing new herbicide combinations. Research was established to investigate these herbicide options to control and reduce seed production of Palmer amaranth and common ragweed with the overarching goal of mitigating herbicide resistance, particularly resistance to protoporphyrinogen oxidase (PPO) inhibiting herbicides, which are a critical part of herbicide options in soybean production. Preemergence herbicides are vital tools in herbicide programs, reducing the number of weeds present at a postemergence application and thereby reducing the risk of herbicide resistance development to the postemergence herbicide. PPO herbicides (flumioxazin, sulfentrazone, or fomesafen) applied preemergence reduced Palmer amaranth and common ragweed density at the postemergence application 82 to 89% and 53 to 94%, respectively. The preemergence herbicide used did not affect control four weeks after the postemergence herbicides were applied. Postemergence herbicides were applied targeting three weed heights: 5 to 10 cm (ideal), 10 to 20 cm, and 20 to 30 cm. Control decreased as weed height increased and larger weeds had greater biomass and seed production, underscoring the importance of proper herbicide application timing. The single site-of-action treatments dicamba, 2,4-D, glufosinate, or fomesafen resulted in greater than 85 and 92% morality of 5 to 10 cm Palmer amaranth and common ragweed, respectively. Palmer amaranth and common ragweed control improved by 19 to 25% and 14 to 19%, respectively, when using two herbicide sites-of-action increased versus using one SOA (mesotrione, dicamba, 2,4-D, or glufosinate alone). The use of two herbicide sites of action resulted in maximum biomass reductions, depending on weed height, of 57 to 96% and 73 to 85% for Palmer amaranth and common ragweed, respectively. Dicamba, 2,4-D, glufosinate alone and in combination with fomesafen reduced seed production (relative to the nontreated) of 5 to 10 cm Palmer amaranth and common ragweed greater than 98 and 76%, respectively. Dicamba, 2,4-D, and glufosinate applied alone or auxin (dicamba and 2,4-D) and glufosinate combinations reduced Palmer amaranth seed production greater than 95% when applied at first visible female inflorescence. This indicates that these herbicides may be useful in soil weed seed bank management. This research reinforces the utility of PPO herbicides for preemergence control and their efficacy postemergence when combined with another effective herbicide, a practice known to reduce herbicide resistance development. This research also reinforces the potential for dicamba, 2,4-D, or glufosinate to reduce weed seed production when applied at a delayed timing. Future research should investigate the progeny of these weeds treated with herbicides at a delayed timing to evaluate the potential for this practice to reduce herbicide resistance development.