Harvest Weed Seed Control (HWSC) concentrates, removes, or destroys weed seeds as they pass through the combine. Seed impact mills are modifications that are mounted directly to the back of a combine and are one way to implement HWSC. Seed impact mills kill weed seeds during harvest, preventing seeds from being added to the soil seedbank. Mills like the Redekop Seed Control Unit (SCU) and the integrated Harrington Seed Destructor (iHSD) could be used in soybean and wheat production in the eastern United States. Understanding the effectiveness and limitations of these mills is important for grower adoption. So, the aim of this research was to evaluate the efficacy of two seed impact mills, the Redekop SCU and the iHSD, in soybean and wheat. The first objective tested general seed kill of problematic species in soybean and wheat and seed kill in adverse conditions, such as high chaff flow rate into the mill and high chaff moisture. Results from objective one indicate that both the Redekop SCU and iHSD killed >98% and >91% of problematic weed seeds in soybean and wheat, respectively. Increases in chaff flow rate and chaff moisture resulted in a decrease in seed kill for specific species depending on the mill. But even at high chaff flow rates, seed kill remained >98% and >77% in soybean and wheat, respectively. At high chaff moisture, seed kill remained >98% and >74% in soybean and wheat, respectively. The second objective evaluated the percentage of weed seeds that bypassed the seed impact mill by exiting the combine in the straw fraction and the percentage of weed seeds that were killed when they entered the seed impact mill during harvest with a commercial combine. Results at field scale indicated that <5% of weed seeds bypassed the seed impact mill by exiting the combine in the straw fraction during harvest in soybean and wheat. Additionally, during a commercial harvest, the seed impact mills killed >99% and >89% of seeds in soybean and wheat, respectively. The third objective monitored population density changes for common ragweed (Ambrosia artemisiifolia) in soybean and Italian ryegrass (Lolium perenne ssp. multiflorum) in wheat following a harvest with a seed impact mill. Results from objective three indicated that in the growing season following a harvest with a seed impact mill, common ragweed density was reduced by 26% and 77% in the spring and fall, respectively, in soybean, and Italian ryegrass density was reduced by 48% in wheat. The fourth objective evaluated Palmer amaranth (Amaranthus palmeri) and its ability to shift its flowering timing in response to HWSC. If weeds flower earlier, they could shatter seeds earlier, and those seeds would bypass HWSC. Through selective breeding, two populations of Palmer amaranth experienced a shift in flowering timing such that the third generations flowered 54.7 and 41.0 days sooner in the greenhouse than the initial generations. In a common garden experiment, the second generations flowered 5.5 and 8.9 days sooner than the initial generations. These results indicate that seed impact mills, like the Redekop SCU and iHSD, can deliver high seed kill rates to a range of weed species at commercial scale in both soybean and wheat. Even in adverse conditions, the mills still killed >74% of seed from tested species. However, weed species can adapt to HWSC selection pressures, resulting in a loss of HWSC efficacy. Overall, this research indicates that seed impact mills are a good tool that growers can implement to reduce the number of weed seeds being returned to the soil seedbank. / Doctor of Philosophy / Herbicide resistance is a growing problem in global crop production systems. Weeds that escape control during the growing season can produce seeds by the time of crop harvest. During harvest, these weed seeds are captured by the combine, separated from the grain, and spread back into the field by the combine. Harvest weed seed control (HWSC) targets these weed seeds as they exit the combine by concentrating, removing, or destroying them. HWSC is a nonchemical weed control method that can be implemented as part of an integrated weed management system. One way to implement HWSC is through the use of seed impact mills, which are aftermarket modifications that can be installed on the back of the combine. These mills process the harvest residue and kill the weed seeds therein. Seed impact mills were initially designed for small grain production systems in Australia, but they have a potential fit in soybean and wheat production systems in the eastern United States as well. The purpose of this research was to evaluate two seed impact mills, the Redekop Seed Control Unit and the integrated Harrington Seed Destructor, for use in soybean and wheat. Stationary mill testing indicated that >98% and >91% of seeds from tested weed species were killed in soybean and wheat, respectively. Additionally, even in adverse conditions, seed kill was >98% for soybean weeds and >74% for wheat weeds. In the field, the results indicated that <5% of seeds were bypassing the mill in the straw fraction and being returned to the field during harvest. Results also indicated that >99% of soybean weed seeds and >89% of wheat weed seeds were killed when they entered the mill during a commercial harvest. Testing the mills in soybean production indicated that common ragweed density was reduced by 26% and 77% in the spring and fall, respectively, in the growing season following the use of a seed impact mill. Likewise, Italian ryegrass density was reduced by 48% following a harvest with a seed impact mill in wheat. The results indicate promise for using seed impact mills in soybean and wheat as a tool to reduce additions to the soil seedbank. However, weeds are known to adapt to management practices, and one way weeds might overcome HWSC is through earlier flowering, potentially leading to weed seeds falling on the ground before harvest and escaping capture by the combine. Through selective breeding, the time to flower for two populations of Palmer amaranth was shortened by 54.7 and 41.0 days in just three generations in the greenhouse, indicating that weeds could potentially adapt to HWSC, making it less effective.