Palmer Amaranth in South Dakota: Growth, Herbicidal Control, and Soybean Yield Loss

Palmer amaranth is a growing concern in the United States. Previously thought to only be able to occupy the southern United States, this plant can now be found throughout the northern states as well. Infestations of Palmer amaranth can now be found in South Dakota and is raising many concerns. Palmer amaranth is characterized by large growth and can be highly competitive with many important crops. Soybean is an important crop in South Dakota, as well as the rest of the world, and has not escaped the detrimental aspects of an infestation of Palmer amaranth. The objectives of this study were to determine the possible impacts Palmer amaranth South Dakota. Surveys were given to applicators and producers from many counties in South Dakota to gauge public awareness of Palmer amaranth and determine other possible infestations of Palmer amaranth. These surveys were made available at commercial applicator recertification classes throughout South Dakota and the Soy 100 meeting in Brookings, SD. Growth rates and plant volume and biomass of Palmer amaranth from several seed source locations and local ascensions of common waterhemp and redroot pigweed were examined and compared in eastern South Dakota. Growth studies were conducted near Aurora, SD over two years using three planting dates from mid-May to late-June. Plant volume was measured every 10 to 20 days until harvest beginning in late-July. At harvest, plants were oven-dried and biomass was recorded. Efficacy of several herbicide treatments were recorded on Palmer amaranth seedlings. Pre- and post-emergence treatments were conducted on Palmer amaranth planted in either sand or potting mix. Post-emergence treatments were applied at the three- to four-leaf stage. Visual ratings of plants were conducted 21 days after treatment. Soybean yield loss due to Palmer amaranth was determined near Corsica, SD. Palmer amaranth in square meter plots were counted and harvested for biomass when soybeans reached R7 to R8. Plots containing two rows of soybeans were harvested several weeks later and yield loss was determined. Survey results indicated that more needs to be done to provide information to the public based on respondents’ ability to correctly identify Palmer amaranth, common waterhemp, and redroot pigweed seedlings and mature plants. Several respondents also indicated possible infestations of Palmer amaranth. Not all counties in South Dakota were represented by the study. Palmer amaranth had greater growth and biomass than either common waterhemp or redroot pigweed. Final volume of Palmer amaranth was greater at lower densities. Growth rates between sampling dates varied among planting dates, which resulted in similarities in final volume among planting dates. Common waterhemp and redroot pigweed shared similar plant volumes and biomass, however, plants in 2015 were larger, possibly due to climatic differences between years. Herbicides tested that offered the best control of Palmer amaranth was a preemergence application of S-metolachlor and a post-emergence application of either dicamba or glufosinate. Glyphosate only provided partial control and mesotrione provided variable control. Atrazine had the little control as a pre- or post-emergence treatment. Thifensulfuron had no control of Palmer amaranth. Soybean yield loss in 2016 determined an incremental loss of 9% at one Palmer amaranth m-2. Maximum yield loss of 45% was seen at 15 plants m-2, however, yield losses at densities slightly lower and higher caused a 35% maximum yield loss prediction. Yield loss in 2017 was variable due to outside factors and a relationship between yield loss and Palmer amaranth density or biomass could not be determined