Werner, Kaisa Marie (2018-08). Management Considerations for Late-Season Escapes of Palmer Amaranth and Common Waterhemp in Texas Cotton. Master's Thesis.
Thesis
Herbicide resistance in weeds is an emerging problem across Texas, and a strong emphasis on weed seedbank management is vital for addressing this problem. Escaped weeds present in the late-season at crop harvest (weeds that escape control measures early-season and/or the ones that recruit after control interventions have been terminated) can tremendously contribute to seedbank addition, but little is known on the level of seed input from such escapes, particularly that of Palmer amaranth and common waterhemp - two of the most problematic and herbicide resistance prone weed species in cotton production in Texas. Moreover, effective methods to reduce viable seed production from late-season escapes also need to be developed. A state-level survey was conducted to quantify seed production in late-season escapes of Palmer amaranth (Amaranthus palmeri S. Wats) and common waterhemp (A. tuberculatus (Moq.) Sauer) across important cotton production regions in Texas (objective 1). The survey revealed that Palmer amaranth densities and seed production were greater in the High Plains region (6.4 to 13.9 million seed ha^-1), whereas waterhemp was predominant in the Blacklands and the Upper Gulf Coast regions, with 12.9 and 9.8 million seeds ha^-1, respectively. In addition, the three most common weeds documented in Texas cotton were Palmer amaranth, Texas millet [Urochloa texana (Buckl.)] and common waterhemp. Further, experiments were carried out in College Station and Lubbock to understand the effect of different cotton defoliants on the viable seed production potential of Palmer amaranth, when applied at different maturity stages (green inflorescence, white seed, brown seed and black seed) (objective 2). Results indicated that paraquat, MSMA, diuron and glufosinate provided the greatest seed mortality compared to a number of other defoliants evaluated. Findings suggest that certain defoliants may be used to provide the dual benefit of crop harvest aid as well as reducing the seed viability of late-season weed escapes. Future experiments in a controlled environment could provide more insights on the impact of these desiccants on seed viability of Palmer amaranth.
