Biology and Chemical Control Strategies for Managing Knotroot Foxtail [Setaria parviflora (Poir.)] in Forage Systems

Abstract

Knotroot foxtail is a perennial grass weed that reproduces by both seeds and rhizomes, making management difficult, as new shoots can emerge from underground rhizomes. There is a knowledge gap that exists on the time it takes knotroot foxtail to develop a rhizome following seedling emergence. Additionally, its close resemblance to yellow foxtail complicates identification and hinders effective management. A greenhouse and field study were conducted to investigate the developmental stages of knotroot foxtail from seed to rhizome and to distinguish it morphologically from yellow foxtail. Results showed that 50% of the knotroot foxtail population emerged by 8 days after sowing, while 95% of the population emerged by 14 days. These emerged seedlings transitioned into rhizome formation between 7 and 13 weeks after planting. At the Beef Unit, 50% of the knotroot foxtail population developed rhizomes at plant heights of 12.7–24.1 cm and the 1–2 leaf stage, while at the Turf Unit, the populations had formed at 40.6–52.6 cm and the 3–5 leaf stage. The average rhizome-root biomass when rhizome formation occurred among 95% of the knotroot foxtail plant population was 37.52 - 92.19 g. Time to seedhead formation among the knotroot foxtail population occurred between 59.8 and 66.8 days after planting. For the morphological differentiation study between knotroot and yellow foxtail, Principal Component Analysis (PCA) identified quantitative traits such as first true leaf width, spikelet width, spikelet length, internode length, as distinguishing features between knotroot foxtail and yellow foxtail. In addition, qualitative traits including leaf margin roughness, leaf color, and seed size also differed between the two species. However, a single morphological trait alone might not suffice for identification as there might be need to combine these traits for effective identification especially when plants are young. Hexazinone and quinclorac are two herbicide active ingredients shown to have efficacy on controlling knotroot foxtail, but their performance is influenced by soil moisture and rainfall activation. Understanding how soon rainfall is needed after herbicide application of these herbicides is essential for maximizing control and to manage for desirable forage species such as bermudagrass, bahiagrass, and tall fescue. Greenhouse studies in 2023 and 2024 at Auburn University evaluated knotroot foxtail response of knotroot foxtail to these herbicides under varied rainfall timings. Knotroot foxtail rhizomes were transplanted into pots and allowed to grow until foliage reached an average height of 28cm before being treated with 0.42 kg ae ha-1 quinclorac and 0.85 kg ai ha-1 hexazinone. Simulated rainfall of 0.63 cm (0.25 inches) was applied at 0, 3, 6, 9, 12, and 15 days after herbicide treatment (DAT). Hexazinone consistently reduced rhizome dry weight more than quinclorac across both years. In 2023, rhizome weight was 1.88 g in hexazinone-treated plants compared to 3.24 g with quinclorac. In 2024, knotroot foxtail treated with hexazinone had lower rhizome dry weight (1.02 g), followed by quinclorac at 2.09 g, while the non-treated control had the highest rhizome dry weight (3.58 g). Similarly, across both years, hexazinone consistently caused higher knotroot foxtail control compared to quinclorac, with control levels of 93% and 82% at 51 days after rainfall treatment (DART) in 2023, and 94% and 79% at 51 DART in 2024, respectively. At 51 days after rainfall treatment (DART), knotroot foxtail plants treated with quinclorac exhibited control levels ranging from 87% to 77% when rainfall occurred within 0–9 days after application. However, control declined to between 67% and 62% when rainfall was delayed until 12–15 days after treatment. Early rainfall (0–6 days) ensured optimal herbicide efficacy, while late rainfall (12–15 days) reduced their effectiveness. This study provides insight into the biology of knotroot and yellow foxtail and highlights how to optimize the efficacy of the limited herbicide options available for knotroot foxtail control in pastures and hayfields.