Differences in selectivity between bermudagrass and goosegrass (Eleusine indica) to low-rate topramezone and metribuzin combinations

Abstract

Goosegrass [Eleusine indica (L.) Gaertn.] remains problematic for bermudagrass [Cynodon dactylon (L.) Pers.] turf managers due to the ineffective, selective control of mature plants with available POST herbicides and lack of sufficient residual activity from those herbicides to control seedling plants. Topramezone controls mature E. indica, but past efforts to suppress potential injury to bermudagrass turf have been inconsistent. We hypothesized that metribuzin at 210 g ai ha−1 in admixture with topramezone would improve bermudagrass tolerance while conserving mature E. indica control. In preliminary field studies, metribuzin mixed with topramezone at 1.2 or 2.5 g ae ha−1 applied twice at a 3-wk interval reduced bermudagrass injury and white discoloration compared to topramezone applied alone, but metribuzin did not safen bermudagrass to mesotrione. Topramezone at 3.7 g ha−1 plus 210 g ha−1 metribuzin applied twice at a 3-wk interval offered improved bermudagrass tolerance while it still controlled mature E. indica during fifteen field and two greenhouse studies in Virginia. This program offered a 10-fold decrease in suprathreshold duration of white-discoloration compared to topramezone alone at 6.1 g ha−1. Bermudagrass absorbed three times less radioactivity than E. indica at timings up to 48 h after treatment with 14C-topramezone. Bermudagrass also metabolized twice as much topramezone compared to E. indica at 48 h after treatment. Metribuzin reduced 14C absorption by approximately 25% in both species. These studies confirm the performance of a novel, low-dose topramezone plus metribuzin program for mature E. indica control in bermudagrass turf, and suggest that selectivity between bermudagrass and E. indica to topramezone is due to differential absorption and metabolism. The fact that metribuzin reduces topramezone absorption in both species suggests that it may help reduce bermudagrass phytotoxic response to topramezone, but its role in altering selectivity between bermudagrass and E. indica may be due to other factors.

Influence of posttreatment irrigation timings and herbicide placement on bermudagrass and goosegrass (Eleusine indica) response to low-dose topramezone and metribuzin programs

“…At this time, only one peer-reviewed article has evaluated topramezone at the low dose of 6.1 g ha −1 plus metribuzin at 210 g ha −1 , but the researchers did not evaluate posttreatment irrigation. During that study, bermudagrass was injured 74% and similar to the results in the present study (Brewer et al 2021). Lindsey et al (2019Lindsey et al ( , 2020 observed that topramezone applied at 10 g ha −1 plus metribuzin applied at 90 or 100 g ha −1 injured bermudagrass 84% and 79%, respectively, which is more than the maximum injury by topramezone plus metribuzin with no irrigation in our study.…”

“…When posttreatment irrigation was not applied, high-dose topramezone and low-dose topramezone plus metribuzin controlled E. indica greater than 90% and equivalently (Table 2). In previous studies, similar results were observed for E. indica control when topramezone at 12.3 g ha −1 or topramezone (Cox et al 2017) at 6.1 g ha −1 plus metribuzin at 210 g ha −1 (Brewer et al 2021) were applied twice at 3-wk intervals, but Kerr et al (2019b) only observed 66% E. indica control when topramezone was applied once at 12.3 g ha −1 . Unfortunately, a substantial decrease in E. indica control resulted from all posttreatment irrigation timings (Table 2), and this effect worsened with irrigation timings that were more effective at reducing bermudagrass injury responses (Table 1).…”

“…Inconsistencies in E. indica control may occur if irrigation occurs rapidly enough to prevent a lethal dose from being absorbed into the foliage. Topramezone absorbs more rapidly into E. indica than bermudagrass, and metribuzin decreases absorption rate by both species (Brewer et al 2021). The reduced absorption caused by metribuzin admixture (Brewer et al 2021) and dependence on foliar uptake by topramezone to reduce E. indica biomass (Table 4) explain why E. indica control was more severely reduced when posttreatment irrigation followed the mixture (Table 2).…”

“…Weed Science 239 also by roots when grown through hydroponics (Grossman and Ehrhardt 2007). Research has shown that when applied directly to soil, topramezone may only be confined to the top layer, reducing the amount of potential root absorption (Shaner 2014), thus the low topramezone rate present in the soil after application may have limited effect on bermudagrass and E. indica (Begitschke et al 2017;Brewer et al 2021). However, the soil-only contribution of metribuzin is not insubstantial.…”

“…Previous research in Virginia refined the herbicide rates of topramezone plus metribuzin admixture to maximize safety while conserving mature E. indica control (Brewer et al 2021). Performance of this combination was documented at 21 Virginia locations.…”