Benzobicyclon Activity on Common Louisiana Rice Weeds

Abstract

A study was conducted at three locations in Louisiana to evaluate the response of common Louisiana rice weed species to different rates of application of benzobicyclon herbicide. Benzobicyclon was applied at 31, 62, 123, 185, 246, 493, 739, 986, and 1,232 g ai ha–1into flooded field conditions when ducksalad was at the first elongated-leaf stage. Barnyardgrass, false pimpernel, and yellow nutsedge control never exceeded 50% from any rate of benzobicyclon applied, averaged across evaluation timing. Ducksalad control, averaged across evaluation timing, was 83% when treated with 493 g ha−1and did not increase when treated with higher rates of benzobicyclon. At 42 d after treatment (DAT), purple ammannia and Indian toothcup treated with 185 and 246 g ha–1of benzobicyclon were controlled 58% and 81%, respectively, and did not differ in control compared with higher rates of benzobicyclon. All weeds were hand-harvested from each plot and separated by species at the conclusion of the study. No differences in fresh-weight biomass were observed for barnyardgrass, false pimpernel, purple ammannia, or yellow nutsedge. Treatment with benzobicyclon at ≥62 g ha–1resulted in reduced ducksalad fresh weight 42 DAT compared with the nontreated sample. Indian toothcup fresh weight was reduced 77% to 96% compared with the nontreated sample when treated with benzobicyclon at 246 to 1,232 g ha–1.

Spray deposition, adjuvants, and physiochemical properties affect benzobicyclon efficacy

“…Control of four- to five-leaf Amazon sprangletop and three- to four-leaf barnyardgrass with benzobicyclon alone at 371 g ha −1 was 67% and 27%, respectively. Research has shown benzobicyclon has considerably more activity on species of sprangletop than on barnyardgrass, and our data further highlight this point (Komatsubara et al 2009; McKnight 2017; Young et al 2018b; CA Sandoski, personal communication). An adjuvant effect was detected in visible control estimates and dry weight reduction of Amazon sprangletop and barnyardgrass (Table 2).…”

“…If bound to soil particles, benzobicyclon activity would be reduced because it would be unavailable for plant uptake, and conversion to benzobicyclon hydrolysate would be significantly hindered. These physicochemical properties may help explain why McKnight (2017) and Norsworthy et al (2014) found significantly reduced benzobicyclon activity when benzobicyclon was applied to saturated soil versus applied to flood water. Thus, we hypothesize an oil-based adjuvant will help keep the lipophilic compound benzobicyclon in suspension when applied to flood water and, additionally, will increase the probability benzobicyclon is converted to the systemic herbicide benzobicyclon hydrolysate.…”

“…Benzobicyclon has POST activity on several problematic and acetolactate synthase inhibitor–resistant grass, sedge, broadleaf, and aquatic weeds commonly found in rice fields, and it is especially effective on sprangletops, annual sedges, and ducksalad (Komatsubara et al 2009; McKnight 2017; Young et al 2018b). Flood establishment in relation to benzobicyclon application timing and flood depth is an important factor in maximizing benzobicyclon efficacy (McKnight 2017; Norsworthy et al 2014; Young 2017).…”

“…Benzobicyclon has POST activity on several problematic and acetolactate synthase inhibitor–resistant grass, sedge, broadleaf, and aquatic weeds commonly found in rice fields, and it is especially effective on sprangletops, annual sedges, and ducksalad (Komatsubara et al 2009; McKnight 2017; Young et al 2018b). Flood establishment in relation to benzobicyclon application timing and flood depth is an important factor in maximizing benzobicyclon efficacy (McKnight 2017; Norsworthy et al 2014; Young 2017). For example, in a greenhouse trial, Young et al (2018b) showed control of one- to two-leaf Amazon sprangletop, barnyardgrass, rice flatsedge ( Cyperus iria L.), and yellow nutsedge ( C. esculentus L.) was improved an average of 47 percentage points to 92% when applied to a 5- or a 15-cm flood versus applications to a saturated field soil (45% control).…”

“…For example, in a greenhouse trial, Young et al (2018b) showed control of one- to two-leaf Amazon sprangletop, barnyardgrass, rice flatsedge ( Cyperus iria L.), and yellow nutsedge ( C. esculentus L.) was improved an average of 47 percentage points to 92% when applied to a 5- or a 15-cm flood versus applications to a saturated field soil (45% control). In addition, McKnight (2017) and Norsworthy et al (2014) found benzobicyclon was most effective when applied 24 h after the establishment of a continuous flood at pegging rice or five-leaf rice, respectively. The half-life (i.e., hydrolysis rate) of benzobicyclon in water at a pH of 7 and a temperature of 25 C is 15 h; under these conditions, the flood would need to be held for >4 d for benzobicyclon to undergo complete hydrolysis (Williams and Tjeerdema 2016).…”