Richard K. Mann

Penoxsulam—Structure–activity relationships of triazolopyrimidine sulfonamides

The discovery of the sulfonamide herbicides, which inhibit the enzyme acetolactate synthase (ALS), has resulted in many investigations to exploit their herbicidal activity. One area which proved particularly productive was the N-aryltriazolo[1,5-c]pyrimidine sulfonamides, providing three commercial herbicides, cloransulam-methyl, diclosulam and florasulam. Additional structure-activity investigations by reversing the sulfonamide linkage resulted in the discovery of triazolopyrimidine sulfonamides with cereal crop selectivity and high levels of grass and broadleaf weed control. Research efforts to exploit these high levels of weed activity ultimately led to the discovery of penoxsulam, a new herbicide developed for grass, sedge and broadleaf weed control in rice. Synthetic efforts and structure-activity relationships leading to the discovery of penoxsulam will be discussed.

Influence of Flood Interval and Cultivar on Rice Tolerance to Penoxsulam

Studies were conducted in 2003 and 2004 over seven environments evaluating rice root growth inhibition (RGI) and foliar injury from penoxsulam at 30 and 60 g ai/ha and bispyribac-sodium at 30 g ai/ha applied to four- to five-leaf rice at three flood timings, 1, 7, and 14 d after herbicide treatment (DAT), for five rice cultivars, ‘Bengal’, ‘Cypress’, ‘Wells’, ‘Cocodrie’, and ‘XP712’. Flooding at 1 and 7 DAT resulted in greater RGI compared with flood at 14 DAT when evaluated 1 wk after flood (WAF). By 2 WAF, RGI was greater with flooding at 1 DAT compared with flooding at 7 DAT for cultivars Bengal, Cypress, and Wells. Analyzing flood timing 1 DAT, bispyribac-sodium reduced root growth of Bengal and Cypress compared with penoxsulam at 30 g/ha at 1 week after treatment (WAT). At 2 WAT, RGI for Cocodrie was higher following penoxsulam at 60 g/ha when compared with bispyribac-sodium. By 3 WAT, RGI was higher following penoxsulam at 60 g/ha when compared with penoxsulam at 30 g/ha for Cocodrie and greater than bispyribac-sodium and penoxsulam at 30 g/ha for Cypress. Foliar injury following penoxsulam at both rates was less than injury following bispyribac-sodium for all cultivars except XP712 at 1 WAT. XP712 resulted in < 5% RGI and < 6% foliar injury at each evaluation. Rice grain yield was not affected by herbicide treatment for any cultivar compared with the standard treatment of propanil plus quinclorac. Nomenclature: Bispyribac-sodium, penoxsulam, propanil, quinclorac, rice, Oryza sativa L. ‘Bengal’, ‘Cocodrie’, ‘Cypress’, ‘Wells’, ‘XP712’

Weed management practices to control herbicide-resistant Echinochloa crus-galli in rice in Mekong Delta, Vietnam

Barnyardgrass (Echinochloa crus-galli) is one of most important weeds in rice field in Vietnam. Seventy-eight barnyardgrass seed samples were collected from rice fields in seven provinces in Mekong Delta of Vietnam. The seedlings grown from collected seed samples were evaluated to determine the level of resistance to bispyribac, penoxsulam and quinclorac. Farmers who managed the fields from which samples collected were interviewed about weed management practices and the cost of weed control per season. The average cost for weed management was 1, 980, 000 VND per hectare, the total cost included 650, 000 VND for herbicide and 1, 330, 000 VND for hand-weeding. Echinochloa crus-galli biotypes resistant to bispyribac, penoxsulam and quinclorac were found and biotypes with resistance to multiple herbicides were detected. The average resistance-score of three tested herbicides was 3.4 in the 78 samples collected. There was a correlation between field size, hand-weeding practice, and the herbicide-resistance level, herbicideresistant weeds were less common in smaller fields where hand-weeding was commonly used. The high cost and limited labour pool for hand-weeding in this region of Vietnam limits the long-term viability of this approach.

Efficacy of Rinskor™ (florpyrauxifen-benzyl ester) on Herbicide Resistant Barnyardgrass ( Echinochloa crus-galli ) in Rice Fields of Mekong Delta, Vietnam

Barnyardgrass (Echinochloa crus-galli) seed samples were collected in rice fields in different locations at Mekong delta in Vietnam for herbicide resistance tests. The ALS-resistant and synthetic auxin-resistant E. crus-galli were confirmed at several locations in the Mekong Delta. The average LD90 value of bispyribac, penoxsulam and quinclorac for assessed weed populations was 33.1, 15.1 and 550.2 g a.i.ha−1 respectively. There were cross resistant barnyardgrass populations to bispyribac and penoxsulam, the LD90 value of the two ALS inhibitors for E. crus-galli was positively correlated at R2=0.39, the cross resistant population was 33.3% of total sample. The correlation analysis was not useful to evaluate the multiple resistance between quinclorac and the two ALS inhibitors, the R2 value was lower than 0.05, however, the percentage of multiple resistance weed was 36.2% of population. There was no cross resistance or multiple resistance among the 3 tested herbicides and the new synthetic auxin herbicide Rinskor™. All tested weed samples, including quinclorac-resistant populations, were effectively controlled by Rinskor™. There was no difference between control from Rinskor™ in the different herbicide resistant populations. Average LD90 value of Rinskor™ in all tested barnyardgrass was 17.1 g a.i ha−1.