Diclofop‐methyl antagonism by broadleaf weed herbicides: the importance of leaf expansion rate

Abstract

Summary: Avena saliva cv. Amuri and A. fatua were sprayed with diclofop methyl (1.0 kg a.i. ha−1) alone and in combination with 2,4‐D (1.1 kg a.i. ha−1), bentazone (1.0 kg a.i. ha−1), chlorsulfuron (15 g a.i. ha−1) or dicamba (0.3 kg a.i. ha−1). Effects of the herbicides on leaf extension rate during the first 8 to 10 days after spraying and subsequent growth (dry weight) after 57–75 days were determined by comparison with unsprayed plants. Diclofop‐methyl applied alone did not cause a decrease in leaf extension rate of A. saliva or A. fatua until at least 4 days after spraying. All broadleaf weed herbicides in combination with diclofop‐methyl caused a decrease in leaf extension rate of both species within 2 days of spraying. Ten days after spraying, leaf extension rates for plants sprayed with a broadleaf weed herbicide plus diclofopmethyl (all combinations) were lower than for unsprayed plants but greater than for plants sprayed with diclofop‐methyl alone. With the exception of A. fatua sprayed with bentazone, long‐term growth of plants sprayed with a broadleaf weed herbicide plus diclofop‐methyl (all combinations) was lower than for unsprayed plants but greater than for plants sprayed with diclofop‐methyl alone. Bentazone applied with diclofop‐methyl caused a substantial decrease in leaf extension rate of A. fatua within 24 h of spraying but at harvest, dry weight of plants from this treatment was similar to or less than that for plants sprayed with diclofop‐methyl alone. Application of diclofop‐methyl with bentazone at a rate of 0.3 kg a.i. ha−1 also caused a reduction in leaf extension rate of A. fatua within one day of spraying. At this rate of bentazone, dry weight of plants at harvest was intermediate to that of unsprayed plants and those sprayed with diclofop‐methyl alone. It is proposed that decreased leaf expansion rate during the first few days afte spraying is the cause of broadleaf weed herbicide antagonism of diclofop‐methyl.

Leaf Extension Rate May Help Determine When Low Wild Oat Herbicide Rates Will Be Effective1

“…Therefore, depending on the spray date, these growth rates represent approximately three-or fourfold differences in wild oat growth rates. Andrews (1990) found that the most rapidly expanding wild oat leaf grew approximately 30 mm/d. At Lethbridge in 1996, wild oat growth rates increased from 8 mm/d on July 4 to 20 mm/d on July 5 (Table 3).…”

“…In this case, wild oat control with 100 g/ha of imazamethabenz increased only 6% for every 10 mm of wild oat growth in the 24 h preceding herbicide application (data not shown). Andrews (1990) found that some broadleaf herbicides reduce wild oat leaf extension rate (growth) faster than diclofop. He postulated that decreased wild oat growth rate during the first few days after spraying is the cause of broadleaf-herbicide antagonism of diclofop.…”

“…It occurred to us that for monocot species, such as wild oat, we could get an indication of growth rate by more simple measurements. Andrews (1990) used calipers to measure daily leaf extension rates in wild oat and cultivated oat (Avena sativa L.) to predict herbicide antagonism. In this study we used a ruler for similar measurements of leaf extension rate to indicate wild oat growth rate.…”

Evidence Against a Direct Membrane Effect in the Mechanism of Action of Graminicides

“…An additional hypothesis to explain the antagonistic effect of auxin-like herbicides on the activity of diclofop was provided by Andrews (2). He showed that 2,4-D and dicamba (3,6-dichloro-2-methoxybenzoic acid) caused a rapid and significant reduction in the rate of leaf extension in wild and cultivated oats.…”

Chlorsulfuron and tribenuron antagonize tralkoxydim activity

“…The mechanisms responsible for herbicide antagonism have been classified by Hatzios and Penner (1985) as biochemical, competitive, physiological andlor chemical. Antagonism of postemergence graminicides (aryloxyphenoxy propionic acids and cyclohexanediones) used for grass weed control in combination with sulfonylureas is well-documented (Baerg et al, 1996;O'Sullivan & Kirkland, 1984;Liebl & Worsham, 1987;Andrews, 1990). According to Penner ( 1989) adjuvants can affect herbicide antagonism in at least two ways, namely to increase herbicide absorption directly and to prevent the formation of less preferred absorption forms of weakly acidic herbicides.…”