Competition of green foxtail [Setaria viridis(L.) Beauv.] was studied in a semi-dwarf wheat (Triticum aestivumL. ‘Norquay’) and in two normal-height wheats (‘Napayo’ and ‘Sinton’) from 1975 through 1978. Green foxtail suppressed wheat growth as well as grain yield. Tiller number, leaf area, and dry weight of wheat were reduced. Green foxtail was more competitive in the semi-dwarf variety than in either normal height variety. The intensity of green foxtail competition could not be determined by density alone. In 1975, as few as 100 green foxtail plants/m2significantly reduced yield of Napayo and Norquay wheat by 21 and 44%, respectively. In 1977, however, 1600 green foxtail plants/m2did not reduce the yield of Sinton wheat significantly. The intensity of green foxtail competition was highly variable from one date of seeding to the next, but there was no correlation between the level of green foxtail competition and the date of seeding. Soil temperature and moisture at the time of seeding and early growth are thought to affect green foxtail competition in wheat critically.

The interaction between diclofop-methyl {2-[4-(2,4-dichlorophenoxy)phenoxy] methyl propanoate} and 2,4-D [(2,4-dichlorophenoxy)acetic acid] was evaluated. Foliar applications of the two herbicides in a tank mixture were antagonistic with respect to wild oat (Avena fatuaL.) control. Application of mixtures of the two herbicides to wild oat roots resulted in an additive herbicidal effect. The acid form of 2,4-D was determined to be the component of the 2,4-D formulation that reduced the toxicity of foliar-applied diclofop-methyl to wild oat. Analysis of diclofop-methyl solutions with and without added 2,4-D revealed no degradation products of dichlofop-methyl, nor any evidence of complexing between diclofop-methyl and 2,4-D. Addition of 2,4-D to the diclofop-methyl spray solution did not affect diclofop-methyl spray retention by, or penetration into, wild oat leaves. Movement of radioactivity to roots and to shoot apices following application of14C-diclofop-methyl to wild oat leaves was reduced by addition of 2,4-D to the treatment solution. Deesterification of diclofop-methyl was inhibited by 2,4-D. The accumulation of diclofop-methyl in these wild oat leaves resulted in membrane damage and leaf necrosis. Symplastic movement of the des-methyl acid, diclofop, to sensitive meristematic areas was reduced. As insufficient toxicant reached meristematic areas to permanently interrupt meristematic activity, the wild oat plants were able to outgrow the contact damage to their leaves.

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The selectivity of {2-[4-(2′,4′-dichlorophenoxy) phenoxy] methyl propionate}, (hereinafter referred to as dichlofop methyl), among wheat (Triticum aestivumL. ‘Neepawa’), barley (Hordeum vulgareL. ‘Bonanza’), wild oat (Avena fatuaL.), and green foxtail (Setaria viridis(L.) Beauv.) was investigated. On an ED50basis, barley, wild oat, and green foxtail were 2, 190, and 1,090 times more sensitive, respectively, to foliar-applied dichlofop methyl at the two-leaf stage than was wheat. Selectivity decreased with increasing maturity of the plant material with the ratio of selectivity between barley and wild oat decreasing from 55 at the two-leaf stage to three at the four-leaf-plus-one-tiller stage. Greater spray retention and more rapid penetration of dichlofop methyl partially explained the susceptibility of green foxtail, but did not explain selectivity between wheat, wild oat, and barley. Root uptake of14C-dichlofop methyl by the four species was proportional to the amount of solution absorbed during the treatment period and to the concentration of dichlofop methyl in the treatment solution but was not related to species sensitivity to this herbicide.

Ethephon [(2‐chloroethyl)phosphonic acid] effectively reduces lodging in barley (Hordeum vulgare L.), but also may have positive or negative influences on grain yield and yield components even if lodging does not occur. The objective of this study was to investigate the effects of ethephon on yield and yield components of barley under western Canadian conditions. In 1987, 1988, and 1989 at Winnipeg MB, ‘Argyle’ conventional height and ‘Samson’ semidwarf barley were grown at 100 and 300 plants m−2 and treated with ethephon at Zadoks growth stage 35 or 45. At harvest, grain yield samples were taken by hand or by combine, and yield components were determined. When lodging occurred, Belgian lodging indices were determined. The two cultivars responded similarly to ethephon application. Hand‐harvested grain yields were either unaffected or reduced by ethephon, but combine‐harvested yield tended to be increased by ethephon in 1989 when lodging was severe in untreated plots. Reductions in hand‐harvested grain yield of ethephon‐treated plots were primarily due to reductions in kernels per spike, which occurred in all 3 yr for barley grown at both plant densities. Ethephon increased spikes per plant for barley grown at both 100 and 300 plants m−2 in 1987, which at the low plant density compensated for reductions in kernels per spike and kernel mass. Barley in ethephon‐treated plots tended to be delayed in maturity compared with untreated plots. This was particularly true in 1987 and 1989, when late tiller emergence was promoted to the greatest extent by ethephon when moisture conditions were favorable. The potential for ethephon to cause yield reductions restricts the use of ethephon in western Canada to situations where the risk of severe lodging is high.