Velvetleaf soybeans

A variety of chemicals may be leached from the aerial portions of plants by rainwater or by fog-drip (16) including organic acids, sugars, amino acids, pectic substances, gibberellic acids, terpenoids, alkaloids, and phenolic compounds. Colton and Einhellig (17) suggested that leaf leachates of velvetleaf (Abutilon theophrasti) may be inhibitory to soybean (Glycine maxT We have recently discovered specialized hairs on the stems of velvetleaf plants which exude toxic chemicals. 

The reported (14) mechanisms of action of allelochemlcals Include effects on root ultrastructure and subsequent Inhibition of Ion absorption and water uptake, effects on hormone-induced growth, alteration of membrane permeability, changes In lipid and organic acid metabolism, inhibition of protein synthesis and alteration of enzyme activity, and effects on stomatal opening and on photosynthesis. Reduced leaf water potential Is one result of treatment with ferulic and p-coumaric acids (15). Colton and Einhellig (16) found that aqueous extracts of velvetleaf (Abutllon theophrastl Medic.) Increased diffusive resistance In soybean fGlycine max. (L.) Merr.] leaves, probably as a result of stomatal closure. In addition, there was evidence of water stress and reduced quantities of chlorophyll In Inhibited plants. 

The remainder of this discussion examines the possibility of a direct feedback mechanism in response to light as an explanation for crop yield reduction from early weed presence. Three types of data will be examined 1) results of our research on velvetleaf interference with light in soybeans 2) a comparison of observed and estimated soybean yield reductions for weed presence versus leaf removal and 3) the poor correlation between weed control and crop yields. 

Velvetleaf interference in soybeans. In 1984, a field study was conducted on the Agronomy farm near Columbia, Missouri to examine the effect of velvetleaf height and duration on soybean yields. 

Figure 8. Effects on Soybean Yields of Three Velvetleaf Heights Maintained for the First Six Weeks after Soybean Emergence Compared to No Height Control (left bar) and Weed Removal for the Entire Season (right bar). Velvetleaf height (15%, 100% and 125%) is relative to soybean height.

Colton, C. E. and Einhellig, F. A. 1980. Allelopathic mechanisms of velvetleaf (Abutilon theophrasti Medic., Malvaceae) on soybean. Amer. J. Bot. 67, 1407-1413... 

Subsequent tests with velvetleaf, Kodkia, Jerusalem artichoke, and cocklebur showed that their allelopathic action altered water balance (55,94,95). Growth reductions in sorghum and soybean seedlings in nutrient solution amended with extracts from these weeds correlated with high diffusive resistances and low leaf water potentials. Stomatal closure occurred in plants treated with the more concentrated extracts. Depressions in water potential were due to a reduction in both turgor pressure and osmotic potential. A lower relative water content was also found in velvetleaf-treated plants. These impacts on water balance were not from osmotic factors. Allelochemicals from these weeds have not been thoroughly ascertained, but the present evidence shows that some contain phenolic inhibitors. Lodhi (96) reported that Kodkia contains ferulic acid, chlorogenic acid, caffeic acid, myricetin, and quercetin. As noted earlier, an effect on plant-water relationships is one mechanism associated with the action of ferulic acid. 

Use of this combination in field trials led to significant velvetleaf control and increased soybean yield. 

The herbicidal activity of FMC 55626 at 1 kg/ha is summarized in Table I. Crops other than soybeans, e.g., cotton, corn, and wheat, are not tolerant. Velvetleaf and lambsquarters were quite susceptible (85-98%) whereas cocklebur and jimsonweed control ranged from 55-80%. Among the grass species, johnsongrass was the least susceptible (20% control). 

Test Species Barnyardgrass, Greenfoxtail Morningglory, Velvetleaf Cotton, Soybean... 

The phenyl tetrahydrophthalimides are primarily post-emergence herbicides for the control of broadleaf weeds, though they will show pre-emergence activity at higher rates of application. Flumiclorac-pentyl is a post-emergence herbicide for the control of broadleaf weeds such as cocldebur, common lambsquarters, jimsonweed, amaranthus, prickly sida, and velvetleaf in soybean and corn at 25-... 

Another area related to fluazolate (17) and pyraflufen-ethyl (18) chemistry is a series of 2,4,5,6-tetrasubstituted-phenyl pyrazoles 89 (Fig. 3.23) from Ishihara Sangyo Kaisha [96]. These compounds differ from previous phenyl pyrazoles in that they have substituents at the 6 position of the phenyl ring. Pre-emergence application of 89 provided 100% control at 63 g-a.i. ha of barnyardgrass, crab-grass, green foxtail, redroot pigweed, prickly sida, and velvetleaf Soybean was reported to have 20% injury for compound 89 at this rate of application. 

The compounds described were tested in the greenhouse pre- and postemergence on various weeds and crops. We found that these pyranopyridylimidazolinones showed herbicidal activity with soybean selectivity comparable to 3, the commercial product imazaquin. Because of the soybean selectivity, the comparison of the herbicidal activity is focused on the control of common weeds found in soybeans. These weeds are crabgrass, foxtail, johnsongrass, cocklebur, morningglory and velvetleaf. The data is summarized in Table I. 

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