Studies were conducted to determine the effect of simulated rainfall on the phytotoxicity of several herbicides. Oil soluble herbicides 4,6-dinitro-o-sec-butylphenol (DNBP), pentachlorophenol (PCP), a 1:1 mixture of the butyl esters of 2,4-dichlorophenoxyacetic acid and 2,4,5-trichlorophenoxyacetic acid (2,4-D: 2,4,5-T), and a 2:2:1 mixture of the isooctyl esters of 2,4–13:2,4,5-T: 4-amino-3,5,6-trichloropicolinic acid (picloram) (hereinafter referred to as M-3140), usually were less affected by artificial rainfall than were water soluble 1,1'-dimethyl-4,4'-bipyridinium salt (paraquat), dimethylarsinic acid (cacodylic acid), and cis-2,3,5,5,5-pentachloro-4-ketopentenoic acid (hereinafter referred to as AP-20), on guava (Psidium guajava L.) and mango (Mangifera indica L.). Washing reduced the activity of some oil and water-soluble herbicides on sorghum (Sorghum bicolor (L.) Moench, var. Combine Kafir-60) and dioscorea (Dioscorea composita Hemsl). As herbicide rates were increased, washing reduced the phytotoxicity of paraquat and 2,4-D:2,4,5-T less on mango and guava. Artificial rainfall applied as a mist spray at different intervals had no effect on the phytotoxicity of paraquat; it reduced the activity of cacodylic acid and increased the effectiveness of AP-20 on mango.
Residues in soil, following application of 0.25 lb/A of 4-amino-3,5,6-trichloropicolinic acid (picloram) to semi-arid rangelands, usually were restricted to the top 12 inches for 60 days. Five ppb or less picloram were detected below 12 inches at 120 to 180 days after application; but picloram usually dissipated from the soil profile within a year. More picloram was detected 5 months after application at 6 to 18 inches deep at the lower ends of plots with 3% slopes than in plots with 0, 1, or 2% slopes. Runoff water from plots irrigated 10 days after treatment contained 17 ppb picloram. Irrigation or rainfall at 20, 30, or 45 days after picloram application resulted in less than 1 ppb picloram in runoff water. No more than 1 or 2 ppb picloram were detected after dilution of runoff water in large ponds.
Granular 4-amino-3,5,6-trichloropicolinic acid (picloram) was effective in controlling live oak(Quercus virginiana(Mill.), huisache)(Acacia farnesiana(L.), Willd.), and yaupon(Ilex vomitoriaAit.), but not honey mesquite(Prosopis juliflora(Swartz) DC. var.glandulosa(Torr.) Cockerell) in south Texas. Granular 5-bromo-3-sec-butyl-6-methyluracil (bromacil) controlled live oak and huisache. Herbicides applied to soil were usually most effective in spring and fall during periods of active brush growth. Picloram granules applied in May to a mixed stand of woody plants in Puerto Rico usually were effective. However, rates up to 30 lb/A were ineffective on some species.
Several herbicides and herbicide combinations were investigated for desiccation, defoliation, and killing of tropical vegetation near Mayaguez, Puerto Rico. Hydroxydimethylarsine oxide (cacodylic acid) was one of the more effective defoliants on guava (Psidium guajavaL.) within 2 weeks after treatment. However, several herbicide combinations including contact: growth-regulator types were more effective 1 month after spraying. Six months after treatment, herbicides containing (2,4-dichlorophenoxy)-acetic acid (2,4-D), (2,4,5-trichlorophenoxy)acetic acid (2,4,5-T), or 4-amino-3,5,6-trichloropicolinic acid (picloram) were superior to contact herbicides in controlling regrowth. In a mixed tropical forest, most rapid desiccation and defoliation of vegetation was produced by 6,7-dihydrodipyrido (1,2-a:2′,1′c) pyrazinediium salt (diquat) within 2 weeks after treatment. At 1 month after treatment and longer, the 1:1 mixture of the butyl ester of 2,4-D:2,4,5-T and a 2:2:1 mixture of the isooctyl esters of 2,4-D:2,4,5-T:picloram were superior to other herbicides.
Acetone, ethanol and water extracts of mature fruits of yaupon (llex vomitoria Ait.) inhibited germination of mesquite (Prosopis juliflora Swartz DC. var. glandulosa (Torr.) Cockerell and sorghum (Sorghum bicolor (L.) Moench). Extracts of guava fruit (Psidium guajava L.) inhibited cucumber (Cucumis sativus L.) seed germination. Water soluble inhibitors were found in fruits, leaves, roots and bark of several tropical species representing 10 different families. Strong inhibition of cucumber seed germination and growth did not occur in sand when water extracts containing inhibitors were applied. Growth of corn, sorghum, cucumber and bean was reduced in soils collected beneath Malay apple (Eugenia malaccensis L.) trees. Plant growth‐inhibitors occurred in all species studied in various plant parts, and some apparently affect the growth and ecology of other plant species.
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