Weed control, population density change and herbicide persistence in soils in no-tillage and conventional tillage soybean-wheat double cropping systems

Experiments were established at Knoxville and Springfield, Tennessee, to monitor weed control, population changes and herbicide persistence in soils under no-tillage and conventional tillage soybeans. In 1977 soybeans [Glycine max (L.) Merr.] were no-till planted into tall fescue (Festuca arundinaceae Schreb.) pasture at Knoxville and into wheat stubble at Springfield, Conventional tillage plantings were established in adjacent areas previously plowed and disked. At soybean harvest both areas were over seeded with wheat.

In 1978 after wheat harvest, soybeans were planted and herbicides were applied on exactly the same plots as in 1977. The same procedures were repeated in 1979.

Herbicides used in tank mixes were: alachlor [2-chloro-2, '6'-diethyl-N-(methoxymethyl) acetanilidel; metribuzin l4-amino-6-tert-butyl-3-(methylthio)-as-triazin-5-(4H)-one); linuron [3-(3,4-dichloro-phenyl)-l-methoxy-l-methylurea]; glyphosate [N-Cphosophonomethyl) glycine and paraquat (1,1'-dimethyl-4,4'-bipyridinium ion) at 2.2, 1.1 and 0.6, 2.2 and 0.6 kg/ha respectively.

The tank mixes used in no-tillage were: alachlor + linuron + paraquat + surfactant; alachlor + linuron + glyphosate; alachlor + metribuzin + paraquat + surfactant and alachlor + metribuzin + glyphosate. The tank mixes used in conventional tillage were: alachlor + linuron and alachlor + metribuzin. All treatments were applied preemergence.

Each year soil samples were taken immediately after herbicide application and at various intervals throughout the year. Atmid-soybean season data were collected on control of: ivyleaf morningglory [Ipornoea hederacea (L.) Jacq]; goosegrass [Eleusine indica (L.) Gaertin.]; prickly sida (Sida spinosa L.); buckhorn plantain (Plantago lanceolata L.); spotted spurge (Euphorbia maculate L.); horsenettle (Solanum carolinense L.); hedge bindweed (Convoluvlus sepium L.); large crabgrass [Digitaria sanguinalis (L.) Scop.] and yellow nutsedge (Cyperus esculentus L.) at Knoxville, and on: common ragweed (Ambrosia artemisiifolia L.); smooth crabgrass [Digitaria ischaemum (Schreb.) Muhl.] and Pennsylvania smartweed (Polygonum pensylvanicum L.) at Springfield. At the end of each season, soybeans were harvested for yields.

Weed control at Knoxville and Springfield under the same tillage system was the same. Tank mixes of alachlor + metribuzin or linuron with any of the tillage systems was more effective than single application of alachlor, linuron or metribuzin in the control of both annual and perennial weeds. However, glyphosate tank mixes were more effective in the control of perennial weeds than paraquat tank mixes. Buckhorn plantain and hedge bindweed showed potential build up in areas where paraquat was used. Yellow nutsedge was completely controlled under the no-tillage systems but built up rapidly in population density over the years under the conventional tillage system at Knoxville.

Cucumber (Cucumis sativis L. 'Japanese cluster') bioassay on soil samples under greenhouse conditions showed no phytotoxic symptoms in soils sampled from field plots at 30 to 48 days after herbicide application. There were no differences in residue persistence in the conventional tillage treatments as compared to the no-tillage paraquat and glyphosate tank mixed treatments. The only treatment that indicated residue in soils at 168 days was metribuzin at 1.1 kg/ha under conventional tillage at Knoxville in 1977.

Soybean yields over the three years did not follow any consistent pattern within or among tillage systems over years. This may have been the result of different planting dates associated with different seasonal climatic conditions.