In the greenhouse, soil applications of primisulfuron (40 g ai ha–1) reduced growth of emerged rhizome johnsongrass plants more than nicosulfuron (40 g ai ha–1). Both herbicides reduced growth more when applied to foliage only; a further decrease in growth did not occur for applications to both soil and foliage. Primisulfuron did not completely prevent regrowth of johnsongrass with any application method. Nicosulfuron prevented regrowth when applied to the foliage and to both soil and foliage. In single-year field studies in corn at four dryland sites and two irrigated sites, 50:50 split applications of primisulfuron (40 g ai ha–1) and nicosulfuron (35 g ai ha–1) approximately 2 wk apart provided the most consistent rhizome johnsongrass control compared with early or late single applications when visually rated 8 wk after the first application. Nicosulfuron treatments were more effective than primisulfuron treatments at both dryland sites the first year and at one of two dryland sites the second year. Primisulfuron and nicosulfuron at the irrigated site the first year were equivalent in efficacy. Nicosulfuron was more effective than primisulfuron at the irrigated site the second year. Primisulfuron or nicosulfuron treatments more than doubled corn yields at the dryland sites both years regardless of application timing. Split applications of primisulfuron or nicosulfuron provided the highest yields (approximately 80% increase over untreated plots) at the irrigated site the first year. All treatments provided equivalent yield increases (approximately 50%) the second year.

An in-row competition study was conducted in 1991 and 1992 near Silver Lake, KS to determine the relationship of noncultivated common sunflower density to soybean yield, PAR at the soybean canopy, and common sunflower dry matter production. Because of environmental differences, year main effect interactions occurred, so results are presented by year. For example, 0.3 common sunflower plant/m2produced 4030 kg/ha of aboveground dry matter in 1991 and 1300 kg/ha in 1992. Soybean yield reduction ranged from 19 and 17% with 0.3 common sunflower plant/m2to 97 and 95% with 4.6 plant/m2, in 1991 and 1992, respectively. Assuming a treatment cost of $35/ha and a soybean market price of $0.21/kg, economic threshold levels were 0.1 common sunflower plant/m2in 1991, and 0.07 in 1992. Common sunflower at 0.3 plant/m2reduced PAR at the soybean canopy by 390 and 300 μE/m2/s, or 24 and 18% in 1991 and 1992, respectively. We conclude that the ability of common sunflower to intercept PAR above the soybean canopy is an important component in its interference with soybean.

Rhizome johnsongrass grown in the greenhouse and treated with glyphosate at 1680 g ai ha−1at an early (3- to 4-leaf) or late (6- to 8-leaf) growth stage displayed injury within a week. Plants treated with CGA-136872 or DPX-V9360 at 40 g ai ha−1at both growth stages displayed injury 1 to 2 weeks later. CGA-136872 did not prevent regrowth at either growth stage. No regrowth occurred from DPX-V9360 or glyphosate-treated plants. Foliar absorption by greenhouse-grown plants within 24 h of application was greater with14C-glyphosate than with14C-DPX-V9360 or14C-CGA-136872. More14C-DPX-V9360 was absorbed than14C-CGA-136872. Growth stage influenced glyphosate absorption (more by younger plants) but not CGA-136872 or DPX-V9360 absorption. Translocation of the14C-CGA-136872 and14C-DPX-V9360 out of the treated leaf was less than 20% of the absorbed label and was less than glyphosate translocation. Growth stage of rhizome johnsongrass at the time of treatment had no effect on the distribution of radiolabeled herbicides within 24 h.

14C-glyphosate [N-(phosphonomethyl)glycine] degradation to14CO2was examined in a Spinks sandy loam, Collamer silt loam, and a Norfolk loamy sand. After 32 days, 40, 9.5, and 3% of the14C-glyphosate was recovered as14CO2in the three soils, respectively. The degradation was primarily microbial. Phosphate additions stimulated14C-glyphosate degradation to a limited extent in the Collamer silt loam but not in the Norfolk loamy sand. Additions of Fe+++and Al+++ions reduced degradation in the Spinks sandy loam. It is postulated that formation of colloidal Fe and Al precipitates in modified soils with concomitant adsorption of14C-glyphosate is responsible for decreased availability of14C-glyphosate to microorganisms. Mn++additions were found to increase degradation. Spinks soil and carbon substrate amendments failed to substantially increase degradation rates in both soils with low degradation rates.

The effects of several concentrations of glyphosate [N-(phosphonomethyl)glycine] and siduron [1-(2-methylcyclohexyl)-3-phenylurea] on germination and seedling growth of Kentucky bluegrass (Poa pratensisL. ‘Nugget,’ ‘Prato,’ ‘Fylking,’ ‘Park’), creeping bentgrass (Agrostis palustrisHuds. Penncross,’ ‘Seaside’), and red fescue (Festuca rubraL. ‘Pennlawn,’ ‘Wintergreen’) were evaluated in petri dishes under controlled environmental conditions. A high concentration of glyphosate (10-4M) reduced shoot growth in all but two cultivars. Siduron had no effect. Glyphosate or siduron applied to the soil and incorporated did not affect turfgrass germination or growth. When the glyphosate was sprayed over the soil and on the seed lying on the soil surface, red fescue germination was reduced and shoot growth of Kentucky bluegrass, creeping bentgrass, and red fescue was inhibited at the high rate of glyphosate application (17.9 kg/ha). Under field conditions, none of the glyphosate treatments reduced turfgrass germination or growth.