Field and greenhouse experiments were conducted to determine the effect of selected insecticides on cotton when applied in combination with pyrithiobac. In the field, acephate, azinphos-methyl, bifenthrin, chlorpyrifos, dicrotophos, esfenvalerate, malathion, methomyl, or oxamyl was applied foliarly with the sodium salt of pyrithiobac at 70 g ae/ha. Pyrithiobac plus malathion resulted in greatest injury when compared with other treatments. There were no differences observed in fruiting characteristics of cotton. Cotton yield was lower with oxamyl, methomyl, and chlorpyrifos when applied in combination with pyrithiobac in one of three experiments. In greenhouse experiments, injury due to pyrithiobac was greater in a cool (21 C day, 13 C night) temperature than in a warm temperature (30 C day, 21 C night), but differences were less than 15%. Pyrithiobac plus malathion in the cool environment injured cotton more severely 7 DAT than either pesticide alone. Adding insecticides to pyrithiobac did not influence injury in the warm environment.

Field experiments were conducted from 1992 through 1994 to determine the effect of selected graminicides when applied in combination with pyrithiobac for johnsongrass control in cotton. Quizalofop-P, clethodim, sethoxydim, and fluazifop-P were applied at rates recommended for rhizome johnsongrass control with or without pyrithiobac at 70 g ae/ha. At 14 DAT johnsongrass control without pyrithiobac was more than 85% for all graminicides all years except sethoxydim in 1993. When graminicides were applied in combination with pyrithiobac, johnsongrass control was 86% or less and was lower than when each respective graminicide was applied alone. At 21 DAT, similar differences were apparent. Seed cotton yield, determined only one year, was not affected by reductions in johnsongrass control when either quizalofop or clethodim were used in combination with pyrithiobac. Pyrithiobac applied alone controlled johnsongrass 20 to 44% 14 DAT depending on year.

A laboratory study was conducted to determine the effects of two adjuvants and temperatures at time of treatment on efficacy, absorption, and translocation of thidiazuron defoliant on cotton. Five days after treatment at 30/21 C day/night temperatures, leaf drop was 17% with no adjuvant, 37% with addition of crop oil concentrate, 40% with ammonium sulfate, and 75% with two adjuvants combined. At 21/13 C day/night temperatures, there was less than 10% leaf drop with all treatments. At 10 d after treatment, leaf drop was not different among treatments at the high or low temperatures. Shoot regrowth at high and low temperature was reduced 55 to 60% with addition of both adjuvants and 44 to 50% with each adjuvant or with no adjuvant when compared to plants defoliated by hand. Absorption of 14C-thidiazuron was not affected by variations in temperature during the time of treatment but was affected by adjuvants. With no adjuvants, absorption was 7 to 10%. With 1.25% by vol crop oil concentrate, absorption was 33 to 46%. Addition of ammonium sulfate resulted in 18 to 19% absorption, and the combination of ammonium sulfate and crop oil concentrate increased absorption to 65 to 68%. There was no movement of radiolabel away from treated leaves as determined by autoradiographs of treated plants.

Field experiments were conducted in 1991 and 1992 near Stoneville, MS and Greenville, MS, to determine the impact of pyrithiobac on cotton fruiting, maturity, and yield. Trials were conducted in areas maintained weed-free with standard production practices to optimize yields. Pyrithiobac was applied at 105 or 210 g ae/ha either PRE or POST to cotton in the cotyledon to 2-leaf, 5- to 7-leaf, and matchhead-square (MHS) stages of growth. At one location, monosodium salt of methylarsonic acid (MSMA) and fluometuron plus MSMA were applied as standard comparisons. At the Stoneville location, injury 1 wk after treatment (WAT) with pyrithiobac ranged from 2.5% with 105 g/ha applied at cotyledon to 2-leaf to 26% with 210 g/ha applied at MHS. Injury was transient in nature and was rarely detectable 4 wk after treatment in either year. Injury was not evaluated at the Greenville location. At Stoneville, a reduction in height occurred with MHS applications of pyrithiobac and MSMA. There was no reduction in yield for any treatment compared to the untreated check in 1991 at Stoneville and in both years at Greenville. In 1992 at Stoneville, treatments of 210 g/ha pyrithiobac, MSMA, and fluometuron plus MSMA at all application timings reduced yield. Pyrithiobac at 105 g/ha applied PRE also reduced yield. Greatest yield loss occurred with treatments of MSMA applied at MHS. There were no yield reductions at either location with postemergence application of 105 g/ha pyrithiobac.