Purple nutsedge Cyperus rotundus is an important weed in cotton (Gossypium spp.) production in Arizona. It is estimated that 15% of the crop acreage is infested. The technology available for selective control of purple nutsedge is not adequate and the infestation level of this weed is increasing. In 1986, the thiocarbamate herbicides, vernolate (Surpass), EPTC (Eptam, Genep) and butylate (Swan +, Genate plus) were evaluated as preplant and postemergence incorporated applications for selective control of purple nutsedge in cotton. There were 25 different trials conducted in Arizona cotton fields. Butylate was more selective to seedling cotton than EPTC and vernolate but all were capable of unacceptable cotton injury. Where preplant irrigations require every furrow irrigation, this concentrated the thiocarbamates in the seed row and resulted in less selectivity. Preplant treatments controlled purple nutsedge for 1 or 2 postemergence irrigations. Postemergence applications were most effective following preplant treatments but were not consistent in controlling nutsedge season -long. This research indicates that purple nutsedge lowered yields only when the cotton was stunted in growth by severe early season infestations. Wherever cotton established dominance over the weeds, yields were not affected. Preplant applications of butylate, if carefully adjusted for the growing conditions, can provide early season control of purple nutsedge.

Research was conducted at Maricopa, AZ with EPTC (S -ethyl dipropyl carbamothioate) and butylate (S-ethyl bis(2-methylpropyl)carbamothioate) in 1986 to identify factors that influence the activity of these two thiocarbamates on cotton (Gossypium hirsutum). Butylate and eptc were applied alone and in combination with other cotton herbicides as either a preplant incorporated or preharrow treatment. The greatest injury to cotton and poorest weed control resulted when butylate or EPTC was applied alone on flat ground and incorporated to a depth of 2 inches or 4 to 6 inches. Adequate weed control and minimal injury to cotton was observed when these two thiocarbamates were applied pre-harrow.

Field research was conducted at Yuma and Goodyear, AZ on fallowed ground to measure the response of purple nutsedge to butylate and EPTC (Goodyear only). At Yuma, the greatest reduction in the number of live tubers resulted when two years of fallow were combined with annual summer application of 3.3 lb/A of butylate. At Goodyear, butylate and EPTC were more effective at controlling purple nutsedge when applied in early July vs early September. Also, one timely application of EPTC or butylate made in July was more successful in reducing the number of live tubers than two applications (July, Sept). September herbicide treatments were not as effective with an irrigation compared to without.

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Research was conducted at Maricopa, AZ with EPTC (S-ethyl dipropyl carbamothioate) (1.0 lb/a) and butylate (S-ethyl bis (2-methylpropyl)carbamothioate) (2.0 and 3.0 lb/a) in 1986 and 1987 to measure the response of cotton to preplant application methods. Butylate and EPTC were applied as either a preplant incorporated or preharrow treatments. The greatest injury to cotton and poorest weed control resulted when butylate or EPTC were applied on flat ground and incorporated to a depth of 2 inches or 4 to 6 inches. Adequate weed control and minimal injury to cotton was observed when these two thiocarbamates were applied preharrow.

Research was conducted at Maricopa, AZ in 1986 and 1987 to measure cotton and weed control response to preplant applications of cyanazine (Bladex) and prometryn (Caparol). Cyanazine was applied in combination with pendimethalin and trifluralin as either preplant incorporated or preharrow treatments. Neither cyanazine or prometryn significantly reduced cotton stands or yields.

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Field experiments were conducted in 1992 and 1993 to determine the crop safety and efficacy of norflurazon applications for control of purple and yellow nutsedge in cotton. Norflurazon was applied preplant-incorporated (PPI) or in two applications, PPI and postemergence (POST) when cotton was 3 to 4" tall. As the PPI norflurazon application rate increased from 0.5 to 0.75, 1.0, and 1.25 lb a.i./A, early season nutsedge control increased from 29 to 49, 58, and 76% of control. Early season weed control declined after about 6 weeks. POST emergence applications of norflurazon prolonged the period of nutsedge control. Data collected 71 and 21 days after the PPI and POST applications, respectively, showed that the 0.5 +1.5, 0.75 +1.25, and 1.0+1.0 lb a.i./A (PPI +POST) treatments resulted in 85, 76, and 73% control of nutsedges. Nutsedge control declined throughout the season with the 0.5 +1.5, 0.75 +1.25, and 1.0+1.0 lb a. i./A split applications all resulting in about 27% control 3 months after the POST applications. PPI rates 1.5 to 2 times the labeled rate for a particular soil type caused cotton injury in several experiments in the 1993 cotton season although no injury was observed in the 1992 season.