Residual Herbicides for Use in the Wheat-Fallow System (1984) Residual Herbicides for Use in the Wheat-Fallow System (1984)

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Several residual herbicides are suitable for controlling weeds during fallow in the wheat-fallow system.The most widely used herbicide for this purpose is atrazine.Atrazine persists in the soil for over a year, and should be applied as soon as possible after wheat harvest to avoid injury to the following wheat crop.Cyanazine (Bladex), metribuzin (Sencor or Lexone), and terbutryn (lgran) do not persist as long in the soil as does atrazine, and are usually applied in the spring.Cyanazine and terbutryn, however, can be tank mixed with atrazine and applied following wheat harvest.
Chlorsulfuron (Glean) is a new herbicide, which obtained full labeling for wheat and wheat-fallow in 1983.Chlorsulfuron is different from the other herbicides currently used in wheat-fallow in that it was developed specifically for use in wheat and barley.Thus, it does not injure wheat and can be applied either to growing wheat (between the 2-3 leaf and boot stages) or anytime during fallow.Its persistence in the soil for weed control purposes is similar to that of atrazine.However, small quantities can remain in the soil for long periods of time and cause injury to following crops.For this reason, chlorsulfuron is currently labeled only for weed control in barley, wheat, and wheat-fallow on soils having a pH of 7 .5 or lower.
All of these herbicides have been tested extensively at the Garden City Experiment Station to determine their suitability for the wheat-fallow system.The results of the experiments reported here are intended  1 and 2. Results from 1982, contained in Table 1, indicate that lib of atrazine gave good broadleaf weed control for 10 months after application, with less control from lower rates.Spring applications of cyanazine and metribuzin controlled witchgrass, in addition to the broadleaf weeds.All rates of atrazine controlled weeds from application through winter freeze-up .Therefore, the best season-long control was obtained from a postharvest application of atrazine, followed by cyanazine or metribuzin in the spring.Data from 1983 are presented in Table 2.Only Russian thistle and kochia were rated, since pigweed and witchgrass were not present in any quantity.Weed control with atrazine, cyanazine, and metribuzin was similar to that obtained in 1982.The control resulting from these herbicides, however, lasted longer into the summer of 1983, because a cool spring delayed weed growth.The plots were rated twice in 1983, and several of the treatments required tillage on July 14, prior to the second rating, including all of the non-sequential atrazine treatments.
Chlorsulfuron was included in 1983, and the 0.375 oz rate, applied postharvest, resulted in longer lasting weed control than did 1 lb of atrazine.Conversely, plots receiving the 0.25 02 rate of chlorsulfuron , postharvest, required tillage on July 14, along with the plots receiving atra2ine.The sequential application of 0.25 oz chlorsulfuron following atra2ine was still giving nearly perfect control on July 21, 1983.Spring (non-sequential) applications of chlorsulfuron resulted in better weed control than t~at obtained with cyanazine or metribuzin .
A comparison of tank mixes containing chlorsulfuron, cyanazine, and metribuzin is given in Table 3. Tank mixes of 0.125 02 chlorsulfuron with either cyanazine or metribu2in resulted in weed control as The LO lb -rateof atrazine applied prior to emergence has given adequate control of volunteer wheat.Lower rates of atrazine, when used on high pH soils have also given satisfactory control.Occasionally, control of volunteer with atrazine has been less than satisfactory.In such instances, controJ•has been improved when 1.6 lb of cyanazine was tank mixed with 0.81b of atrazine.
The use of chlorsulfuron alone will result in essen• tially no volunteer control; tank mixing chlorsulfuron with atrazine will aid in controlling volunteer.In the event that an excessive amount of volunteer escapes the residual herbicide, tillage or postemergence herbicides should be used.Delaying the application of the residual herbicides until the volunteer has emerged, and tank mixing the residual herbicide with a postemergence herbicide also can be effective.

Conclusions
Herbicides for wheat-fallow can be applied postharvest, in the spring, or sequentially.In addition, chlorsulfuron can be applied to growing wheat.All of these methods result in good weed control for varying periods of time.The length of time of weed control depends on the persistence of the particular herbicide in the soil, the application rate, and climatic conditions.Atrazine and higher rates of chlorsulfuron can be applied after harvest for control that will last into the spring, while short-term residual herbicides such as metribuzin, cyanazine, or a low rate of chlorsulfuron are best suited for application in the spring, or for se-~uential treatments following a postharvest application.
Ideally, herbicides should be selected to control weeds during the entire fallow period.Unfortunately, there is no single herbicide that will control weeds for the entire time.The best control, therefore, will result from an in-crop or postharvest application followed by a spring application.
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Table 1 .
Fallow weed control from fall and spring appliedherbicides, 1982.

Table 2 .
This publication from Kansas State University Agricultural Experiment Station and Cooperative Extension Service has been archived.Current information: http://www.ksre.ksu.edu.Fallow weed control f rom fall and spring applied herbicides, 1983.

Table 3 .
Fallow weed control from spring applied herbicides, 1983.Of the herbicides suitable for reduced tillage, chlorsulfuron is unique in that it can be applied to growing wheat.It will control most broadleaf weeds in This publication from Kansas State University Agricultural Experiment Station and Cooperative Extension Service has been archived.Current information: http://www.ksre.ksu.edu.
Although not evaluated in this test, such tank mixes can be expected to provide control of certain grassy weeds, in addition to broadleaf weeds.Extende d Control from In-Crop Applications.