D. C. Bridges, P.L. Raymer, J.L. Day
Dept. Crop & Soil Sciences
Griffin, Georgia

In recent years Georgia wheat farmers have increasingly experienced difficulties controlling Italian (annual) ryegrass in wheat with Hoelon (diclofop). An option is to use Sencor (metribuzin) as a postemergence treatment for control of annual ryegrass, wild radish, and other broadleaf species. Approximately 10 years ago metribuzin use among Georgia wheat growers was fairly common, but use had declined over the intervening years. However, the current difficulties in controlling ryegrass with Hoelon have spurred interest in once again using Sencor.

Some wheat cultivars are sensitive to Sencor (metribuzin). If Sencor is applied to sensitive cultivars severe injury and/or stand loss may occur. The Sencor label contains a list of known tolerant and sensitive wheat and barley cultivars. However, the label does not contain all varieties that are currently grown in Georgia. Until about 10 years ago field experiments were routinely conducted to determine cultivar sensitivity to metribuzin. Due to the increased interest in using metribuzin for winter wheat weed control we resumed cultivar evaluations using metribuzin during the 1997/98 wheat season and are conducting trials again during the 1998/99 season.

Field trials conducted at Griffin, Georgia, included approximately 40 wheat cultivars were submitted to the Statewide Variety Testing Program for testing and consideration for recommendation. This included commercially available cultivars and a limited number of advanced experimental lines, both private and public, that have not yet been released. A replicated experiment that included paired metribuzin-treated and untreated plots was conducted each of the last two years. Each year wheat was drilled into a well-prepared seedbed with an experimental plot drill at approximately 2 bu/acre during mid-November. Georgia Cooperative Extension Service recommendations were used for production of the wheat each year. Metribuzin was applied postemergence at a rate of 0.375 lb ai/acre in water at 20 GPA. Application was made with conventional hydraulic nozzles with a tractor. Details on planting date, application date, stage of wheat growth at application, and environmental conditions are presented in Table 1.

1997/98. Little wheat injury occurred with any wheat cultivar (data not shown). In fact, wheat yields were not reduced are a result of metribuzin application, even with cultivars known to be sensitive to metribuzin (data not shown). Several factors, other than cultivar sensitivity, determine wheat injury potential with metribuzin. Stage of growth is important. Apparently wheat senstivity to metribuzin declines with age and advanced stages of development. Wheat was planted in mid-November and based on wheat growth metribuzin should have been applied at the 4-leaf stage in late-December or early-January. However, due to excessive rainfall metribuzin was not applied until 2 February 1998 when wheat was fully tillered. It appears that the advanced stage of growth may account for the lack of injury in 1997/98.

1998/99. Substantial differences in cultivar tolerance were observed in the 1998/99 test. Severe wheat injury and stand reduction was observed with many cultivars, including previously-known sensitive cultivars and some previously untested cultivars (Table 2). Of the 37 cultivars tested 17 were categorized as "Tolerant", which indicates that total crop injury did not exceed 10%, nor was there detectable stand reduction based on the untreated paired plot (Table 2). Four (4) cultivars were categorized as "Moderately Tolerant", which indicates that crop injury ranged from 11 to 20% and that stand loss was insignificant. Sixteen (16) cultivars were categorized as "Sensitive", which indicates that crop injury exceeded 20% and/or significant stand loss was observed. Seventeen of the 37 cultivars tested are currently listed as either "Tolerant", "Moderately Tolerant", or "Sensitive" on the Sencor label. Preliminary data from this 1998/99 trial are in good agreement with Sencor label data for these 17 cultivars with the exception of NK Coker 9704. We observed an average 5% crop injury with NK Coker 9704 and categorized it as "Tolerant". The Sencor label indicates that NK Coker 9704 is "Sensitive", meaning that in at least one field trial severe injury or stand loss was observed with this cultivar. Therefore, it is prudent to regard this cultivar as "Sensitive" until further testing proves otherwise.

Why were result so different between the two years? The apparent, and most likely, explanation relates to the stage and size of the wheat at the time of treatment. Wheat was fully tillered when the application was made in the 1997/98 test, whereas wheat was in the 2-3 leaf stage when the application was made in the 1998/99 test. Excessive rainfall immediately following application can increase metribuzin injury to sensitive wheat cultivars. More than 7" of rainfall occurred in February of 1998, but little injury was observed, presumably because the wheat was very large and well established. Only 2.6" of rainfall occurred during the 30 day period after application in December 1998, but injury was significant. Environmental variables such as rainfall and temperature after application do not account for the differences in injury.

Should metribuzin applications be delayed to reduce the chance for wheat injury? These data appear to indicate that farmers can reduce the chance for crop injury by delaying metribuzin applications until after wheat is fully tillered. While this may be true, they will also reduce the level of annual ryegrass control achieved. More research is needed to determine the optimum rate and timing for metribuzin application to achieve maximum weed control and minimum crop injury.

Conclusions. While one should be cautious about drawing final conclusions on limited data sets these studies provide some useful information even at this preliminary stage. They confirm (at least from this year's data) that several cultivars previously thought to be sensitive to metribuzin (reference the label) are in fact sensitive to metribuzin. They also provide evidence that several other widely planted, but previously untested, cultivars are also sensitive to metribuzin. Farmers and seedsmen are advised to follow the Sencor label and consider these data when using Sencor for weed control in wheat. When applied at the proper time and rate Sencor can provided excellent control of annual ryegrass and winter annual broadleaf weeds in wheat. For more information on using Sencor on wheat CLICK HERE

Table 1.Application and environmental particulars for wheat cultivar by metribuzin field tests, 1997-1999.