Warm and Dry Winter Causes Downy Brome to Head Out Early
Fabian Menalled, Revised 2004
Alvin J. Bussan and Meghan Trainor, Original
Numerous questions have arisen regarding how long to wait to spray Roundup®, once downy brome (Bromus tectorum) heads out to prevent viable seed production. Downy brome is a winter annual typically germinating in the fall. However, if adequate fall moisture is not available downy brome can act like a spring annual. Thus, after an exceptionally warm and dry winter, downy brome may develop heads earlier than average.
Research published by Oregon State University showed a simple growing degree-day (GDD) model predicted seed production of downy brome. Results support the hypothesis that time of downy brome seed production can be predicted based on historical weather records coupled with current temperature monitoring.
In 1999, downy brome panicles were sampled weekly at nine locations from winter wheat growing regions of the western U.S. Three of these locations were in Montana including Bozeman, Havre and Mocassin. The samples taken from the nine sites produced seed after 1003 GDD on average, ranging from 854 to 1161 GDD. Table 1 shows the cumulative GDD for the first viable seed, the actual dates for the first viable seed produced in 1999, and the predicted date of seed production based upon a 10-year average from the three sites in Montana.
Table 1. Downy brome collection sites in Montana and cumulative GDD for first viable seed. GDD are in Celsius and used a base temperature of 0.
Collection Site |
GDD for first viable seed in 1999 (C) |
Date for first viable seed in 1999 |
Predicted date of seed production based on 10-year average |
|
Bozeman |
907 |
June 7 |
June 10 |
|
Havre |
1022 |
June 9 |
June 11 |
|
Mocassin |
923 |
June 10 |
June 23 |
How soon after downy brome heads out should you spray glyphosate in order to prevent seed set? Basically, when heads first appear, Roundup must be applied within 3 to 5 days to prevent seed production.
Studies have shown that if treatment is delayed by 2 weeks after seed heads first appear, seed production is high. Furthermore, neither mowing nor burning are effective controls to prevent seed set. Seed will continue to mature on cut-off plants. One option is mowing and removal of plants from the field. Burning is not effective, as many of the seeds drop to the ground before the heat can kill them.
Several people have claimed they can determine germinability of downy brome seed by looking at it. To our knowledge, there is no known technique whereby looking at the seed indicates germinability. The only way to test for viability is by actually subjecting the seed to conditions under which germination may be induced. This involves the use of a growth chamber or water vapor. Using the tools available at home, you can wrap seeds in a damp paper towel and place them in a ziplock bag, leaving the bag on a counter at 55-60 °F. Within one week, germination should occur. This is not a perfect test as seed dormancy can mask viability of seed.
In conclusion, if you have fields in which you were not able to apply chemical fallow and significant downy brome seed set occurred, options still exist. First, try not to seed winter wheat in those fields; rather rotate to a spring crop. If you must grow winter crops, delay seeding until the first flush of downy brome emerges. This allows you to control the weed as you prepare a seedbed. Selective herbicides for downy brome are available such as Maverick®, Far-Go®, Amber® and Finesse®. Refer to all labels for specific information regarding proper rates and usage.
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Downy brome seedling. Source: Weed Science Society of America photo herbarium |
Categories: Weed, Downy Brome, Weed Seed, Drought
Date: 04/28/2005