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Pasture Grass Injury from Herbicides (06/18/15)

The following is information provided by Dr. Rod Lym, NDSU Weed Scientist.

Pasture Grass Injury from Herbicides

The following is information provided by Dr. Rod Lym, NDSU Weed Scientist.
                Question: Last year a producer applied Milestone at 5 fl oz/A plus Plateau at 7 fl oz/A to his pasture to control leafy spurge, Canada thistle, and buckbrush. This year the pasture grass was killed and weed control was good except for buckbrush. He wanted to reseed his grass now. The weed officer and I can’t figure out what killed his grass since the grower used labeled herbicide rates. Is it safe to reseed? Also, the weed board was wondering why they have many spots on the right of way that last fall look good with good spurge control but this spring it caused the grass to be brown and just now it is starting to green up and grow.  What caused the grass to turn brown?   

                Answer:  Cool season grass species injury to Plateau in not unusual. I have seen it the year after treatment, even at application rates as low as 6 fl oz/A.  The grass usually comes back slowly and by the end of July or early August, the pasture looks normal again.  I would tell the producer to hold off on reseeding as it may not be necessary because the grass is likely suppressed and not dead.  Neither Plateau nor Milestone are effective on buckbrush. 2,4-D works well and is the least expensive option for buckbrush control.

                The cool season grass species were probably under stress last fall from dry conditions which likely magnified the injury response you have seen.  I have seen more injury in fields treated with Plateau following a dry compared to wet fall.  I have not seen grass injury from Plateau at 4 fl oz/A applied with Tordon and 2,4-D in June, a treatment used by many and dubbed the “North Dakota 3-way”. Another option for areas that are not hayed or grazed is Perspective applied in the spring or fall.  Some people use Tordon plus Overdrive in the spring and many still use the old standby Tordon plus 2,4-D. 

Rich Zollinger

Extension Weed Specialist

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