Effective rate for early post crabgrass control?

Weed question of the week: What is the ramification of backing down the rate of dithiopyr from 0.38 lbs ai/acre to 0.25 lbs ai/acre due to the cooler weather? Response by Dr. John Street

The answer to your question is not clear-cut as there are many variables involved.  With the warm spring so far and higher than normal soil temperatures, crabgrass began germinating in most areas of Ohio around March 15-20 (50-55 degree F soil temps. for several consecutive nights at the 2" depth).

Now, those areas with high quality, dense turf cover, of course, are less likely to be as immediately impacted as marginal quality, less dense turf cover areas relative to crabgrass germination.  My weekly OTF clippings article (see ohioturfgrass.org) a week or two ago suggested that if dithiopyr (pre-post) or any preemergence is being applied that the normal rate this early in the season with these air and soil temperatures should be increased by 25-30%.  This is because of the greater likelihood for more rapid dissipation of the herbicide, again due to higher than normal air and soil temps. If the turf managers expect, via experience, climatological data or programs, or scouting observations of indicator spots, that crabgrass has germinated over most of their location, then I recommended a switch to dithiopyr (pre and early-post activity).

Dr. Gardner indicated we are 3-4 weeks ahead of schedule across the state. So, my response to play it safe would be to stay at 0.38 lbs ai/A and certainly not reduce the rate for this year and stick to my 25-30% rule. This is only my opinion and I clearly understand the economics involved.