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Years ago we didn't really think about that aspect much, had a very slow travel speed type older then the hills sprayer and took forever to get any acres done so we would be up at 4 in the morning getting going and at least on wheat we never had a problem. Probably Buctril M and some sort of MCPA being used before that but expect all chemicals don't work the same in dripping wet crop. We still use a pull type sprayer but because we are traveling at 13 mph and the tires getting wet, the dirt builds up on the tires and the plants stick onto the tires and makes a heck of a mess of the plants and can see lots of damage effects long after its sprayed if we do push it in the dew. I have to wonder though if it might be worth it to do that anyway with roundup to help reduce the dust problem a bit if its dry as that certainly does nothing for the effectiveness of that chemical producing a dust cloud. That was the only good part about traveling at 5 mph, can't create too much dust that way.
 

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If spraying roundup wait for dew to burn off and plants to dry before spraying. Heavy dew will dilute droplet concentration and can cause run off on plants. Have had poor results with roundup and wet plants
 

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If spraying roundup wait for dew to burn off and plants to dry before spraying. Heavy dew will dilute droplet concentration and can cause run off on plants. Have had poor results with roundup and wet plants
Same. We usually always try to rule of thumb an hour of dry leaves for us. We aim for atleast if spray is on 1 hour before rain and after dew.

This was off a Roundup FAQ page

Will rain or watering wash Roundup brand products off treated plants and hurt other plants nearby?

Roundup brand products are quickly absorbed by plants after you spray them, becoming rain proof in 30 minutes or less. Because the active ingredients in Roundup Weed & Grass Killer products bind to the soil or are quickly absorbed by treated plants, it is very unlikely to affect surrounding plants.
 

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Well that doesn't sound good then, what I was thinking in terms of reducing dust. I know roundup will sure become ineffective if rained on right after its sprayed, had a field where it suddenly poured right as I was spraying and could see round by round in previous passes that the chemical was working better and better farther up the field as it had more time before that downpour.
 

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If you are running glyphosate and you are worried, put a little extra ams and surfactant in the tank. Dew is distilled water.

I never felt I lost kill because of spraying on a dew. But inversion conditions are often present as well, and thats another issue.
 

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I sprayed generic roundup once when it was wanting to drizzle but never rained and I had really good results. Even the buckwheat was getting smoked. The atmosphere was so heavy that every once in a while you had to wipe some water off the w/s! Last year, I sprayed stubble/weeds with Glyphos about 24 hours after a heavy downpour. I could barely move in the field and had issues going through low spots with the SP sprayer due to wetness. I thought this was going to be my best kill ever. No dust, lots of moisture ....... Turns out that was my worst kill ever. I was spraying at night with no wind. I believe the plants were shutdown after the rain and refused to take in the chemical. I had never seen poor results like that with a Glyphos 2-4-D/express/ams mix before. There was damage but no smokingly good results like I was expecting. I had crop alongside where I was spraying and I didn't see any signs of inversion which at the time of spraying I had feared because it was so quiet. No crop in the area was damaged at all. Weeds were slowed down but not smoked. The spray did seem to hang but I assumed it had to eventually go down. As I said, I saw no crop damage and I had fields on either side of me which remained unscathed.
 

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I just sprayed some soybeans with fomosafen, cadet, and crop oil on a field with a good dew and noticed the cadet didn't burn the beans as bad as the field I sprayed the afternoon before. I'll be seeing if my weed control is as good.
 

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Always thought the problem of dew was one of surface tension of the droplets. Where the spray breaks the tension and causes the drop to roll off without depositing chemical. I used to get a blade of grass and push the tip into a dew drop; if that didn't cause the drop to fall off the plant I figured it was ok to go spraying.
 

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Discussion Starter · #18 ·
There's a big difference between plants wet due to a rain and plants wet due to condensation (dew). Plants wet with rain water tend to have large drops of water on them and when they are hit with spray, they tend to roll off the feaf taking much of the chem with it. Dew, OTOH tends to cover the whole leaf with a mass of micro drops and when the spray hits the leaf, the chem spreads over the whole surface of the leaf and does not create a runoff situation. This improved dispersal is why I think most chemicals work as good or better on a dew which is why I asked the question "which chemicals DON'T work". What nig71 describes is what I experience with rain water but not dew. After a rain, I will give a plant a light flick with a finger, if the water runs off, don't spray, if it hangs on, good to go. Not the best test and I would sure like to hear of some better ones.
 

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There's a big difference between plants wet due to a rain and plants wet due to condensation (dew). Plants wet with rain water tend to have large drops of water on them and when they are hit with spray, they tend to roll off the feaf taking much of the chem with it. Dew, OTOH tends to cover the whole leaf with a mass of micro drops and when the spray hits the leaf, the chem spreads over the whole surface of the leaf and does not create a runoff situation. This improved dispersal is why I think most chemicals work as good or better on a dew which is why I asked the question "which chemicals DON'T work". What nig71 describes is what I experience with rain water but not dew. After a rain, I will give a plant a light flick with a finger, if the water runs off, don't spray, if it hangs on, good to go. Not the best test and I would sure like to hear of some better ones.
I may have to have another look at that, funny thing is although I don't intentionally spray in the rain unless its atrazine I find it takes a lot more than just having the windscreen wipers on. My knowledge on this stems from spraying in the early nineties with a lay farming system. We were typically spraying roundup on large plants later in the season for knockdown. Now I am disappointed if I see ryegrass tillering, meaning I got there too late. These days the weather when we are spraying knockdown (glysophate) it's earlier in the season and there seems to be less dew. Maybe also bigger boomsprays mean we are getting to the paddock a little later.(getting slacker)
 

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Discussion Starter · #20 ·
funny thing is although I don't intentionally spray in the rain unless its atrazine I find it takes a lot more than just having the windscreen wipers on.
My rule of thumb is that when the hood is all wet, it's time to shut 'er down. The amount of rain in the window is irrelevant as you are driving into the rain so more rain hits the window (per square inch) than the ground and it's not enough for the plants to get a bit wet, it has to WASH the chem off. That takes quite a bit. Of course, some chemicals I shut 'er down if it only LOOKS like it might rain.
 
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