Pro Tips for Pre-Harvest and Desiccation Sprays

A version of this article was originally written by @nozzle_guy as a guest blog for Farm At Hand, and is reproduced with permission.

One of the smartest decisions a grower could make is to consider a late-season harvest-aid application. Particularly in years with thinner stands, weeds can maintain a foothold. Late season moisture can give new life to late emerging plants or branches.  When the crop is ready to cut, this could mean all sorts of cutterbar, pickup reel, feederchain, and sieve headaches.

A desiccant or pre-harvest herbicide application can help avoid those problems.  The challenge is to get the spray into, or through, a mature crop canopy.  Here are some pointers to do it right.

1. Evaluate where within the canopy the spray needs to go to do its job. If you’re considering a pre-harvest herbicide, are you looking to control dandelions or buckwheat near the bottom of the canopy, or are you trying to get thistles or quackgrass, whose leaves are near the top? If you’re mostly trying to accelerate drydown with a contact product, where in the canopy are the green stems and leaves that you need to contact?
2. Take a bird’s eye view of your canopy. That’s how the spray sees it.  If you can clearly see your target, the spray application is pretty straightforward because most droplets will make their way there easily. But if the target is obscured by a lot of foliage, or if it’s vertical, the job is much more challenging and will require some combination of more water, slower speeds, angled tips or finer sprays.
3. To hit plant parts that you can’t see, one of the main tools is finer sprays. The smaller droplets have an easier time changing direction to get around obstacles like leaves, and they are also much more likely to be intercepted by petioles and stems, and to stick to them. This can be both an advantage and disadvantage – for example, the awns in bearded cereals are notoriously effective at capturing the smallest droplets before they can do any good further down.  If you don’t want to install a different nozzle to get a finer spray, simply increase the spray pressure of your low-drift nozzle to 80, 90, even 100 psi.  This will create enough fine droplets. But don’t expect the higher pressure to push the spray into the canopy.  Only air-assist can do that.
4. To get more spray deeper into the canopy, slow down, add water, and point nozzles backward. The backward orientation helps offset the forward travel speed, giving the droplets a slower net forward velocity that helps their downward movement.
5. If you’re using contact products like diquat, paraquat, saflufenacil or carfentrazone, use generous amounts of water, and slightly finer sprays. Make sure that spray drift control remains a priority and pay attention to water quality.
6. Test your water and make sure your water doesn’t have turbidity (suspended clay or other organic matter), for glyphosate and diquat or paraquat, and hardness, for glyphosate. Aluminum sulphate can help get rid of turbidity in a pond, but it takes time (treat turbid water at least 24 to 48 h before you need it).  If treating a storage vessel, expect a layer of sediment. Ammonium sulphate (AMS) and other water conditioners can remove antagonizing hard water ions like magnesium and calcium. This is especially important as we increase water volumes with glyphosate to get better coverage. The higher water volumes give a concentration advantage to the hardness minerals.
7. Diquat and paraquat’s mode of action benefits from being applied in the evening. The absence of the sun allows it to be taken up and slightly moved (by diffusion, not true translocation) within the leaf before morning sunlight activates it. Once activated by the sun, these products exert their activity and movement stops. If you’re not careful, the tighter window of evening-only applications could get you behind. And of course, be aware of the signs of inversions and know when to quit.
8. Plan ahead and make sure you give yourself enough time, because to do the job right you’ll be using more water and driving a bit slower. Focus on productivity tools like a fast, efficient fill to make up the lost time.

A good job with a pre-harvest herbicide or a harvest-aid can save many harvesting headaches, and can help dry down during less than ideal conditions. It’s another reason why the sprayer may be the most important implement on the farm.