Growers tend to build their own formula for good vine killing management from years of experience. Over that period of time they learn how factors such as soil moisture, the percent of mature vines in the field and the level of nitrogen in the soil can affect the potential volume of high-quality potatoes.
Growers are walking a tight rope when it comes to good vine killing management, said Dr. Brian Hopkins, cropping systems soil scientist at the University of Idaho.
The most important thing is timing,” Hopkins said.
Before you kill the vines, there are two conditions that need to be met: appropriate nitrogen levels in the soil and the breed-specific percent of mature vines.
“Coming into vine kill with adequate Nitrogen (N) management is very important,” Hopkins said. “Levels can be monitored through petiole testing, which is very important to do because if the N level gets too low the plants die too soon or if it’s too high you end up with lushness.”
The second condition, which is based on a bruise management standpoint, requires vines to be killed based on the recommended percent of matured vines in the field for that breed.
“For Russet Burbank, you want to kill the vines when 70 percent of the vine is still green,” said Bill Bohl, University of Idaho-Extension educator in potatoes. “Ranger Russet needs the field to be 90 percent green. Allowing a crop to almost be dead is going to increase your potential for black spot bruising.”
Bohl said if a farmer has had troubles in the past with bruising, the farmer should re-think his or her vine killing management.
“Most of the time that’s the problem when black spot bruising occurs,” he said.
The actual process of vine killing can either be done mechanically, chemically or a combination of the both.
If vine killing is done mechanically, you just go through and chop up the vines, Hopkins said.
“Generally, you also split the vines and move them off into the furrows because it can make harvesting easier,” Hopkins said. “It just gets them out of the way. If they are off to the side, it’s that much less coming up over and slowing the operation down or plugging things up.”
If the mechanical method is used for vine kill, Hopkins wants to remind growers to be aware of the soil moisture.
“You don’t want it too wet because it induces soil compaction,” he said. “You could make clots, which makes the digging process harder. And you don’t want it too dry either because it’s not good for tubers.”
Chemicals are also used to kill vines, and range in strength in a variety of products.
Sulfuric acid is commonly used in Idaho, Hopkins said.
“It adds sulfur for the crop next year, but it’s a pain to work with and you need special equipment for it,” Hopkins said.
Applying chemicals can take longer for the vines to die, though, which is why many farmers use a combination of mechanical and chemicals, Bohl said.
“Some products may take several days to a couple weeks to take full effect,” he said. “Combining mechanical and sulfuric acid is quicker.”
Bohl said he doesn’t think that mechanical or chemical methods of vine kill used alone is better than another.
“I don’t think any one method is better than another as far as how rapidly it kills the plant,” Bohl said. “Most of the time it’s a matter of convenience. Let’s say a farmer has 800 acres and can harvest 50-60 acres a day.”
If he uses sulfuric acid for vine kill, which has to be commercially applied, the person who comes in will want to do it all at once.
“Instead, the farmer could mechanically kill and/or use a product that doesn’t need to be commercially applied,” Bohl said.
And he or she can do it at whatever pace he or she wants because it may be the intention to let some of the potatoes grow longer because they were planted later.
Whatever method, or combination thereof, a farmer uses for vine killing management, Bohl said farmers should trust to what has worked in the past.
“From a management standpoint, it’s better for farmers to manage their own vine killing,” Bohl said.