The Right Dose

Published online: Mar 23, 2021 Articles, Fertilizer Mike Thornton, UI Southwest Idaho R&E Centers
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This article appears in the April 2021 issue of Potato Grower.

Like many people, I like spicy food. I have several types of hot sauce I use to add just the right amount of heat to many dishes I cook. The problem is that there is a very fine line between just the right amount to flavor a dish perfectly and too much that burns my mouth.

That situation reminds me of what can happen when growers first attempt to use plant growth regulators in their potato operations. When used in just the right amount, they can do desirable things like break dormancy of tubers, speed emergence, optimize stem number and tuber set, and as a result increase payable yield. However, when applied at too high a rate, they can have negative effects, such as slowing plant development, causing deformed plants and tubers, and reducing yield.

To better define what constitutes the “right amount,” we have been studying the response of red potato varieties to two commercially available plant growth regulators applied to seed just prior to planting. ProGibb 4% (a gibberellin) is labeled to break dormancy and promote uniform emergence, while Rejuvenate Seed Treatment (an auxin) helps restore apical dominance on aged seed and control stem number. One of the first challenges we had to address was the fact that ProGibb is labeled as a dip application to seed at a rate of 0.2 to 0.4 fluid ounces in 100 gallons of water, while Rejuvenate is labeled as a spray application to seed at up to 0.16 fluid ounces per ton (depending on variety). Not too many operations are set up to apply products via dipping, so we decided to apply all the treatments as a spray to cut seed in a total application volume of a half-gallon per ton of seed. We applied the ProGibb 4% at rates of 0 (water check), 0.5, 1.0 and 2.0 milliliters per ton of seed alone or in combination with 4 milliliters of Rejuvenate. The main responses we were interested in were emergence rate, stem number, tuber number, tuber shape, yield and grade.

Results for the Chieftain variety really illustrate some of the key lessons from this project. For example, it was apparent very quickly that ProGibb applications to cut seed does help speed plant emergence, reaching nearly complete emergence by 33 days after planting compared to 39 days for the water check (Figure 1). All three rates of ProGibb also increased stem number per plant compared to the water check (Figure 2). Adding Rejuvenate tended to moderate the response by about 0.5 stem per plant compared to what was seen with ProGibb alone. The number of tubers per plant pretty much mirrored the changes in stem number, with higher rates of ProGibb producing the most tubers per plant.

That might all sound great if you are looking to speed emergence and increase tuber number so that your crop produces more small red potatoes that tend to command higher prices. However, this is where some words of caution are warranted. The higher rates of ProGibb also caused a lot of deformed stems to emerge, which took several weeks to grow out of those symptoms. This delayed growth resulted in about a 10 percent reduction in yield at the higher application rates compared to the water check. The tubers were also elongated and not the typical round shape that buyers expect from a variety like Chieftain. Luckily, most of the desirable responses to ProGibb (more rapid emergence, higher stem number, smaller tuber size distribution) were attained at the lowest application rate (0.5 milliliters per ton of seed), without the negative effects on yield and tuber shape. Therefore, we can pretty confidently say, that under our application conditions, there was no benefit to applying more than 0.5 milliliters per ton of seed of ProGibb 4% to Chieftain. Even lower rates might be optimal, but we did not include those in this project. In our studies, Rejuvenate helped moderate the effects of ProGibb, and it seems advisable to consider using both products together initially until finding the exact combination that works best for a given situation.

Emergence rate of Chieftain plants in response to treatment of cut seed with ProGibb 4% prior to planting

Stem number of Chieftain plants in response to treatment of cut seed with ProGibb 4% and Rejuvenate prior to planting.

Unfortunately, it would not be wise to take these results and utilize them for all other varieties, application techniques and market segments. Past research has shown that each variety has its own specific sensitivity to applications of growth regulators, so each operation will need to find the best application program that fits its particular situation. With that in mind, here are some recommendations for working with these types of plant growth regulators:

  • Start small by treating at most a few hundred pounds of seed to help gauge the rate that produces the optimal response.
  • Double-check your calculations each time to make sure you are putting on the desired rate. In the example above, 0.5 milliliters per ton of seed would translate to 3.4 fluid ounces of the ProGibb 4% formulation in 100 gallons of water if applying at a volume of a half-gallon of water per ton (treating 200 tons of seed). This is one case where you can’t afford to fudge on measuring or take a “good enough” approach, as a little extra in the tank can have a lot of negative consequences.
  • Be very specific about the formulation of the product you are using. Gibberellin also comes in a ProGibb LV formulation that has double the amount of active ingredient per fluid ounce as the ProGibb 4% formulation. Using the more concentrated formulation could result in twice the desired application rate.

Despite the challenges of working with plant growth regulators, growers who have taken the time to monitor their applications and study the variety-specific responses, as well as track incremental changes in stem numbers, tuber counts, and size profile, are reporting that the results are adding dollars to their bottom line.

Mike Thornton is a professor of plant sciences and chair of the University of Idaho’s Parma Research & Extension Center. He can be contacted at