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Andy Robinson

Potato Extension Agronomist

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Understanding Potato Stem Number

Knowing your stem number will likely affect your production and profitability. Because potatoes are inherently quite variable, seed will vary across lots and within the same lot. Research clearly shows that as stem number increases, so does tuber number. With fewer stems there are fewer tubers, which result in a larger tuber profile. Think about this for a minute: What do you get paid for in your production system? The answer to this is what you need to focus on in order to maximize profit. Finding the best number of stems for the cultivar and desired outcome is essential to an economically profitable potato operation.
Understanding Potato Stem Number

Difference in tuber number and size when comparing a 2 vs. 6 stem Russet Burbank plant.

As you prepare to plant your potato crop this year it would good to take some time to review the effect of stem number on potato production. This is important because knowing your stem number will likely affect your production and profitability. Because potatoes are inherently quite variable, seed will vary across lots and within the same lot. Growers most typically use the average stem number to determine the expectation of each lot. Certainly the desired size and number of tubers will depend on if you are growing for seed, the fresh market, process market, or chip market.

The main factor affecting stem number is the physiological age of the tuber. Potato seed is aged in two ways, chronologically and physiologically. The chronological age is the number of days that have passed since the tubers were harvested. The physiological age refers to the internal age of the seed as affected by biochemical changes that take place within the tuber. Potato tubers age at a greater rate when grown in stressful environments such as temperature stress, moisture stress, fertility stress, or disease pressure, when stored under fluctuating temperatures or warm temperatures, or are bruised. Aged tubers will have more stems, more tubers and smaller average size as a result of competition.

Now, let’s focus more on some principles in stem number. Research clearly shows that as stem number increases, so does tuber number. High stem number results in a smaller tuber profile and can reduce overall yield and, even more important, marketable yield (Table 1). The opposite is also true. With fewer stems there are fewer tubers, which result in a larger tuber profile. Think about this for a minute: What do you get paid for in your production system? The answer to this is what you need to focus on in order to maximize profit. Finding the best number of stems for the cultivar and desired outcome is essential to an economically profitable potato operation.   

The best way to determine stem number is to do a grow out before planting. The more representative seed sampled and the more uniform the method of sampling, the better the chance of accurate predictions (100 seed pieces would be a good number to start with). Grow outs can be done in soil or in bags, and require 3 to 4 weeks.  The good news is that moderate success in predicting stem counts can increase the bottom line if managed properly. Keep in mind that seed in storage will age slightly more that the seed tested, so expect a small increase in stem number from what was counted. Practice will help improve your predictability.

By having an accurate idea of the number of stems prior to planting you can manage the seed. If the stem number is too low, increase the storage temperature to 50-55 °F (to age the seed), delay planting, or decrease within-row spacing. If the stem number is too high minimize the aging by keeping the bin temperature cool (38 °F), plant as early as possible without subjecting your seed to increased disease pressure, increase within-row spacing, or treat seed with auxin (commercially called Rejuvenate). For example, the effects of planting date and storage temperature on Russet Burbank stem number are shown in Table 2. Another example, in Table 3, shows the effect of Rejuvenate on stem number.

As we go into the 2015 growing season my hope is that it can be a successful year. Paying attention to the small and simple details in the spring can result in large benefits at the end of the season.

Table 1. Average stem number of Russet Burbank seed pieces and effect on graded yield. Adapted from Knowles and Knowles, 2006.

 

Avg. stem number

 

3.2

5.4

< 4 oz (cwt/a)

67

106

4-6 oz (cwt/a)

108

134

6-10 oz (cwt/a)

203

189

10-14 oz (cwt/a)

127

86

> 14 oz (cwt/a)

113

60

Total yield (cwt/a)

618

574

Marketable yield (cwt/a)

576

544

% > 6 oz

72

58

% > 10 oz

39

25

Table 2. Effect of planting date and storage temperature on Russet Burbank stem number in Washington. Adapted from Iritani and Thornton, 1992.

 

Planting date

Storage temperature °F

March 31

April 20

May 12

 

----------------------- Average stem number -----------------------

38

2.4

2.7

2.9

42

2.5

3.4

3.3

48

2.8

3.6

3.4

Table 3. Reduction in stem number by applying Rejuvenate on Russet Burbank and Umatilla in Inkster, ND in 2014.

 

Treatment

Average stem number

Russet Burbank

Non-treated

4.9

 

0.16 oz Rejuvenate/ton seed

4.2

Umatilla

Non-treated

4.6

 

0.16 oz Rejuvenate/ton seed

3.8

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