It's well known that within the range of seed-piece sizes and spacings normally used by growers, planting larger seed pieces at the same in-row spacing and planting equal-size seed pieces at closer in-row spacings will generally increase yields. However, the interaction between seed-piece size, spacing and total seed weight planted per acre is a topic that has not received a lot of attention in the past. This was the issue we wanted to address in a study conducted at the University of Idaho Aberdeen R & E Center using Alturas, Russet Norkotah and Ranger Russet potato cultivars.
Seed tubers of the above three cultivars were hand-cut into seed pieces averaging 1.5, 2.25 and 3.0 ounces, with each seed piece weighing within one-quarter ounce of the average. All seed size categories for each cultivar were planted at a spacing of eight, 12 or 16 inches in rows spaced 36 inches apart.
Seeding rates ranged from 10.2 to 40.5 cwt per acre; however, the 1.5-ounce seed planted at eight inches, 2.25-ounce seed planted at 12 inches and 3.0-ounce seed planted at 16 inches all resulted in an equal amount (20.4 cwt) of seed planted per acre. It is these three planting rates that are the focus of this article. All cultivars responded similarly to the treatments, so the data were combined for this article.
As expected from previous reported research, total and U.S. No. 1 yields increased with increased seed-piece size, and total and U.S. No. 1 yields were higher when seed pieces were planted with a closer in-row spacing (Table 1). An interesting outcome was the effect of planting equal amounts (20.4 cwt/ac) of seed with the three combinations of seed-piece size and in-row spacing. Note in Table 2 that equal quantities of seed all produced equal total and U.S. No. 1 yields.
For the fresh market, tubers in the 10- to 16-ounce size range generally tend to be more valuable than larger or smaller tubers. Comparing harvested tuber size profiles from the equal seeding rates, planting 2.5-ounce seed pieces at 12 inches or 3.0-ounce seed pieces at 16 inches resulted in fewer harvested tubers weighing less than 10 ounces compared with planting 1.25-ounce seed at eight inches. However, there was no difference in the yield of 10- to 16-ounce tubers harvested from planting equal quantities of seed per acre.
A fresh-market economic analysis (gross returns minus seed costs) was performed on Russet Norkotah. Increasing seed-piece size increased gross returns only for the 16-inch seed-piece spacing. Gross returns were lower when 1.5- and 2.25-ounce seed were planted at 16 inches compared to eight or 12 inches.
Spacing had no effect when planting 3.0-ounce seed. Additionally, planting equal quantities of seed per acre resulted in equal gross returns per acre of $2,944, $2,848 and $2,840 for 1.5-oz seed planted at eight inches, 2.25-oz seed at 12 inches and 3.0-oz seed at 16 inches, respectively. Keep in mind, however, that economic return will depend on cost of seed and price received for harvested potatoes when sold.
This study demonstrates that equal quantities of seed obtained by varying seed-piece size in inverse proportion with in-row spacing resulted in equal total and U.S. No. 1 yields. For the cultivars used in this study, planting seed pieces weighing approximately 2.25 ounces at a spacing of 12 inches in 36 inch-wide rows produced a crop with optimal yield and quality. Keep in mind, however, that although these cultivars responded similarly to seed-piece size and spacing, other cultivars may respond differently, particularly when grown under different environmental conditions.