“During the past decade there has been a constantly increasing demand for good seed potatoes.” This could very easily have been a recent quote, but it’s from the March 1926 issue of the American Potato Journal, a scientific publication now named American Journal of Potato Research, published by the Potato Association of America. Selecting quality seed was a concern nearly 90 years ago and should still be today.
In Idaho, seed expenditures—including cutting and seed piece treatment—can account for 15 to 19 percent of the total operating costs of raising potatoes. It seems prudent that potato producers carefully cut and plant their seed to minimize costs.
The total cost to purchase, cut, treat and plant potato seed on any particular farming operation depends on the potato cultivar, transportation fees, seed treatment purchased and cutting and planting costs. However, planting seed pieces that are larger than they need to be, which is under the control of the potato producer, will certainly increase seed costs.
For many potato cultivars, seed pieces should be in the range of 1.5 to 2.5 ounces with an average size of 2 ounces to produce a crop with acceptable yield and quality. Increasing seed piece size generally results in higher yield, but the law of diminishing returns eventually kicks in. This means that for every incremental increase in seed piece size, there is a steadily reduced yield increase. Furthermore, planters may not be able to accurately plant seed pieces that are larger than 3 ounces or so.
With that said, here’s an example to illustrate how seed piece size will affect seed costs: Assume seed costs $14.50 per cwt. by the time you get it to the field ready to plant, and you are planting seed pieces 12 inches apart in rows spaced 36 inches apart. All potato planters will have some skips—missing seed pieces—so we’ll assume 90 percent displacement accuracy. That is, each acre will have 13,068 seed pieces out of the intended 14,520 that would have been planted if the planter had never missed placing a seed piece.
Now, let’s go through the calculations to determine seed cost per acre using an average seed piece size of 2 ounces:
2 oz./seed piece × 14,520 seed pieces/acre
÷ 1600 (to convert oz./acre to cwt./acre)
× 0.9 accuracy × $14.50/cwt
If the average seed piece size is increased to just 2.25 ounces, cost per acre would be $266.46, an increase of $29.61.
For every 500 acres of potatoes, increasing the average seed piece size one-quarter ounce adds $14,805 to your seed bill. The increased cost for seed on a whole-farm basis will be even more dramatic as the cost of seed increases.
Most potato producers pay close attention to the accuracy of their planters. However, producers need to carefully monitor seed cutting operation as well to assure the desired seed piece size is being achieved. Be sure the person overseeing the cutting operation is very familiar with operating the seed cutter, and check cutting accuracy often.