Which seed to use

The purpose of crop
To find out which seed stock is suitable, we must first establish the purpose of the crop. Are they to be seed potatoes, ware potatoes, a large-sized potato for the processing industry, or production for early markets perhaps? The combination of the required production purpose and harvesting time determines which seed stock should be used.

Stem density
Both the yield of the crop as well as the average tuber size are strongly influenced by the number of stems per hectare. Each main stem can be regarded as an independent production unit, which is why a sufficient number of strong stems should develop per seed tuber. A greater stem density leads to higher yields but a finer grading, so the preferential density of a crop may be expressed in the number of stems per square metre and depends on the expected financial return. In addition to the number of tubers planted per hectare, the stem density is determined by:

• the physiological stage of the seed tuber;
• varietal characteristics;
• the number of sprouts per tuber;
• soil conditions;
• planter adjustments.

Relations between number of main stems per m² and tuber yield in the various grades (according to figures compiled by Reestman and Bodleander).













Tuber size
If a reasonably high plant density is achieved, seed size has, under normal growing conditions, no great influenceon yield. As a grower, you have the choice of different seed grades. This choice depends on economic factors, the purpose of the crop and local growing conditions. A large seed potato will produce more stems. Nevertheless, a tonne of large seed potatoes will produce fewer stems than a tonne of small ones. A large seed potato has the advantage of having extra reserves in times of drought, cold, heat or in an inferior seed-bed. Under such growing conditions, the large seed potato will be more reliable than the small one. Small and large sizes usually differ in price. The grower can calculate which is the most economical: multiplying the price by the required number of kilograms will give the investment in seedstock. Given the right pre-treatment and depending on the size, the seed tubers should produce between three and six firm, short sprouts at the time of planting.

Seed required
How does the grower calculate the quantity of seed potatoes required? In addition to the surface area of the field, two factors play a role in this:

* the required number of stems per square metre;
* the size of the seed tubers.

Examples
Those wishing to harvest a small size (28-45 mm) of seed should aim at a high stem density. Depending on variety, soil and climate, approximately 30-45 stems per square metre will suffice for this purpose under Dutch conditions. To obtain a high proportion of large tubers (>55 mm) to be sold to the processing industry forfrench fries, around 15-20 stems per square metre can suffice under Dutch conditions. In the latter case, four seed potatoes with four good sprouts will be sufficient for one square metre. If the average weight of a seed potato amounts to 50 grams, 2.000 kg of seed potatoes will be required per hectare (4 seed potatoes x 50 grams x 10.000 square metres).