Thinking of chitting seed potatoes

Late harvests, low dry matter, poor fry colour and skin quality are serious issues facing potato growers in Northern Ireland. Late harvesting can result in poor ground conditions and low soil temperatures increase the threat of bruising, additionally extending the interval from tuber initiation until harvest increases the risk of skin blemishing diseases detrimental to skin quality.

Altering the physiological age or chitting seed potatoes by controlled sprouting provides the opportunity to promote earlier crop emergence, tuber initiation and bulking. Encouraging early emergence with the aim of producing good canopy cover by mid June will extend the growing season, in the early part of the year when light quality and intensity are high. This increases the potential for light entering the leaves to be converted into plant dry matter and partitioned into tubers.

Physiological age measures the ‘ageing’, of seed during chitting and is determined by the cumulative number of day degrees above 4 degrees Celsius following dormancy break. Physiological aging begins when the sprout has grown greater than 3 mm in the eye.

The aim in chitting late maincrop varieties such as cv. Navan would be to accumulate 250-300 day degrees prior to planting. Working back from a target planting date of, for example, 20 April and allowing seven days for dormancy to break, seed set up on 1 March assuming an average daily temperature of 10 degrees Celsius, would accumulate 264 day degrees, that is, 44 days x (10 degrees Celsius – 4 degrees Celsius).

Traditional sprouting trays remain labour intensive and difficult to incorporate into mechanical planting systems without damaging sprouts. At Greenmount Campus we chit maincrop potatoes using the Pregerm system where seed is placed into a flat net bag rather than a tray. Each Pregerm bag is divided into six long pockets that stop the bag bulging at the bottom and maintain its wide flat shape. This ensures that the layer of potatoes in the bag is relatively thin and that light and air can penetrate to all tubers. Each bag typically holds 125 KGs of seed. This promotes the growth of short robust sprouts approximately 1 cm in length capable of withstanding the mechanics of the cup planter. This is achieved by chitting in a cool, well-ventilated store with both natural and artificial light. Daily day degree accumulation averages approximately 4 to 6 degrees Celsius during March and April resulting in relatively slow sprout growth.

The major impact of chitting is producing an acceptable yield of quality tubers which can be harvested earlier in the season. This provides options in planning for harvest at the beginning of the growing season and a means of advancing the average harvest date with advantages to tuber appearance, processing quality and crop value.