Most potatoes are harvested by solid set harvesters that dig two or more rows at a time and load directly into bulk trucks, which carry the tubers to a packinghouse, processor, or storage facility. Where soil is free of rocks and clods, windrowers are frequently used, allowing the harvest of up to twelve rows at a time.
Preventing bruising is one of the most important considerations in a well-managed harvest operation. Blackspot and shatter bruise can seriously affect marketable yield if precautions are not taken to reduce them. Several factors are important in controlling bruising.
Proper soil moisture at harvest helps reduce bruising. The water content of tubers affects their sensitivity to bruising. If their water content is low, they are sensitive to blackspot bruise; if it is high they are sensitive to shatter bruise. The right combination of moisture and temperature helps to minimize both kinds of bruising. Soil moisture of 60 to 80% of field capacity is generally recommended.
The lower the soil temperature, the more susceptible tubers are to shatter bruising, especially when the temperature is below 50ºF. Whenever possible, harvest when the soil temperature is above 50ºF. Soils are warmest between 11 a.m. and 11 p.m. Harvest during these hours if soil temperature is likely to fall below 50ºF at night. However, avoid harvesting when tuber pulp temperatures are above 60ºF to reduce tuber rots in storage.
Follow the recommendations for harvester operation in your area to reduce bruise injury. Use equipment that is in good repair, correctly adjusted, and operated by an experienced person familiar with local conditions. Important considerations are digger blade depth; reduction of "spill-out" losses at the digger blade; apron pitch, speed, and agitation; travel speed; drop heights; and control of "undersweep." Operate the primary chain at 1.1 times the travel speed (1.5 feet per second for each 1 mile per hour of tractor speed). Operate the other harvester chains at 0.6 to 0.8 times the travel speed (1 to 1.2 feet per second for each 1 mile per hour). Keep chains filled so the tubers have little space to bounce around. Use padding on equipment wherever bruising might occur. When unloading at storage facilities, keep drop heights to a minimum; use straw-filled bags on baffle boards and use mats underneath hoppers to cushion spilled tubers.
Instruments that measure bruising forces in harvesting and handling equipment can be used to correlate the measured forces with resulting bruising, and thereby identify specific locations where modifications are needed to reduce bruising.