Cull or waste potatoes are any tubers deemed to be unsaleable in the fresh, processing or seed-potato markets. Cull potatoes are the result of damage caused by disease (both in field and storage), bruising, adverse environmental conditions, unacceptable size and lack of markets. These potatoes eventually need to be destroyed. It is always in producers' best interests to reduce cull rates in their crops to enable them to increase their bottom lines. This fact sheet is intended to provide methods to dispose of potatoes safely so that they do not pose a threat of spreading diseases, especially late blight.
When do culls accumulate
Cull potatoes will accumulate whenever potatoes are graded. It may become necessary to grade potatoes at any time during the year. Culls occur during grading in fall or spring, due to disease in the storage season or inability to market a certain variety. Culls also accumulate from seed cutting operations and during harvest. A cull potato can be anything from a whole tuber to the slivers that are left after cutting seed. If these slivers have an eye, they will have the ability to sprout a new plant.
Why are culls a problem
Due to the fungal disease late blight (Phythopthora infestans), culls can become an increasingly serious problem. Late blight is potentially very destructive and can be spread very quickly across a field, and from field to field. Late blight produces spores that can be spread by wind, water, and mechanical or human transmission. Once a plant is infected, it will begin to exhibit symptoms in two to three days and can be dead in a matter of days. Potatoes culled due to late blight damage represent a potential source of late blight if they sprout and produce a plant. Although tubers near the top of the pile will normally break down and rot due to weather exposure and different environmental conditions, tubers near the middle of the pile will sprout and grow in the spring. Potato plants growing from cull piles may also accelerate the spread of late blight, as they are seldom included in growers late blight spray programs.
The smell from a cull pile can become a nuisance problem in the spring. Rotting, wet tubers produce a foul odour that can cause problems if the pile is close to a residential area. The decaying pile also attracts insects, including the Colorado potato beetle. The beetles will eat the foliage from the pile and then move on to nearby or adjacent fields. Aphids can also be a problem. They may suck juices from plants in the pile, then move to a nearby field. There is a risk of disease transmission if the pile had either potato mosaic virus or potato leaf roll virus.
Cull piles near waterways also pose a problem as tuber breakdown produces nitrates that could seep into nearby water sources.
Disposal of Culls
Disposal is the next step, once potatoes have been culled. Some possible methods include:
Use as Livestock Feed
Potatoes are 80 per cent water and 20 per cent dry matter. Growers must take this into account when using potatoes as a ration. Raw, ensiled or cooked culled potatoes can be fed to cattle. It is estimated that two kg (4.5 pounds) of potatoes equals .45 kg (one pound) of barley in nutritional content. It has been shown that the addition of limestone to a diet consisting of potatoes is beneficial to cattle. Limestone helps reduce the amount of acid that has accumulated in the rumen due to a large intake of potatoes. Cattle will eat large quantities of potatoes, if given the opportunity. Information from Prince Edward Island indicates that a cow will eat 10 per cent of its body weight per day in potatoes. If feeding cull potatoes is an option, contact your local livestock agrologist to determine a ration that is suitable for your herd.
Bury the Cull Pile
It may be necessary to bury the pile, if there is a large amount of culls. There are certain steps that must be followed to ensure the culls are disposed of safely. Make sure the pile is not buried near a water source. When selecting a site, make sure the water table is low and that there is no access to streams nearby. Also make sure that the water table is not shallow and that the pile does not drain into nearby streams to avoid contamination.
Cull piles should have at least 1.8 metres (six feet) of soil on top of the potatoes to prevent regrowth. Transportation of potatoes to a burial site can be expensive; therefore, keep the distance travelled as short as possible. The cull site should be monitored during the summer for any volunteer plants. If any plants appear, they should be rouged (pulled) out or treated with a herbicide. Extra soil may need to be added as the cull pile settles during decomposition.
Spread the Pile on a Field
Spreading cull potatoes on a field is one of the most common disposal practices in North America. It is normally done in late fall or winter to ensure that the potatoes freeze. Freezing causes cells to rupture within the tuber, accelerating breakdown and rot once it thaws. Culls can be piled up to six inches deep in the field, if spread in late fall or early winter. In a normal prairie winter, there should be sufficient freeze/thaw action to break down all the tubers before spring. If tubers are being spread on fields in early spring, they should be placed uniformly on the field and only one layer thick. Regular discing is necessary to ensure that new root systems do not form.
Cover the Pile
This method is not popular in many areas, but can be used if piles occur late in the spring or when immediate action needs to be taken. The pile should be evenly distributed over an area and covered with plastic, preferably black polyethylene. This causes the tubers to heat up and eventually break down. Breakdown is aided in a hot summer, while a cool summer may leave some tubers intact. These intact tubers should then be spread on fields in late fall to ensure complete breakdown by the next spring.
Make certain the entire pile is covered so there is no chance of plant growth or spore escape. One of the problems with this method is the watery mess that will form in and near the pile once the pile has broken down. Sawdust or other absorbent material can be used to soak up the potato runoff and then removed from the site. Covered piles should not be placed near waterways.
* Culls can also be sold to companies that produce dehydrated potato flakes or other processed potato products. There are currently no companies that dehydrate potatoes in Saskatchewan (September 2015).
Mandatory Removal of Cull Pile
Late blight is a very serious disease in the potato industry. Millions of dollars are spent each year trying to prevent or control late blight outbreaks.
Due to the extreme seriousness of this problem, the industry requested that late blight be named a pest under Saskatchewan's Pest Control Act. As a result, regulations require all culled potatoes to be disposed of in an approved manner on or before June 15 each year. Failure to comply with this Act will result in fines. This is just one further step to ensure that growers do everything possible to make sure that late blight does not become an annual occurrence in the province.
Written and Edited by Saskatchewan Agriculture with the generous assistance of the following individuals: Dr. Doug Waterer, Dr. Jill Thomson, University of Saskatchewan