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Rotational options being studied

Rotation is often first considered from an above-soil perspective, says Dr. Claude Caldwell, professor of crop physiology at the Nova Scotia Agricultural College in Truro, Nova Scotia. “However, you also must be concerned about what’s going on underneath the soil surface,” he stresses.

Caldwell is currently involved in two studies on how rotational crops can help secure root health at the same time profits are maximized. One is an oilseed crop project involving canola led by Don Smith at McGill University for which it is hoped funding will soon be secured. “With the new crushing plant up and running about 14 hours’ drive away in Becancour, Quebec, they’re expecting a large 2010 canola crop,” Caldwell notes. “The research is needed.”

The new plant will process approximately one million metric tonnes of oilseeds (600,000 tonnes of canola and 400,000 million tonnes of soybean), and generate annual sales of $450 million.

Caldwell will be co-ordinating basic agronomy experiments to determine the best planting date and seeding rate for canola at eight sites across Nova Scotia, New Brunswick, Ontario and Quebec. “We will also be measuring how canola affects the base rotational crop, whether that’s soybeans, corn or potatoes,” he notes, “Rooting depth and rooting pattern is very important in rotation to break up the soil and stimulate invertebrate and micro-organism activity.”

He will also study how canola can be used to increase nutrient use efficiency and break disease cycles. “Potatoes leave a fair amount of nutrients in the soil,” says Caldwell, “and the hope is that canola can use these effectively.”

Camelina is the focus of his other research. “I’ve been working with it for the last five years,” Caldwell says. “We’re seeing it as a very positive crop for potatoes as it’s very resistant to diseases and doesn’t share diseases with potatoes.”
While camelina is being grown for jet fuel production in other parts of the world, Caldwell says it is being looked at in the Maritimes for its oil for human consumption and for aquaculture feed. “We want it to be a profitable cash crop for farmers here,” he says. “We’ll also be examining its effects on nematodes and root disease.”

New best practice advice
Dr. Christine Noronha, who studies pest resistance at the Agriculture and Agri-food Canada Crops and Livestock Research Centre in Charlottetown, Prince Edward Island, has a few tips for growers with regard to controlling pests with rotation. “The mandated use of this practice is definitely beneficial in disrupting the life cycle of insects,” she says, “but because pests move around, distances are critical. That is, when a grower plants a field for potatoes that has been in rotation, it needs to be as far away as possible from a field where potatoes just grew.”

Even a few hundred metres, says Noronha, can make a difference. “The further you put a food source away from the pest’s stronghold, the more you will reduce the pest population,” she explains. For this strategy to really be effective, however, area-wide collaboration is needed. “If you have an area free of a pest, you can expand that area with collaboration. Talk to other growers and do what you can to make it better for all of you.”

Some pests will turn to weeds as an alternative food source when their food-source field is planted with something else in the rotation. “So this means if you can control nightshade and other weeds, you can reduce the pests because they have no alternative to feed on,” says Noronha. “Delaying planting also helps, because if the pests have nothing to eat, they’ll leave.”

Weed scientist Dr. Jerry Ivany, also at AAFC Charlottetown, says that traditional rotation options of barley under-seeded with clover and timothy mix have now been replaced by animal grade soybean and some food grade soybean crops by growers who want a higher value cash crop in the rotation. He cautions, however, that before a new rotational crop is chosen, growers need to ensure that there are herbicides registered for use on the crop, or they may be sorry in terms of weed management. “However,” he notes, “if there is no herbicide registered for that crop, you can reduce weed problems by choosing a field with a low weed density or by reducing the weed density before planting by cultivating the field, letting weeds grow for seven to 10 days, cultivating again, and then seeding. Make sure you have no broadleaf perennials and quack grass.”

With regards to controlling broadleaf perennial weeds such as field mint, perennial thistle and mugwort that are problems in the potato crop, Ivany advises controlling them before you plant. “None of the potato-registered herbicides control broad-leaf perennial weeds very well, so growers have to think ahead and control the year before, and some are not doing this,” he says. “Fall application of herbicide products that control broadleaf-perennial and quack grass on barley stubble is recommended, and the use of Roundup Ready crops such as soybeans will also mean you’ll have less of a problem with the perennial weeds in your next year’s potato crop. Growers who buy or rent new land for potatoes need to find out what perennial broadleaf weeds are present, and if those weeds are in the field, then plan to control them in the rotation before the potato year.”

Dr. Rick Peters, who studies vegetable diseases at AAFC Charlottetown, says his research has indicated that length of a rotation is a key consideration for reducing soilborne diseases in potatoes. In a long-term trial in Harrington, PEI (1994-2008), Peters compared two-year (potato-barley) with three-year (potato-barley-red clover) rotations. “The potatoes grown in three-year rotations were consistently significantly less diseased than those grown in two-year rotations, particularly with respect to diseases caused by rhizoctonia (stem and stolon canker; black scurf),” he says. “Growing non-host crops between potato crops for successive years causes a natural decline in pathogen populations in the soil.” Peters notes that in some situations (organic growers and conventional growers in some parts of the world), rotation cycles are extended so that potatoes only appear once every four or five years.

The type of crops grown in a potato rotation can also impact on disease severity. “Our research has shown that growing brassica crops, like canola, prior to potatoes can reduce the incidence and severity of soilborne diseases caused by rhizoctonia,” says Peters. “This is particularly effective when canola is used as a green manure crop prior to potatoes.”
The breakdown of the canola tissues in the soil releases chemicals that are toxic to pathogens and also stimulates increased soil microbial activity that leads to biological control of pathogens.

Peters is also experimenting with using sorghum-sudan grass as a green manure crop prior to potatoes. “In some production areas, it has been shown to reduce Verticillium wilt in subsequent potato crops,” he says.