Pea-Wheat Rotation Works in S. Alberta

EDMONTON - Jun 2/08 - SNS -- Using diverse crop rotations that include three or four different crops is generally the most desirable rotational plan, while southern Alberta dryland farmers often find a simple pea-wheat rotation can often be very profitable.

"In the drier regions of southern Alberta, a pea-wheat rotation offers a number of significant advantages," says Dr. Ross McKenzie, P.Ag, with the agricultural research division, Alberta Agriculture and Rural Development, Lethbridge. "Wheat yields after a pea cropping year are often higher because of the increased soil nitrogen (N) availability, higher amounts of stored soil water, and reduced disease potential. As well, the need for nitrogen fertilizer inputs for pea and wheat are greatly reduced and often eliminated. The longer a field is in pea-wheat rotation, the greater the benefits."

In the Brown and Dark Brown soil zones of southern Alberta, crop production has historically focused on spring wheat. In the past 20 to 30 years, crops such as barley and canola have increased in areas where there is sufficient precipitation. However, summerfallow and monoculture wheat systems remain a significant cropping practice for many farmers.

Both the Brown soil zone and the Dark Brown soil zone are semiarid, with average annual precipitation in the Brown soil zone at approximately 350 mm (14 in) and growing season precipitation at about 150 mm (6 in), and the Dark Brown soil zone average annual precipitation of approximately 400 mm (16 in) and growing season precipitation at about 175 mm (7 in). With these relatively low precipitation levels, retaining soil moisture through summerfallowing has been an important agricultural practice in the semiarid regions of the Prairies. However in the long-term, the practice of using summerfallow leads to a decline in soil quality as a result of declining soil organic matter levels, increased salinization, increased wind and water erosion and depleted soil nitrogen and other nutrient reserves.

"To reduce the negative effects on organic matter loss and erosion, producers have shifted from the use of conventional cultivation for weed control in fallow fields, to the use of herbicides to control weed growth," says McKenzie. "This practice is referred to as chem-fallow. Over the years, dryland producers have also adopted the use of commercial fertilizers to optimize wheat yield and quality, as soil nutrient reserves have declined. However, in the long-term, environmental sustainability of cropping systems that include summerfallow remain in question."

McKenzie initiated a dryland research cropping study in 1992, titled Long-Term Dryland Crop Rotation Study in the Brown Soil Zone at the Bow Island substation of Alberta Agriculture and Rural Development’s Alberta Crop Diversification Centre South. It is the only long-term dryland crop rotation study in the Brown soil zone in Alberta. This study was designed to determine the effects of different cropping practices on soil quality in the long-term and to determine the economic performance of the various crop rotations that included:

- reduced summerfallow use

- legumes in the rotation

- use of inorganic fertilizers

- use of manure/compost applications

"One of the initial rotations researched was a wheat-legume plow-down rotation with the intention to replace fallow with a plow-down legume to reduce the need for nitrogen fertilizer," notes McKenzie. "In 1996, the rotation was modified to have pea as the legume crop, but rather than terminate the crop at flowering, the crop was grown to maturity and harvested for grain. It was felt that this would be a more realistic agronomic and economic alternative."

An economic evaluation of all the rotations (Walburger and McKenzie, 2003) showed that the pea-wheat rotation had the highest net annual income of the seven different crop rotations tested. The rotations in the study were: continuous wheat, fallow-wheat, fallow-wheat-wheat; pea-wheat, fallow-flax-wheat, continuous grass; each with various nitrogen and phosphorus or manure treatments.

"One of the major concerns with the two-year pea-wheat rotation is the potential for increased disease pressure," says McKenzie. "However, to-date, increased disease has not been an issue in the long-term dryland trials at Bow Island in southern Alberta."

Work conducted at Indian Head, Saskatchewan in the thin-black soil zone by Agriculture and Agri-Food Canada showed that in terms of plant establishment, plant numbers after 11 years without the use of seed treatments and by using no-till were similar among a wheat-wheat-pea, a wheat-pea and a continuous pea rotation (Lafond et al 2007), which suggested that root diseases may be less important than anticipated for field pea production on the Canadian prairies.

"It is important to provide some break between successive pea crops to reduce potential disease pressure," says McKenzie. "In the drier regions of southern Alberta, a one-year break normally seems to be adequate."

Ultimately, with the rising cost of nitrogen fertilizer, the pea-wheat rotation can be a very profitable crop rotation when used over a period of years in southern Alberta.