Pea-Wheat Rotation Explained

EDMONTON - Jun 2/08 - SNS -- Having only pea and wheat crops in a rotation is not a true crop rotation, note Alberta Agriculture agronomists. Ideally, having three or more crops in a rotation is best from a crop diversity standpoint and is preferred to help manage pest issues, particularly disease. However, a short two-year pea-wheat rotation does offer a number of advantages in the drier regions of southern Alberta.

A pea crop will fix about 80% of its nitrogen (N) requirements. Generally, no additional N fertilizer is required with pea, however, pea seed must be inoculated with the proper rhizobium bacteria (Rhizobia leguminosarum) to ensure optimum N fixation (McKenzie et al. 2001).

Residual N remains in the surface residue, roots and nodules of pea after harvest. As the pea residue degrades, N is released for subsequent crops. It is estimated that field pea contributes about 1.0 to 1.5 lb/ac of N for every bu/ac of pea grain produced. At the long-term crop rotation site at Bow Island, wheat has been grown successfully without any additional commercial N fertilizer in the pea-wheat rotation, since inception in 1992.

Pea is not very responsive to phosphate fertilizer and could be reduced or even eliminated when soil test levels are greater than 30 kg P/ha (McKenzie et al 2002).

The need for phosphate and other fertilizers in the crop rotation can be eliminated if composted manure is applied at a sufficient rate once every four years.

Some insect problems such as the wheat stem sawfly and foliar disease problems can be controlled using a pea-wheat rotation as opposed to growing continuous wheat crops.

Using the pea-wheat rotation allows for rotation of herbicide groups with different modes of action for weed control, reducing the potential for development of herbicide tolerant weeds.

Pea is well adapted to no-till direct seeding. Elimination of tillage leads to conserved soil water which results in increased grain yield and higher water use efficiency. Other benefits of no-till include the lower fluctuations in surface soil temperature, which favors increased nitrogen fixation.

Generally, pea is shallower rooted and draws most its moisture from the top 60 cm of soil (McKenzie et al. 2004). When pea is seeded in early spring in southern Alberta, it is often harvested in early August. This leaves an extended period from August to November to store precipitation for crop use the next year. As a result, following a pea crop, there is often more stored soil moisture available for the next crop season.

In summary, wheat yields following a pea crop are often higher because of the increased soil nitrogen availability, higher amounts of stored soil water, and reduced disease potential. With the pea-wheat rotation, the need for nitrogen fertilizer inputs is greatly reduced and often eliminated resulting in a very productive and economical cropping system.