Replacing summer fallow with an annual legume is an option being considered by some producers. Possible benefits include reduction of fertilizer inputs (due to biological fixation of nitrogen), reduction of pesticide inputs (due to disruption of pest life cycles), improved labor and machinery efficiency, and risk reduction through crop diversification.

This study was started in 1995 at the Williston Research Extension Center. The crop sequences being compared are wheat grown continuously or alternately with summer fallow, safflower, field pea, grain lentil, or forage lentil. For each legume species both harvested and green-manured treatments are included. 

Two sets of the continuous wheat treatments as well as each of the green manure treatments were included to allow for different N fertilizer rates on the subsequent wheat. One set of the continuous wheat treatments receives the recommended N rates based on soil testing, while the second receives less than the recommended rate of N. This lesser rate is the determined by using the N recommendation for wheat following fallow. In one set of the green manure treatments the subsequent wheat crop receives the recommended N rates based on soil testing, while the second receives more than the recommended rate of N.  This greater rate is determined by using the N recommendation for continuous wheat. All the other wheat treatments receive the recommended N rates based on soil test results for a 40-bu/a wheat crop. The actual rates of N applied this year are given in Table 3. In addition, all crops received 20 lb/a of diammonium phosphate (DAP; 18‑46‑0) with the seed.

Safflower, field pea, grain lentil, and forage lentil were planted between April 30 and May 3. Wheat was planted on May 20 and 21. Field pea was green manured on June 30; grain lentil and forage lentil were green manured on July 17. Forage lentil was swathed on July 31. Field pea and grain lentil were harvested on July 31, wheat was harvested August 18, and safflower was harvested on September 5. Above ground biomass was determined for all crops at the time of green manuring and at harvest. Protein content was determined for wheat and oil content was determined for safflower.

The biomass at green manuring was greater in 2003 than the 8‑yr average for forage lentil and grain lentil but less for field pea (Table 1). The biomass at harvest was greater in 2003 than the 8-yr average for forage lentil but less for grain lentil and field pea (Table 1). 

Grain yields of grain lentil, field pea, and safflower in 2003 were greater than the 8-year average (Table 2). Safflower oil content was the second lowest during the 8-year period.

Average wheat yield was 34.3 bu/a, just 1.2 bu/a greater than the 8-yr average (Table 3). Seven treatments had yields greater than the 8-yr average; six treatments had yields lower than the 8‑yr average. Yield of continuous wheat with insufficient N fertilizer was 11.6 bu/a greater than its corresponding 8-yr average, all other treatments were within 3.5 bu/a of their corresponding 8-yr averages. The improved performance this year of the continuous wheat with insufficient N fertilizer is unexplained.