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. Most years, this lesser rate is determined by using the N recommendation for wheat following fallow. In years such as 2004 when the recommended N rates for wheat following fallow and for continuous wheat are the same, a lesser rate is chosen. 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.

Forage lentil was planted April 30, grain lentil and field pea on May 4, wheat on May 6, and Safflower on May 10. Field pea and grain lentil were green manured on July 2 and forage lentil was green manured on July 14. Forage lentil was swathed on July 20. Cool temperatures during summer delayed the grain harvest. Field pea was harvested on August 16, grain lentil on August 21, wheat on September 15, and safflower on October 10. Above ground biomass was determined for all crops at the time of green manuring and at harvest. Protein content was determined for wheat, field pea, and grain lentil. Oil content was determined for safflower.

For forage lentil, the biomass at green manuring was greater this year than the 9‑yr average, but at harvest it was less than the 9-yr average (Table 1). The trends were reversed for grain lentil and field pea; the biomass at green manuring was less this year than the 9-yr average, but at harvest it was greater this year than the 9-yr average. The biomass at harvest for field pea was the greatest ever.

This year, the grain yields of grain lentil, field pea, and safflower were all greater than the 9-year average (Table 2). The grain yield of field pea was the greatest ever. Safflower oil content near the 9‑yr average this year.

Wheat yields this year average was 48.9 bu/a, the second greatest during the 9-yr period (Table 3). All treatments had yields greater than the 9-yr average.

¹NDSU Williston Research Extension Center, Williston, ND