Wheat Performance Following Annual Legumes Compared to Summer Fallow

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 monoammonium phosphate (MAP; 18‑46‑0) with the seed.

All crops were planted during the week of May 14-20. Peas were green manured on July 3; grain lentil was green manured on July 8 and forage lentil was green manured on July 18. Forage lentil was swathed on July 25. Peas were harvested on Aug. 2; wheat and grain lentil were harvested on Aug. 27; and safflower was harvested Sept. 25. 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 of all annual legumes varieties at green manuring in 2002 was much less than the 7-yr average (Table 1). This may have been due to the later planting date this year without a corresponding delay in the date of green manuring. This also is the likely reason for the reduction in forage lentil biomass at harvest. Grain lentil and pea biomass at harvest did not seem to be affected, as the values this year were similar to the 7-year averages (Table 1). 

            Grain yield of both grain lentil and field pea was similar in 2002 to the 7-year average (Table 2). Safflower yield in 2001 was greater than the 7-year average. Safflower oil content was the second greatest during the seven-year period.

Wheat yields averaged 33.9 bu/a, just 0.9 bu/a greater than the 7-yr average (Table 3). The continuous wheat with deficient nitrogen had a wheat yield of 19.1 bu/a, which was significantly less than all other treatments. There were no other significant differences in wheat yields. Eight treatments had yields greater than the 7-yr average; five treatments had yields lower than the 7‑yr average.