Yield and Nitrogen Recovery of Biennial Canola Forage Following Peas in the Northwest US.

When planted as double- or multi-purpose crop, canola can play a significant role in renewable energy, food and feed production in the Northwest US by providing additional annual farm income with the production of green pea and canola forage, and preventing the decline of soil health while enhancing soil and water quality. The objectives of the current study were to assess the effect of pea precursor crop, and N and S fertilizers on biennial canola forage yield and nitrogen recovery from ¹⁵N enriched pea residue and ¹⁵N labeled urea fertilizer. To produce  ¹⁵N enriched pea residue, peas were raised in sand media in a greenhouse receiving ¹⁵N labeled 10 atom% urea. Other nutrients were supplied from a special formulation. Treatments were 3 x 3 factorial combinations of N (0, 112, and 224 kg//ha and S (0, 34 and 64 kg/ha) fertilizer rates arranged in a randomized complete block design with four replications. Plots measured 6.1 m wide by 15.2 m long. The ¹⁵N enriched pea residues, and ¹⁵N labeled urea were applied to selected treatments in 0.36 m² micro-plots during the canola phase of the experiment. Sixteen-percent moisture adjusted biennial canola dry matter yield from growth stage 28–29 (BBCH scale) was 2183 and 1285 kg/ha in 2012/13 and 2013/14, respectively. Poor establishment reduced the forage yield in 2013/14. In 2012/13, there were no significant differences in forage yield between plots that received N and the check. No trend was observed for forage yield with increases in N rates. The lack of difference to applied N in 2012/13 compared to the check indicated that N mineralized from pea residue presumably contributed significant quantities of N. In 2012/13, the inorganic N in the soil prior to pea planting was approximately 2.2 kg/ha in the first 15 cm of the soil, much lower than what was documented before biennial canola planting (24.6 kg/ha). Unlike 2012/13, in 2013/14, biennial canola forage yield increased with increases in N rates.  In both years, no response to S fertilizer was observed. The ¹⁵N enriched pea residue recovered 70% of applied 10 atom% ¹⁵N labeled urea. Biennial canola forage recovered 15% of ¹⁵N from the pea residue. The recovery of ¹⁵N labeled urea in canola forage was 42%. Biennial canola  forage recovered significantly  lower ¹⁵N from pea residue compared with that of ¹⁵N labeled urea fertilizer plots. The recovery of N from pea residues by the subsequent biennial canola forage could be improved by managing application time and proper incorporation. Additional work is underway to validate these preliminary results.