Growing soybean (Glycine max L.) prior to corn (Zea mays L) can enhance corn grain and nitrogen (N) use efficiency compared to continuous corn. This two year study (2007-2008) was conducted at 62 sites in Quebec (Eastern Canada) to assess the effect of crop rotations [soybean-corn, soybean-wheat (Triticum aestivum L.,)-corn and corn-corn] on corn yield, N uptake, N fertilizer efficiency (NFE), and the economic optimum N rate (EONR). Plots within each crop rotation received N fertilizer rates from 0 to 250 kg N ha-1 to assess the N contribution from the preceding soybean crop. Corn grain yields ranged from 8.4 to 10.8 Mg ha-1 and were lower in continuous corn than in the crop rotations. Corn N uptake and NFE varied from 89 to 164 kg N ha-1 and from 45 to 80 kg grain per kg N fertilizer, respectively. A significant interaction of crop rotation and year on corn N uptake and NFE was obtained implying that annual variations influenced soil N supply. The EONR for corn was lower under crop rotations than continuous corn in 2008 only. No difference in corn yield, NFE and EONR was observed for soybean-corn and soybean-wheat-corn crop sequences. In conclusion, crop rotations including soybean increased soil N availability and reduced EONR from 32 to 45 kg ha-1 for corn grown in 2008.


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