An agronomic and economic analysis of a long-term wheat-based crop rotation trial in Ethiopia

Cropping systems in the- Ethiopian highlands consist primarily of cereals in rotation with grain legume and oilseed cropsthe proportional allocation among crop species varies with altitude, rainfall, and soil type. Barley (Hordeum vulgare) tends to dominate in the highest altitudinal zones, while bread wheat (Triticum aestivum) is more common at medium altitudes on well-drained soils. A trial was established in 1992 at the Kulumsa and Asasa research sites in southeastern Ethiopia to evaluate interactions among wheat- based cropping sequences and annua1 app1ications of inorganic nitrogen (N) and phosphorus (P) fertilizer. Rotational crops included Ethiopian rapeseed (Brassica carinata), faba bean (Vicia faba), and barley. The results indicated significant rotational effects on wheat grain yield (GY), including enhanced GY in dicot vs. cereal rotations, in two year vs. three year rotations, in first year wheat after any break crop, and in rotation with faba bean vs. rapeseed. Interactions among cropping sequences and N and P application were also significant. Response to N was markedly reduced in two year rotations with any break crop, in first year wheat after any break crop, and after faba bean, in particular. Conversely, P response was occasionally enhanced in two year rotations and in the first wheat crop after any break crop, and in dicot-based rotations, particularly with faba bean. Presumably, this enhancement was the result of simultaneous improvement in soil N status and a reduction in wheat root pathogen and grass weed populations in these cropping sequences. Across all fertilizer and seed price scenarios considered in the economic analysis, the faba bean-wheat 3 year rotation (with an annual application of 60 kg N/ha) was economically optimal at Kulumsahowever, this recommendation may have to be modified to minimize the anticipated long-term impact of mining of soil P. In the drier environment at Asasa, the faba bean-wheat 3 year rotation (with an annual application of 30 kg N and 20 kg P/ha) was economically optimal. Alternate recommendations are discussed for farmers with lower levels of cash to invest in crop production.