EFFECTS OF COMBINED RATES OF NITROGEN AND PHOSPHORUS ON YIELD, YIELD COMPONENTS AND GRAIN PROTEIN CONTENT OF BREAD WHEAT (Triticum aestivum L) CULTIVARS: THE CASE OF ROBE FARMS, BALE HIGH LAND, SOUTH EASTERN ETHIOPIA.

Field experiment was conducted during 2014-15 to assess the effect of combined NP rate on yield, yield components and grain protein of bread wheat varieties on Robe Farms, Bale Zone of Oromyia, South-eastern Ethiopia. The NP rates combinations were N 0P0, N46P23, N92P46, N138P69, N184P92 and N 46P46 kg/ha, .whereas varieties were Kakaba, Pavon-76 and Dandaa. Experiment was designed in factorial RCBD with eighteen treatments and three replications. There was significant response of wheat to fertilization. The higher levels of N138P69 and N 184P92 kg ha -1 gave significantly higher grain yields and grain protein than lower levels, including state level recommendation of N46P46 kg/ha. Dandaa produced significantly higher grain yield than Pavon-76 and Kakaba. The protein content was similar for Dandaa and Pavon-76, but significantly higher than Kakaba. The quadratic yield responses showed high correlation between yields and fertilizer rates for varieties (R 2 0.99, 0.98, 0.84) and wheat in general (R 2 0.98). The yield potentials of Dandaa, Pavon-76 and wheat in general were 71.4, 61.5 and 67.1 q ha -1 , achieved at N 147P74, N133P66 and N 191P96 kg ha -1 , respectively. The optimum yields of Dandaa, Pavon-76 and wheat in general were 71.2, 61.2 and 66.8 q/ha, achieved at N 135P68, N121P60 and N 170P85 kg ha -1 , respectively. Kakaba had still not reached maximum yield. The profit was higher for Dandaa (25,659 birr/ha) compared to Pavon-76 (20,608 birr/ha) and wheat in general 23,872 birr/ha. Dandaa and pavon-76 were more nutrient-use efficient than Kakaba at low to high application rates with higher marginal rate of returns (MRR) and agronomic efficiency. Accordingly, Dandaa and pavon-76 varieties may be more universal for adoption under varied situations. The Kakaba variety, on the other hand, showing promise only at very high rate of fertilization may be less preferred for adoption in areas with low to medium fertility and resource-poor farmers. The results indicating the need for fertilization three to four times the state level recommendations may be further validated through verification trials on farmers’ fields with same agro ecology, in cooperation with Commercial Agricultural Development enterprises and private investors. Keywords: Bread Wheat, Grain Protein Content, Marginal Rate of Return, Nitrogen and Phosphorus, Yield-Response Functions.