Barley is one of the most important cereal crops widely grown by small-scale farmers under rain fed conditions in Ethiopia. For millennia it has been supplying the basic necessities of life (food, feed and beverages) for many in the Ethiopian highlands. Barley ranks fourth in  worldwide production of all cereals (FAO, 2004).

In Ethiopia, barley is ranked fifth of all cereals, based on area of production, but fourth based on yield per unit area (CSA, 2014).

The factors  constraining  the  increased  production  of barley in the different barley production systems are various (Chilot et al., 1998). The most important abiotic stresses include low soil fertility, low soil pH, poor soil drainage, frost and drought. Soil fertility depletion is a key problem of cereal production in Ethiopia. Low soil nitrogen is often the major factor limiting crop production (Andrews et al., 2004). Application of inorganic nitrogen fertilizer, crop rotation and intercropping are some of the important tools used to increase crop yields and grain quality in intensive agricultural system. Crop rotation is necessary and a desirable management option to restore, maintain, enhance soil fertility, and maximise yield (Tolera et al., 2011). Legumes contribute to increased productivity of other crops when incorporated into cropping systems as intercrops and through crop rotation (Giller, 1991).

Rotations with legumes build up the N status of the soil. Patwary et al. (1989) reported that available soil N was increased by about 32 and 40 kg ha^-1 following chickpea and lentil crop compared with wheat. The grain yield of barley monoculture with an application of 41/20 kg ha^-1 N/P was lower than the yield of barley following linseed or faba bean with an application of 18/20 kg ha^-1 N/P, which saved 23 kg N ha^-1 in the form of urea (Tolera et al., 2011). Farmers in the mid-highlands and highlands of Ethiopia are well aware of the importance of crop rotation to replenish soil fertility and skill fully used this option for nitrogen management (Bereket et al., 2011). Depending on agro-ecology, farmers grow legumes such as chick pea, faba bean, field pea, grass pea and fenugreek as rotation crop with cereals to improve soil fertility of cereals. Farmers usually reduce the N requirement of next cereal crop after legume. For example, farmers in Ude and Hatsebo reduce their N fertilizer rate for teff after leguminous crops, mostly for one year based on their personal judgments (Bereket et al., 2011). However, the N rate required after a specific legume crop for barley is not well studied in Ethiopia specifically in northern Ethiopia. Therefore, a study was conducted to evaluate agronomic efficiency, yield and yield components of barley response to nitrogen rates after faba bean precursor crop.

On farm field experiments were conducted at district of Emba-Alaje at Te'ka and Ayba Tabias  sites in 2014. The experiment consists of six nitrogen treatments that were 0, 11.5, 23, 34.5, 46 and 69 kg N ha^-1. Each treatments were supplied with basal application of phosphorous, potassium and sulfur at rates of 69 kg P₂O₅ ha^-1, 80 kg K₂O ha^-1 and 30 kg S ha^-1. The design was Randomized Complete Block Design treatments with three replications in a plot size of 3 m by 4 m. Nitrogen, phosphorus, potassium and sulfur were applied in the form of urea, Triple Super Phosphate (TSP), potassium chloride and calcium sulfate, respectively. Phosphorus, potassium, sulfur and half of the nitrogen rates were applied at planting. The remaining half of the nitrogen rates were applied during tillering.

The initial experimental soils (0-20 cm) were analyzed for texture, organic carbon, total nitrogen, CEC, available P, pH and EC. The methods used for samples physicochemical analysis were pH (Jackson, 1967), EC (Bower and Wilcox, 1965), organic carbon [modified Walkley and Black method (Jackson, 1967)], texture [hydrometer method procedure of Bouyoucos (Day, 1965)], available phosphorous (Olsen et al., 1954), total nitrogen [Kjeldehal method (Bremner and Mulvaney, 1982)] and CEC [Neutral ammonium acetate method (Black et al., 1965)]. Agronomic efficiency was calculated following Fageria and Baligar (2001).

Where: Gf is the grain yield in the fertilized plot (kg), Gu is the grain yield in the unfertilized plot (kg), Na is the quantity of N applied (kg).

The barley variety used in this experiment was HB1307 and was planted at a rate of 150 kg ha^-1 in row planting. Above ground biomass from whole plots were sun-air dried before weighing. The spikes were threshed and cleaned and grain yield was weighed. The straw yield was calculated by subtracting grain yield from the above ground biomass.

Data collection

Soil and agronomic data were collected following the standard procedure.

Data analysis

The collected data were subjected to analysis of variance (ANOVA) using SAS software program version 9.1.3 (SAS, 2002).Significant difference among treatment means were assessed using the least significant difference (LSD) at 0.05 level of probability (Gomez and Gomez, 1984).

Soil properties before planting

The chemical and physical property of the soils of the experimental sites after the precursor legume crops is indicated in Table 1. Textural class of Te'ka and Ayba sites was sandy clay and clay loam, respectively. According to Tekalign (1991) rating of soil pH, the pH of soils of both Te'ka and Ayba was slightly acidic. The organic carbon content of soils of the experimental sites was low at both sites (Tekalign, 1991). According to Birhanu (1980) total nitrogen content at Te'ka was low and medium at Ayba. Available phosphorous (Olsen P) for fababean- barley was medium at Ayba and high at Te'ka (Olsen, 1954). According to Landon (1991) rating, CEC was very high at both sites.

Growth parameters

Application of different nitrogen rates for barley production after faba bean was significantly influenced the plant height at Tek’a kebelle. The reverse was true for increasing     nitrogen     rate.    Application    of   nitrogen significantly influenced spike length at Ayba kebelle. This study were in line with the findings of Sofonyas et al. (2016) and Bereket et al. (2016).

Effect of nitrogen on barley grain straw yields after fababean

There is a significant difference in grain and straw yields of barley due to application of nitrogen after fababean crop rotation at Te'ka (Table 2) but not at Ayba probably due to the initial moderate level of soil total nitrogen. Grain and straw yields of barley increased with an increase of nitrogen application at Te'ka up to 69 kg N/ha though it was not significant with nitrogen rates of 23, 34.5, and 46 kg N/ha. Although it was not statistically significant, grain and straw yield of barley  increased with nitrogen application till 34.5 kg N/ha with further decreasing trend at the higher rates. Most of the farmers at Tek’a and Ayba kebelles reduce the amount of nitrogen for barley production after faba bean. This may be similar with the findings of Bereket et al. (2016) reported that half of the recommended nitrogen rate was optimum for wheat production after legume crops at Emba   Alaje.   Maadi   et   al.  (2012)   reported   that  the preceding leguminous crop increases the grain yields of wheat significantly. Benefit of legumes insertion in the cropping sequence is the yield advantage associated with extra soil N availability to a cereal crop succeeding a legume; for example, under pea-wheat rotation, approxi-mately 50 kg ha^-1 more N was accumulated compared to wheat-wheat rotation (Evans et al., 1991) (Table 4)..

Agronomic efficiency of nitrogen

Agronomic efficiency is the amount of additional yield obtained for each additional kg of nutrient applied (Fageria and Baligar, 2001). The highest agronomic efficiency was recorded from the addition of 23 kg N ha^-1 (31.52 kg kg^-1) and the lowest from 11.5 kg N ha^-1 (3.65 kg kg¹), respectively at Ayba kebelle (Table 4). At Tek’a kebelle the highest and lowest agronomic efficiency were obtained at 34.5 kg N ha^-1 (31.44 kg kg^-1) and 11.5 kg N ha^-1 (8.45 kg ka-1), respectively (Table 4).

Partial budget analysis of nitrogen rates

At  Emba  Alaje district Tek’a kebelle the highest marginal rate of return was obtained from plots treated with 46kg N ha^-1. The identification of a recommendation is based on a change from one treatment to another if the marginal rate of return of that change is greater than the minimum rate of return (100%). According to the marginal rate of return 46 kg N ha^-1 was found economically profitable compared to other treatments for barley production after faba bean (Table 5).

Grain and straw yields of barley increased significantly with application of nitrogen after faba bean precursor crop at Tek’a. There was no significant effect of nitrogen application for barley after faba bean indicating the precursor crop (faba bean) had contributed for fixation of nitrogen  in  the  soil.  This  is  in-line  with  the  initial  soil nitrogen in Ayba kebelle. Partial budget analysis revealed that nitrogen application rate has to be 46 kg N/ha in Tek’a kebelle. But, application of 34 and 46 kg N ha^-1 was statistically the same, consequently 34.5 kg N ha^-1 was optimum rate for barley production after faba bean.

The authors have not declared any conflict of interests.

The authors would like to extend their sincere thanks and appreciation to Tigray Agricultural Research Institute (TARI), Ethiopia for sponsoring this study and special thanks to Mekele Soil Research Center staffs.