Introduction
Grain legumes are a primary source of amino acids providing about a third of all dietary protein (Kudapa et al., 2013) and a third of processed vegetable oil for human consumption (Graham and Vance, 2003). Legumes are second to cereals in providing food for humans worldwide (Kudapa et al., 2013). Being a source of nutritionally rich food, grain legumes complement cereals or root crops, the primary source of carbohydrates. Grain legume crops represent an important component of agricultural food crops consumed in developing countries and are considered a vital crop for achieving food and nutritional security for both poor producers and consumers. Grain legumes are also known to play a pivotal role in nutrient cycling and nutrient enrichment in various cropping systems. They are considered as engines of sustainable farming as they intensify the productivity and interaction of the soil, crop, livestock, people and other components. Improving production of grain legume in smallholder farming systems will lead to improved food security in these systems.

Grain legume also provides essential minerals (Grusak, 2002), and health-promoting secondary compounds that can protect against human cancers (Madar and Stark, 2003), and also protect the plant against the attack of pathogens and pests (Ndakidemi and Dakora, 2003). Furthermore, legume grain has blood cholesterol-reducing effect as well as a hypoglycemic effect, reducing the increase in blood glucose after a meal (Jenkins et al., 2003). Whole-grain is a good source of bioactive proteins, peptides and functional fiber; also it may have beneficial combinations of many micronutrients, antioxidants, vitamin E, polyunsaturated fatty acids and phytochemicals (Liu, 1999). Many of these constituents have been independently associated with reduced risk of coronary artery disease (CAD) (Liu, 1999; Anderson and Hanna, 1999). In addition to reducing insulin resistance, the beneficial effects of whole-grain consumption on lipid peroxidation may be another possible explanation in the significant inverse relation of the whole-grain intake to the risk of CAD. Thus, these protective effects are likely due to multiple mechanisms, such as fiber, antioxidants, and many constituents of grain legumes.

Though grain legumes have several benefits, their productivity is very low and far below the potential production of the species (IFPRI, 2010). This low productivity in grain legumes is often associated with declining soil fertility of the farmland and reduced N₂-fixation. Yield reduction of grain legume can be improved through inoculation of adaptable effective rhizobia (Jida and Assefa, 2014; Desta et al., 2015). Despite the fact that inoculating legumes with rhizobia can achieve substantial increases in nodulation, grain and biomass yield, nitrogen fixation and post-crop soil nitrate levels, there is no doubt that specificity exists between rhizobial strain and the legume variety, and compatibility between the two is essential for successful nodulation and nitrogen fixation (Emam and Rady, 2014).

Biological N₂ Fixation (BNF) in Agriculture
The earth’s atmosphere contains the largest global pool about 10