Crude protein and mineral composition of major crop residues and supplemental feeds produced on Vertisols of the Ethiopian highland

This study assessed the concentrations of crude protein (CP) and minerals in tef (Eragrostis tef) and grass pea (Lathyrus sativus) residues, noug seedcake (Guizotia abyssinica) and grass pea grain produced on Vertisols of the Ethiopian highland. Soil and feed N and minerals were determined from samples obtained from tef and grass pea crop fields as well as oil extracting plants. The concentration of Ca (16 g/kg), Na (274 mg/kg) (P < 0.001) and Fe (152 mg/kg) (P < 0.05) in grass pea haulm exceeded that of tef straw, while the reverse was noted for P and Mn (P < 0.001). Noug seedcake contained high concentrations of CP (328 g/kg DM) and P (12 g/kg DM), but had low concentrations of other minerals. Grass pea grain had high concentrations of CP (280 g/kg) and Fe (78 mg/kg). The K/(Ca + Mg) ratios of the evaluated feeds were favourable but their Ca:P ratios, with the exception of grass pea grain (1.06), were wide. A strong positive correlation was found between soil Na and the Na content of tef and grass pea residues. Soil P was also positively correlated with tef straw P. For N and the remaining minerals, the correlations between soil and crop residues were not significant (P > 0.05). It was concluded that the observed CP and mineral concentration differences between the examined feed sources can be exploited to ensure optimum supply of CP and most minerals, but Na and Cu deficiencies would likely limit production of animals relying on these feeds.