Amino acid composition, protein content and accurate nitrogen-to-protein conversion factor for sheepgrass (Leymus chinensis)

The protein content of plants is commonly estimated by multiplying the total nitrogen content (Kjeldahl; KN) by a nitrogen-to-protein conversion factor of 6.25. This method is based on the incorrect assumption that all nitrogen in the ammonia/ammonium and organic substances in plants is protein nitrogen, usually resulting in overestimation of protein content. We have examined amino acid composition, amino acid nitrogen, total nitrogen (KN), and actual protein content (AP) determined from amino acid residues in 16 accessions of perennial sheepgrass (Leymus chinensis (Trin.) Tzvelev). We determined a new nitrogen-to-protein conversion factor, k(p), as the ratio of AP to KN, and applied this factor to estimate the total protein content (TP) as KN x k(p). The non-protein nitrogen accounted for 40.5% to 62.4% of the total nitrogen. The average k(p) value was 3.17 overall, 3.20 in the accessions sampled at the jointing stage, and 3.15 in the accessions sampled at the flowering stage. The TP, calculated as KN x 3.17, was about half that of crude protein contents, calculated as KN x 6.25. Our study suggests that the AP-based k(p) of 3.17 can be used to more accurately estimate the total protein content in sheepgrass.