For peanut, a factor of 5.46 is used for converting nitrogen concentration into protein concentration because the peanut proteins arachin and conarachin contain 18.31% nitrogen. Using published reports on the amino acid composition of peanut genotypes, the nitrogen-to-protein conversion factors (NPCF) for arachin and conarachin, as well as for whole peanut kernel were calculated. The value of NPCF for arachin varied between 5.271 and 5.563 and that for conarachin from 5.076 to 5.496. The location of crop growth may significantly affect the value of NPCF for whole kernels. For the samples of various genotypes grown at various locations, the values of NPCF varied between 5.3 and 5.7. Thus, the protein content obtained by using the conventional factor of 5.46 would have a precision of ± 0.7% for samples having 3.5% N (approx. 18% protein). As the nitrogen content increases, the precision decreases to reach ± 1.1% for samples having 5.5% N (approx. 30% protein). This implies that the protein contents of peanut calculated on the basis of a fixed conversion factor would be significantly different only when they differ by 1.4 to 2.2% in the calculated values. Therefore, use of a fixed NPCF for comparison of concentration of protein in samples emanating from breeding or agronomic trials may be erroneous, especially when samples differ only marginally in their protein concentrations. Research will be required to determine the relationship between the nitrogen concentration of peanut kernel samples and their protein concentration.

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