Evaluating Two Methods for Measuring Cowpea Seed Protein Content

Cowpea (Vigna unguiculata (L.) Walp), a legume that is grown in the United States and worldwide, is beneficial for human consumption and animal feeding. The health benefits include high protein, vitamin, and mineral content. The goals of this research are to evaluate seed protein content, study genetic diversity, and develop high protein content varieties in cowpea. Elementar Rapid N III has been widely used in plants to measure seed protein through the percentage of total nitrogen in the seed’s grain powder. Near-Infrared Transmittance (NIR) has commonly been used to measure protein content in soybean seed using whole seeds without the need of grinding. It is possible to achieve fast result and delivers low cost-per-analysis than traditional method by combustion for total nitrogen. The specific objective of this study is to evaluate the two methods of measuring cowpea seed protein content using the NIR analyzer and by Elementar Rapid N III a. A total of 143 cowpea samples, including 54 USDA GRIN germplasm accessions and 89 advance cowpea breeding lines were used in this study. Whether cowpea seed coat color affects the measurement of cowpea seed protein content by NIR is a concern because cowpea has a colorful seed coat including black, brown, blue, cream, green, grey, mix (two or more colors), purple, red, tan, and yellow. Comparing two measurement methods, we found linear relationship exist between LECO CHN 2000 analyzer and Elementar Rapid N III. The correlation efficiency (r) between the seed protein content from NIR and from a LECO CHN 2000 analyzer is very high for blackeye seed (r = 0.70), brown eye (0.68), cream (0.73), and pinkeye (0.70), respectively, indicating NIR analyzer can be used to measure protein content for cowpea seeds with the four types of seed coat. However, the seed protein content measured from NIR showed a little higher. A regression model is being built to adjust to protein content from NIR.