Batch-to-batch variation in protein molecular structures, nutritive value and ruminal metabolism in corn coproducts

The objectives of this study were to investigate batch-to-batch variation in 1) crude protein (CP) chemical profile; 2) the Cornell Net Carbohydrate and Protein System (CNCPS) CP subfraction profiles 3) ruminal degradation kinetics of CP; 4) the DVE/OEB₂₀₁₀ model estimated metabolizable protein supply to dairy cows; 5) molecular structure spectral characteristics of protein primary (amide I and II) and secondary (α-helix and β-sheet) structures among different types of coproducts and among three batches of corn coproducts using parent corn kernels of each batch as reference; and 6) to correlate the batch-to-batch variation in protein molecular structures to the corresponding variation in CP chemical composition, digestibility and metabolism in dairy cows. Corn coproducts, namely, corn gluten feed, corn germ, corn germ cake and corn gluten meal from the same wet milling manufacturer were evaluated. The parent corn kernel was used as control. The results showed large differences in the CP chemical profile (P < 0.001), CNCPS subfractions (P < 0.001) and ruminal degradation (P ≤ 0.01) except undegraded-fraction, between corn kernels and the coproducts. Batch-to-batch variation (P < 0.05) was also observed for some of the CP chemical components, subfractions and ruminal degradation parameters, however, the pattern of variation among the batches was not consistent for the different coproducts. The metabolizable protein supply to the small intestine from microbial protein, rumen undegraded feed protein and endogenous protein from the corn coproducts were different (P < 0.001) from that of corn kernels. Except corn germ, differences (P < 0.05) were observed in degraded protein balance among the three batches, with marked ranges in corn gluten feed (58.6–101.8 g/kg dry matter) and corn germ cake (16.9–76.8 g/kg dry matter). All the intrinsic protein primary (amide I and amide II) and secondary (α-helix and β-sheet) structures spectral parameters varied (P < 0.001) among the coproducts. Regression equation showed that amide I and II areas, β sheet and area ratio of amide I to II were strong predictor (R² ranged from 0.54 to 0.98) of protein chemical profile, degradability and metabolizable protein supply to small intestine. Overall, corn processing and processing-batches caused significant variation in CP chemical composition and digestibility that was strongly related to the changes in protein molecular structures.