Molecular basis of protein structure in combined feeds (hulless barley with bioethanol coproduct of wheat dried distillers grains with solubles) in relation to protein rumen degradation kinetics and intestinal availability in dairy cattle

The objectives of this study were to reveal protein molecular structure in relation to rumen degradation kinetics and intestinal availability in combined feeds of hulless barley with bioethanol coproduct [pure wheat dried distillers grains with solubles (DDGS)] at 5 different ratios (100:0, 75:25, 50:50, 25:75, and 0:100) in dairy cattle. The parameters assessed included 1) protein chemical profiles, 2) protein subfractions partitioned by the Cornell Net Carbohydrate and Protein System, 3) in situ protein degradation kinetics, 4) truly absorbed protein supply in the small intestine (DVE), metabolizable protein characteristics and degraded protein balance (OEB), 5) protein molecular structure spectral profiles, and 6) correlation between protein molecular structure and protein nutrient profiles and metabolic characteristics. We found that 1) with increasing inclusion of wheat DDGS in feed combinations, protein chemical compositions of crude protein (CP), neutral detergent-insoluble CP, acid detergent-insoluble CP, and nonprotein N were increased, whereas soluble CP was decreased linearly; CP subfractions A, B₃, and C were increased linearly, but CP subfractions B₁ and B₂ were decreased; truly digestible CP increased but total digestible nutrients at 1× maintenance decreased linearly; protein degradation rate was decreased without affecting potentially soluble, potentially degradable, and potentially undegradable fractions, and both rumen-degradable protein and rumen-undegradable protein were increased; by using the DVE/OEB system, the DVE and OEB values were increased from 98 to 226 g/kg of dry matter and -1 to 105 g/kg of dry matter, respectively; 2) by using the molecular spectroscopy technique, the spectral differences in protein molecular structure were detected among the feed combinations; in the original combined feeds, amide I and II peak area and ratio of amide I to II were increased linearly; although no difference existed in α-helix and β-sheet height among the combinations, the ratio of α-helix to β-sheet height was changed quadratically; 3) in the in situ 48-h residue samples, amide I and amide II peak area intensities were increased linearly and the ratio of amide I to II peak area was decreased linearly from 4.28 to 2.63; α-helix and β-sheet height of rumen residues were similar among 5 feed combinations; and 4) the ratio of α-helix to β-sheet height in original feed combinations was strongly correlation with protein chemical and nutrient profiles, but the ratio of amide I to II area had no significant correlation with all items that were tested; no correlation was found between the ratio of α-helix to β-sheet height of the in situ rumen residues and protein chemical and nutrient profiles. In conclusion, by integration of hulless barley with bioethanol coproduct of wheat DDGS, feed quality in combined feeds was improved and more optimized. Adding wheat DDGS increased linearly CP, truly digestible CP, rumen-degradable protein, rumen-undegradable protein, DVE, and OEB values in combined feeds. The molecular spectral differences of protein molecular structures (amide I and II area intensities, the ratio of amide I to amide II, and the ratio of α-helix to β-sheet height among feed combinations) were detected among the combinations. This may partially explain the biological differences in protein chemical profiles and protein utilization and availability in dairy cattle. In the original combined feeds, protein α-helix-to-β-sheet ratio had significant correlations with protein chemical and nutrient profiles, but in in situ 48-h residue samples, protein amide I-to-II ratio had significant correlations with protein chemical and nutrient profiles. This study may provide an insight into how to more efficiently use hulless barley grain (high energy and high degradation rate) and wheat DDGS (high metabolizable protein and low degradation rate) in beef and dairy production systems.