Relationship of feeds protein structural makeup in common Prairie feeds with protein solubility, in situ ruminal degradation and intestinal digestibility

• Differences in protein molecular structures affect protein digestion in dairy cows.
• Amid II area and β-sheet height were good predictors of digestible protein contents.
• Advanced molecular spectroscopy can characterize feed protein molecular structures.

The objectives of this study were to determine the relationship between the intrinsic molecular structures of protein feeds and protein solubility, in situ rumen degradability and intestinal digestibility. The feeds investigated were barley, corn, oat, wheat, lentil, peas, canola meal, expeller meal (extruded canola meal), soybean meal, mill feeds (pelleted byproducts from cereal grains), lantic sugar beet pulp, blood meal and meat meal. The protein molecular structures were quantified from spectral data on unique bands such as amid I and amide II, and protein secondary structures such as α-helix and β-sheet and their ratios using attenuated total reflectance-Fourier transform infrared molecular spectroscopy (ATR-FTIR). The feeds were analyzed for protein solubility, in situ ruminal degradation and intestinal digestibility. The water-based (10.5–70.3 g/100 g crude protein (CP)) and buffer-based (4.4–76.3 g/100 g CP) protein solubility had a large variation among the feeds. The effective ruminal degradability (13.8–77.0 g/100 g CP), intestinal digestibility of rumen undegraded protein (RUP; 57.0–89.1 g/100 g RUP) and total protein digestibility (76.1–96.4 g/100 g CP) had a large variation among the feeds. Significant differences were observed in protein amide I and amide II area, model-fitted α-helix and β-sheet height and in the ratio of amide I to II area. The protein molecular structural characteristics were strongly correlated with protein solubility, ruminal degradation and intestinal digestibility. Amide II area had the highest correlation with the contents of CP (r = 0.713), water-soluble CP (r = 0.730), buffer-soluble CP (r = 0.436) and intestinal digestible CP (r = 0.735). Whereas the area ratio of amide I:II showed strong correlation (r = 0.707) with the content of rumen degradable CP. The protein amide II area and β-sheet height could be used to predict the contents of intestinal digestible RUP (R2 = 0.826) and total digestible CP (R2 = 0.808) of prairie feeds in dairy cattle. This study report a novel data on feed protein molecular structures and showed that protein structural makeup was associated with protein nutritional value and digestive behaviour in dairy cattle.