Use of spectroscopy to predict nutrient digestibility in pigs and to identify in vitro digestion limits

Abstract Text: Co-products are increasingly used as alternative feedstuffs for pigs, but their quality varies widely. Rapid evaluation of apparent total tract digestibility (ATTD) of energy of co-products using, e.g., near-infrared spectroscopy (NIRS) is thus important; however, digestibility data are required for its calibration. In vitro digestion (IVD) models may predict ATTD of energy, but have not been tested rigorously for co-products. In Exp. 1, we predicted ATTD of energy of canola meal, corn dried distillers grain with solubles (DDGS), soybean meal, and wheat millrun using an IVD model and chemical analyses. Unlike for cereal grains (R²>0.90), the IVD model predicted variation in ATTD of energy among co-products not accurately (R²=0.69) while chemical analyses had greater accuracy. The IVD model underestimated ATTD of energy in corn DDGS and wheat millrun, poorly described variation of ATTD of energy within the other co-products, and must be improved. The small quantity of IVD residues restricts using chemical analyses or NIRS to identify IVD limits. Mid-infrared spectroscopy can obtain accurate scans of 200-mg IVD residues, but has not been calibrated for ATTD of energy. In Exp. 2, a novel approach of using functional group digestibility (FGD) predicted apparent ileal digestibility (AID) of CP of wheat accurately (R² = 0.99). Absorbance in the amide I region (1,689-1,631 cm^-1) of mid-infrared spectra and ratio of inorganic indigestible marker in diet and digesta was used. In Exp. 3, the FGD predicted fat digestibility. The AID of total fatty acids (R² = 0.75) and ATTD of ether extract (R² = 0.90) of flaxseed and field pea were estimated with FGD at 2,923 cm^-1 and 1,766-1,695 cm^-1 of mid-infrared spectra, respectively. Using spectroscopic methods, evidence of poor enzymatic digestion of fat and fiber for co-products in IVD models was identified. Our findings will assist further development of IVD models to predict ATTD of energy of co-products.

Keywords: energy digestibility, in vitro, spectroscopy