Modeling Soluble Protein Fractionation in Feedstuffs in Equine Rations Using Crude Protein and Fiber Composition

Current National Research Council equine protein requirements are based on feedstuff crude protein (CP) content; however, protein distribution in feedstuff cell contents and cell wall affects foregut protein digestion. Neutral detergent soluble crude protein (NDSCP) is the readily digestible protein fraction available to meet equine protein requirements. Updated models of equine nutrient requirements use NDSCP to quantify pre-cecal digestible crude protein (pcdCP) to determine if protein requirements are met. Thus, our study sought to develop linear regression models to predict NDSCP and pcdCP using feedstuff CP and fiber. Data were collected from the 2023 cumulative Dairy One© Feedstuff Composition Library (Ithaca, NY, USA; accessed 2 February 2024). Feedstuffs were selected when n ≥ 50 within each nutrient analysis used for model development [CP, neutral detergent fiber (NDF), acid detergent fiber, acid detergent lignin]. Feedstuffs were categorized into four groups: forages, grains, grain byproducts, oilseeds and meals. Data were analyzed using multiple linear regression in R Statistical Program© (v4.5.1). Predictor variables were selected using Pearson's correlation with models ranked using Akaike's information criterion (AICc), AICc weights (AICcWt), and Adjusted R². Significance was set at p ≤ 0.05. The model using CP+NDF was the highest-ranked model for both NDSCP (AICcWt = 0.95; Adjusted R² = 0.9868) and pcdCP (AICcWt = 0.95; Adjusted R² = 0.9867). Overall, NDSCP and pcdCP were predicted with high precision using CP and NDF; both have previously predicted CP digestibility. Overall, incorporating CP and fiber fractionation in linear models improves prediction of estimated dietary available protein.