Protein Evaluation of Feedstuffs for Horses

SIMPLE SUMMARY: To formulate rations for horses, feeds must be evaluated for their supplies of nutrients. For this purpose, the German Society of Nutrition Physiology (GfE) proposed a new protein evaluation system that based the protein evaluation of feeds on predicted digestibility in the small intestine depending on chemical properties. A total of 71 feeds for horses were chemically tested and evaluated according to the new protein evaluation system. In a feeding trial, four feeds were randomly tested for post-prandial plasma lysine responses to determine the lysine supplied by the respective feed in horses. Chemical-based feed evaluation confirmed protein-rich feeds as protein suppliers with high amounts of small intestine-digestible protein. Feeding with synthetic amino acids, including lysine, induced a high post-prandial plasma lysine response in horses. Although limited by a low number of horses, high plasma lysine responses after feeding lucerne seemed not to correspond to the chemical feed evaluation, as chemical protein evaluation indicated low pre-cecal protein availability of lucerne. Protein evaluation by chemical parameters seemed to be limited, at least for forages such as alfalfa, as the impact of chewing and bacterial fermentation processes were neglected. Post-prandial plasma lysine levels may provide information regarding the chemical protein evaluation of feeds. ABSTRACT: The German Society of Nutrition Physiology has proposed a new protein evaluation system for horse feeds to estimate pre-cecally digestible crude protein (pcdCP) and amino acids (pcdAA) from chemical properties. A total of 71 feeds for horses were chemically tested and evaluated according to the new protein evaluation system. A feeding trial with eight horses tested whether differences in estimated pcdAA and neutral detergent soluble CP (NDSCP) in the diet were reflected by post-prandial (ppr) kinetics of plasma lysine (Lys) by feeding a complementary feed (control = CTRL) with 1.02 g Lys/100 kg body weight (BW) as well as three diets with 3.02 g Lys/100 kg BW, as follows: (i) CTRL with synthetic AA (CTRL + synAA); (ii) CTRL with soybean meal (CTRL + SBM); and (iii) lucerne pellets (LUC). In comparison to CTRL, the areas of curves (AUCs) of ppr plasma Lys differed: CTRL < CTRL + SBM (p < 0.01) < CTRL + synAA (p < 0.05). For 71 feeds, the estimated pcdCP was correlated with the CP content (p < 0.001), NDSCP (p < 0.001), and ash-free neutral detergent fiber (p < 0.001). A mean neutral detergent insoluble CP content of at least 3–5% can be assumed in horse feed. It is speculated that the predicted availability of Lys from LUC seems to be underestimated by the new protein evaluating system. The influence of chewing and microbiota in vivo needs to be considered in horses.