By Leoni Martins
Ruminal protein degradation of feed ingredients can be described by first-order mass-action models. These models consider that crude protein (CP) consists of multiple fractions that differ widely in their rates of degradation, and that ruminal disappearance of protein results from the combined effects of degradation (Kd) and passage rates (Kp; NRC, 2001). Degradation represents the proportion of nutrient that disappears at a given moment in time, whereas Kp represents the proportion of nutrient that flows from the rumen to the intestine. Therefore, both processes can be expressed as %/h, and both Kd and Kp can be used to calculate rumen degradable (RDP) and rumen undegradable protein (RUP) fractions in feed ingredients (NRC, 2001).
To determine Kd, researchers typically incubate feedstuffs in the rumen of cannulated cows and model the degradation and disappearance of nutrients over time. Based on these degradation kinetics, protein can be classified into three main fractions: A, B, and C (expressed as % of CP). Fraction A is considered rapidly degradable, whereas fraction C is considered undegradable in the rumen. Fraction B degrades more slowly than fraction A but can still be degraded, provided it remains in the rumen long enough and does not pass to the intestine. As the reader can infer, the determination of protein fractions in feeds is labor- and resource-intensive, limiting the routine estimation of ruminal degradation kinetics of nutrients in feed ingredients by researchers and nutritionists. Therefore, diet formulation typically relies on modeling approaches and library values for protein fractions.
According to NASEM (2021), fractions A, B, and C in raw soybeans are 26%, 74%, and 0%, respectively, whereas in roasted soybeans these fractions are 18%, 77%, and 5% (Table 1). Although these numerical differences may appear small, they can translate into substantial differences in ruminal degradation kinetics. Using the NASEM (2021) library values and assuming these soybeans have the same particle size, it is possible to model degradation and disappearance (i.e., 100 – degradation) for both soybean types (Figure 1A and 1B, respectively). Assuming a Kp of 7%/h for both soybeans, the calculated Kd values are 8%/h for raw soybeans and 5%/h for roasted soybeans. Correspondingly, RDP is 65.5% and 50.1%, and RUP is 34.5% and 49.9% for raw and roasted soybeans, respectively.
Source : psu.edu