Have you ever felt overwhelmed after reading the term "first-order fiber digestion kinetics" in a manuscript or textbook? I definitely have — several times! Throughout our research on fiber digestion at the Dairy Forage Research Laboratory, I have discovered that while some terminology may sound intimidating, the meanings are often quite straightforward. Let’s break down this term in a friendly way.
In pharmacokinetics — a term that can also be daunting — first-order kinetics refers to situations where a drug or substance is absorbed, distributed, or eliminated at a constant rate. For now, let’s focus on that idea of a "constant rate." In the context of ruminant nutrition, when we say fiber degrades according to first-order kinetics, it means the fiber degradation rate (also known as Kp) remains constant over time. But what does that really entail?
Imagine we fill 24 porous bags with 4 grams of alfalfa stems containing 25% potentially degradable neutral detergent fiber (pdNDF), and we place them in the rumen of a cow. At the beginning, each bag has 1 gram of pdNDF. Over the next 24 hours, we will remove one bag each hour to measure the remaining pdNDF. If the degradation rate is 3.7% per hour, then after one hour of fermentation, the remaining pdNDF would be 0.963 grams. This is calculated as follows: 1 gram of pdNDF × (100 - 3.7)% = 0.963 grams. You could also express this as 1 gram of pdNDF × 0.963 = 0.963 grams, making 0.963 a constant.
Since the degradation rate is constant, after two hours, the residual pdNDF would be 0.963 grams × 0.963 = 0.927 grams. Similarly, after three hours, the remaining pdNDF would be 0.927 grams × 0.963 = 0.893 grams. Notice that we are simply multiplying the existing amount of pdNDF at a certain time by the constant degradation rate. The figure below illustrates how much pdNDF remains over time in this hypothetical scenario. You will observe that every hour, the remaining amount declines by 3.7% compared to the previous hour, illustrating that the degradation of pdNDF adheres to first-order kinetics.
To ensure this explanation is clear, consider this analogy: When you request a loan, would you prefer a variable interest rate or a fixed interest rate? If you favor the latter, you are essentially choosing a debt payment structure that reflects first-order kinetics, where the interest rate remains constant over time.
