Feed & Additive Magazine Issue 25 February 2023

ISSUE FOCUS FEED & ADDITIVE MAGAZINE February 2023 27 dox potential) are necessary to ensure the maximum colonization. Many species of bacteria can produce these enzymes and are broadly categorized as fibrolytic bacteria. Furthermore, other microorganisms can similarly degrade fiber including protozoa and fungi. The extent of ruminal fiber digestion is influenced by the digestibility of the fiber (NDFd - see boxed text), the rate of degradation of the fiber (Kd of NDFd) and the passage rate (Kp) of the fibrous material from the rumen. 1. The digestibility (NDFd) of the forage is influenced by morphological factors such as maturity (leaf:stem ratio) and lignification, plant anatomy and plant species. 2. The rate of digestion (Kd of NDFd) is largely a function of species differences, lignin complexes and ratios, and maturity. 3. The passage rate (Kp) can be influenced by theoretical length of cut, rumen dysfunction (acidosis and SARA), moisture content and dry matter intake of the animal. Fiber degradation in the rumen is leading to production of VFAs that represents up to 70% of the energy required for milk production. Thus, the efficacy of fiber degradation in the rumen is an important driver of milk production yield. Oba and Allen have determined that, for every 1% unit increase in NDF digestibility, milk production increases by 0.24 kg/d, and Fat Corrected Milk (FCM) increases by 0.25 kg/d (Oba and Allen, 1999). WHAT INFLUENCES FIBER DIGESTIBILITY? There are several factors that can alter the population of the rumen microorganisms (rumen microbiome), including the influence of diet. 1. Rumen pH Since fibrolytic bacteria need a pH above 6.2, decreases in pH can inhibit their growth and performance. If rumen pH falls below 6.0, fiber digestion in the rumen begins to decline. When ruminal pH falls below 5.8 to 5.9, the rumen is mildly acidic, and fiber digestion in the rumen is disrupted. Bacteria • Adhere to fibers • Important enzymatic activity (cellulase and xynalase) Ciliate protozoa • Engulf plant particles • Digest cellulose and xylan in digestive vacuoles • Very complex enzymatic equipment Fungi • Attached to plant particles • High cellulose and hemicellulose activities • Mechanical fibre weakening THE FIBER DEGRADING MICROFLORA Figure 1. Fiber digestibility depending on rumen conditions and fiber type (MB Hall, 2014). Figure 2. Fiber degradation in the rumen: kinetics and associated microbiota

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