ARTICLE FEED & ADDITIVE MAGAZINE July 2025 95 strated that the particle size of the bolus (cud) was very similar between diets regardless of the particle size of the forages. CUD CHEWING AND PARTICLE SIZE Cud chewing is a critical component of the dairy cow rumination process because it allows the dairy cow to access the nutrients locked within the forages. The majority of cud chewing is done during the resting period for the cow – when the cow is lying down. As previously noted, the forage particle size is reduced through the eating process. Cud chewing abrades the fibre particles through a grinding action that is crucial for bacterial attachment. Bacteria are opportunists and seek out the most rapidly available nutrient source – they quickly attach to the cut end of the forage particle. Ruminal bacteria attach to cells on the inside of the fibre and digest towards the outside until they are inhibited by a lignified wall. A thin lignified residual wall always remains even after extremely long rumen exposure times (Engels, 1989). The rumen bacteria can easily attach to the cut end of the forage particles but have difficulty attaching to the outside wall, partially due to an impermeable waxy coating. This waxy coating is a defense mechanism for plant survivability through improved water retention and lower water loss. This is especially important during drought situations but also increases with maturity. Fungi, another key group within the rumen microbiota, have the ability to penetrate the outside wall of the plant and start the digestion process. This degradation of the outside plant wall structure through fungal digestion will allow for adhesion of primary bacterial colonizers - but this is a slow process. Cud chewing will quickly improve attachment by abrading the fiber and allowing the bacteria to enter the plant cells. Increasing particle size of the diet seems to have less of an impact on cud chewing time but more of an impact on eating time. Beauchemin (Journal of Dairy Science Vol. 101 No. 6, 2018) reported that “increasing particle size of silage increased eating time (minutes per day and minutes per kilogram of DM) of dairy cows in many studies but not all”. Decreasing particle size of the diet may have more of an impact on cud chewing than increasing particle size. The regurgitation of the bolus (cud) occurs as a result of a contraction of the reticulum. Small particle size may not illicit the tactile response in the reticulum to contract and regurgitate the cud. Small particle size may also decrease cud chewing by increasing passage rate of the material and subsequent exit from the rumen; leading to reduced ruminal fibre digestion. Fibre that exits in the rumen “untouched” is primarily lost to further digestion in the hind-gut and will eventually pass into the manure. RECUMBENT CUD CHEWING Research would indicate that approximately 80% or more of the cow’s ruminating behavior occurs while she is lying down or sternally recumbent. Any management practice that reduces lying time Photo: Lallemand Animal Nutrition
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