Mechanical Factors Involved in Ruminants' Digestion

The mechanical factors involved in ruminant’s digestion include mastication, deglutition, rumination, and eructation (Fig. 14.2). Rumination is a procedure of retrieving the food from upper part of rumen to the mouth for mastication.

14.3.1 Regurgitation

Heavy substances such as grains, rocks, or nails settle into the reticulum after ingestion, whereas lighter substances (rough­age) enter the rumen. The saliva and fermentative gases accompany the lighter substances. Based on the specific gravity, the ruminal substances partitions into three zones viz. gas (upper), lighter roughage pieces (middle), and grain and fluid-saturated roughage (Bottom) (Fig. 14.3). Freshly eaten forages whose particle size is too great to be suspended in the rumen fluid for extensive maceration are not regurgitated immediately. The fermentation led constantly proliferating microbes reduce the feedstuff into micro-sized pieces. The continuous ruminal contractions push lighter roughage pieces into middle layer and denser substances into cranial sac of rumen and reticulum. The elevated soft palate closes glottis and the inspiratory effort with tongue drops intra-esphageal and intrathoracic pressure. The nega­tive intrathoracic pressure opens cardia and caudal esopha­geal sphincters and forces the cud from middle layer of rumen into esophagus. The retrograde peristaltic wave originated from the terminal part of the thoracic esophagus carries cud to oral cavity. The lighter roughage pieces return to the mouth in cud form causing remastication. The regurgitated cud consists of small particulate matter highly mixed with liquid, which sinks to bottom layer within the rumen.

Fig. 14.2 Mechanical factors involved in ruminant digestion.

Fig. 14.3 Rumen ecology with the three partitions. [Rumen consists of complex ecosystem with diverse microorganisms such as bacteria, protozoa, and fungi. The gaseous phase of rumen contains CH₄ and NH₃, liquid phase comprises of free-floating low-dense feed, and the solid phase consists of high dense feed particles]

14.3.2 Remastication

After regurgitation, the rigid portion of cud is masticated for about 30-70 s. Entire cycle is repeated with an interval of

2- 4 s between the remastication of two boluses. While chewing, the liquid portion is swallowed spontaneously. Remastication reduces the particle size and provides more surface area for the attack of microorganisms. Optimum remastication time is essential to lessen the risk of acidosis and improve the fiber degradation within the rumen. The chewing time is related to physically effective NDF portion of the diet.

14.3.3 Reinsalivation

As the cud is remasticated, parotid glands secrete saliva, facilitating reswallowing of the chewed cud. The remastication of the solid cud is accompanied by reinsalivation and redeglutition. Saliva is a significant buffering agent for rumen and hence reinsalivation plays a pivot role in maintaining optimum rumen pH of 6-7.

14.3.2 Redeglutition

Redeglutition is an act of reswallowing the cud. The reswallowed cud directly reaches rumen for increased micro­bial action on the complex carbohydrates including cellulose and hemi-cellulose.

Several intrinsic factors such as sex, age, and body size, and extrinsic factors including diet, time, and season affect the time of rumination. On an average, the rumination time for cattle is 10 h per day on complete hay-based diet. Grind­ing the roughage may decrease rumination time to 3 h per day.

14.3.3 Eructation

Eructation is the expulsion of fermentation gases like carbon dioxide and methane accumulated in the rumen.

14.4