SALIVARY SECRETION AND ITS ROLE IN RUMINANT DIGESTIVE SYSTEM
Komal Chauhan and Harneet Kour
PhD Scholar, Division of Animal Nutrition, ICAR-National Dairy Research Institute, Karnal-132001
e-mail: chauhankomal603@gmail.com
Introduction
The ruminant animals have specially adapted digestive systems that allow them to efficiently use high roughage feedstuffs. One of the integral components of the ruminant digestive system are salivary glands which secrete saliva. While ruminants are consuming feed, saliva is secreted by the parotid, the submaxillary, and the sublingual glands along with other minor salivary glands. Saliva aids in chewing and swallowing, contains enzymes for breakdown of fat (salivary lipase) and starch (salivary amylase), and is involved in nitrogen recycling to the rumen. Besides these functions, saliva’s most important function is to buffer pH levels in the reticulum and rumen.
Saliva and its composition
Saliva is a biofluid containing water, electrolytes, mucus and enzymes produced mainly by major salivary glands as well as by minor submucosal salivary glands. Whole saliva also contains ingredients that do not originate from the salivary glands (Malathi et al., 2014) like bacteria and products of their metabolism, desquamated epithelial cells as well as extraneous debris. Saliva has around 99% water and 1% solids. The water part of saliva has organic substances dissolved in it which include enzymes like amylase, maltase, lingual lipase, lysozyme and carbonic anhydrase. Besides enzymes some other organic substances are also present eg., proteins- mucin & albumin, blood group antigen, free amino acids, non protein nitrogenous substances- urea, uric acid and creatinine. The solid portion of saliva has ions like sodium, calcium, potassium, bicarbonate, chlorine, fluorine and phosphate and gases like oxygen, carbondioxide and nitrogen.
The amount of saliva produced varies depending on the animal, type of feed, amount of time spent chewing feed, as that stimulates saliva production. (Argenzio, 2013). The estimated per day secretion of saliva is around 1 litre in adult pig, 100-200 liter in adult cows, and 10-20 liter in sheep.
Salivary Glands
Salivary glands are exocrine glands which synthesize and secrete their secretions over an epithelial surface through a system of ducts. These glands are present in and around the oral cavity. The salivary secretions play an important role in the physiological processes of the animal’s oral cavity. There are three major salivary glands:
- Parotid (serous saliva) – Parotid gland is a serous and monostomatic gland having the stenson duct. It produces serous secretion having amylase (not in ruminants), and lipase.
- Submaxillary/mandibular (mixed saliva) – Sublingual gland is a mucous and polystomatic gland having two ducts namely the Bartholin and Rivinus duct . The mucin present in the secretions of sublingual gland helps lubricate the bolus as it passes down the esophagus.
- Sublingual (mucous type saliva) – Submaxillary gland is a mixed salivary gland and is monostomatic in structure because of presence of single duct i.e., the wharton duct. It produces a mixed secretion that has both serous and mucous attributes. The saliva produced is hypotonic in nature and thus helps to reduce the osmotic concentration of the ingesta.
- Functions of saliva
Saliva plays a significant role in the ruminant’s digestive processes by performing the following functions:
Lubrication of bolus – It lubricates the feed bolus with the fluid and the mucus. It also provides water to dilute the osmolarity of the ingested material.
Maintenance osmolarity of ingesta – The feed bolus is hyperosmotic when ingested and saliva provides water and electrolytes to dilute the osmolarity of the ingested material.
Enzymatic activity – Salivary secretions contain small amounts of α‐amylase which can break the α-1,4 linkages between glucose molecules in starch. Saliva also contains lingual lipase, an enzyme that begins the process of fat digestion.
Neutralization of acid content of ingesta – Saliva is slightly alkaline in nature and thus, provides some ability to neutralize acids that might have been consumed. Salivary pH in ruminants can be considerably higher than in non-ruminants, thus helps in neutralizing and buffering acids produced during bacterial fermentation in the rumen.
Water balance – Saliva maintains water balance, as almost 70% of the fluid entering rumen is saliva.
Control over intake – Saliva infused into abomasum increased reticular contractions and DM intake in cattle (Froetschel, 1995).
Nitrogen and mineral recycling – 15% to 50% of total nitrogen recycled in a ruminant is via saliva. Minerals which are recycled by saliva include sodium, phosphorus (80% of the endogenous P recycled to the gut) and sulphur.
Modulation of ruminal microbes – Saliva is considered as main vehicle for introducing immune active metabolites in to the rumen (Yáñez-Ruiz et al., 2015). Immunoglobulins, especially secretory immunoglobulin A (IgA), modulate the proliferation of symbiotic microbiota, by either inhibiting or stimulating their growth. Some recent studies have also shown that some specific protein components have the ability to modulate rumen fermentation in goats (Palma-Hidalgo et al., 2021a).
Buffering action – Salivation is estimated to buffer 37% of total acids in dairy cattle (Allen, 1997). Saliva is slightly alkaline which provides some ability to neutralize acids that might be consumed in feed as well as neutralize and buffer acids produced during bacterial fermentation in the rumen. In particular instances, salivation can also help in buffering the adverse or undesirable effects from a variety of toxic compounds in feed.
Perception of taste – Together with major and minor salivary glands, other glands serve the perception of taste. Von Ebner’s glands pour small amounts of serous saliva into the cleft of circumvallate and foliate papillae to refresh taste cells and start new taste perception. Moreover, saliva is also responsible for maintaining the number of taste buds, as taste cells display a turnover of about 10 days. Salivary secretions, thus, are of absolute importance to allow the animals to feel the taste of the feed, select feedstuffs, and sort flavours.
Conclusion
Salivary secretions because of their chemical composition and nature play an important role in the digestive processes of an animal. The action begins in oral cavity as saliva lubricates the feed bolus as well as starts the amylase activity on the ingesta. The activity continues in the rumen as well since it helps to modulate the ruminal microbes and hence, the ruminal fermentation.
References
Allen, M. S. 1997. Relationship Between Fermentation Acid Production in the Rumen and the Requirement for Physically Effective Fiber. Volume 80, Issue 7, p1447-1462.
Argenzio, R.B. 2013. Digestive And Absorptive Function Of Intestines. Digestion, Absorption and Metabolism, Duke’s Physiology of domestic animals,12th edition . 999: (419-37).
Froetschel M. A. 1995. Effect of Abomasal Infusion of Saliva on Reticular Motility and Ruminal Liquid Contents of Steers. Journal of Dairy Science. 78:2395-2401.
Malathi, N, Mythili S, Vasanthi HR. 2014.Salivary Diagnostics: A Brief Review. ISRN Dent. 158786.
Palma-Hidalgo, J. M., Jiménez, E., Popova, M., Morgavi, D. P., Martín-García, A. I., Yáñez-Ruiz, D. R., & Belanche, A. 2021.Inoculation with rumen fluid in early life accelerates the rumen microbial development and favours the weaning process in goats. Animal Microbiome, 3(1).
Yáñez-Ruiz D.R , Abecia L. , Newbold C.J. 2015. Manipulating rumen microbiome and fermentation through interventions during early life: a review. Frontiers in Microbiology, 6 (2015), p. 1133.
