A Review of a Few Key Factors Regulating Voluntary Feed Intake in Ruminants1

ing the integration of multiple feedback signals in the control of feeding behavior (Forbes, 1995, 1996). Because of During the history of ruminant nutrition many factors have been the experimental difficulties in the study of the central proposed as regulators of voluntary feed intake. In some cases, the implicit assumption has been that a factor acted independently and nervous system, the use of empirical mathematical methexclusively of other mechanisms. A knowledge of ruminant digestive ods are likely to remain critical to synthesizing the current anatomy aids in understanding both the ecological niche and the understanding of intake regulation (Fisher, 1996). feeding behavior of the ruminant animal. Factors controlling ruminant This brief review will cover some key aspects of rumiintake should be assumed to function with multiple interactions. A nant anatomy, physical and metabolic feedback mechanumber of feedback regulators such as distension, protein, and energy nisms, and some practical applications of theoretical should be considered in the context of their interacting regulatory feedbacks in the context of impacts on voluntary feed effects when attempting to predict intake. Behavioral aspects also intake and feeding systems. influence voluntary feed intake through associations formed via postingestive feedback. Ruminants can learn to identify particular feeds and alter intake on the basis of past experiences. An integrated apANATOMY proach is proposed as a means of understanding ruminant feed intake regulation and eventually to improving prediction of intake. Empirical In the investigation of voluntary feed intake in rumimathematical methods are likely to be fundamental to developing nants, it is instructive to consider the anatomy of domesunderstanding and models of feed intake because of the difficulty of tic ruminants in relation to the anatomy of all ruminants studying the central nervous system. In spite of these difficulties, and the ecological niches occupied by ruminants in the knowledge of theoretical feedbacks has already been used to develop wild. This should seem obvious but ruminant anatomy is practical ruminant feeding strategies. not generally considered a key to understanding forage utilization. For example, the two-volume textbook Forages (Barnes et al., 1995) in its 5th edition has 60 chapF as long as mankind has kept ruminant livestock ters and 91 authors but it has no chapter on herbivore there has probably been interest in their digestive anatomy. This is not to say anatomy should be regarded tract and their ability to thrive on herbaceous material. as trivial; however, anatomy is obviously not generally This interest naturally leads to speculation on manageperceived as fundamental to understanding forage utiliment strategies designed to improve ruminant perforzation. This is true of herbivores in general and rumimance and more efficiently meet human needs (Van nants in particular although there are general comments Soest, 1994). Present day ruminant production systems on the anatomy of herbivores in the textbook (Ely, 1995; have stimulated research to improve the prediction of Evans, 1995; Fisher et al., 1995). feed intake to increase the efficiency of management Domestic ruminants have been selected and mainsystems (Tedeschi et al., 2000). Researchers have identitained by humans for thousands of years but the niche fied many factors that are correlated with intake and occupied by the wild ancestors of domestic ruminants have proposed many of these factors as regulators of has a direct bearing on the digestive anatomy present intake. One key feature of a regulatory feedback is today and utilized in modern production systems. An the discovery and description of a physical feedback animal may be fed a diet that is very different from mechanism. Even when this difficult process is accomthe diet that characterizes the ecological niche of the plished, the task remains of understanding and describruminant’s adaptation. With appropriate management, animal performance may be excellent on a diet that is 1USDA prohibits discrimination in all its programs and activities more characteristic of a monogastric animal. However, on the basis of race, color, national origin, gender, religion, age, human management of domestic ruminants interacts disability, political beliefs, sexual orientation, and marital or family status (not all prohibited bases apply to all programs). Persons with with the natural limitations and specializations still presdisabilities who require alternative means for communication of proent in domestic ruminants. gram information (Braille, large print, audiotape, etc.) should contact There are approximately 180 species of ruminants USDA’s TARGET Center at (202) 720-2600 (voice and TDD). To existing in the world today. The digestive tracts of rumifile a complaint of discrimination write USDA, Director, Office of Civil Rights, Room 326-W, Whitten Building, 14th and Independence nants are well known to contain multiple stomachs and Avenue, SW, Washington, DC 20250-9410 or call (202) 720-5964 the term rumination is used in modern English to indi(voice or TDD). USDA is an equal opportunity provider and emcate the process of chewing the cud as well as to meditate ployer. Use of trade names in this publication does not imply endorseor turn something over in the mind. However, ruminant ment by USDA, ARS of the products named or criticism of similar ones not mentioned. digestive tracts are not uniform. On the contrary, they are highly specialized to process specific diets (HofUSDA-ARS, 1420 Experiment Station Road, Watkinsville, GA, mann, 1988, 1998). The ruminant’s digestive tract is fit 30677-2373. Received 29 May 2001. *Corresponding author (dsfisherto a particular ecological niche and ruminants range in @arches.uga.edu). specialization from concentrate selectors through intermediate feeders to species adapted to grazing. A generPublished in Crop Sci. 42:1651–1655 (2002).