Physical and Metabolic Constraints on Feed Intake in Ruminants

Abstract

Feed intake control in ruminants is mediated through physical and metabolic constraints. Rumen fill, dietary fiber concentration, and fiber digestibility are important physical constraints. Rumen Volatile Fatty Acids (VFA) concentrations, post-rumen nutrient assimilation and absorption, and hepatic and systemic nutrient balance (and imbalance) are important metabolic constraints on feed intake. Blood levels of glucose and some ketones and fatty acids are other significant players in feed intake regulation in ruminants. Research is needed to elucidate how to optimize feeding strategies and feeding systems to improve feed intake in high-producing ruminants.

Keywords: Feed Intake; Metabolism; Constraint; Ruminant

Objective and Physical Constraints

The objective of this review article was to elucidate the main physical and metabolic constraints on feed intake in ruminants. Over the last 25 years, several major physical and metabolic regulators of feed intake in ruminants have been emphasized. Ruminal fill [1-4] is one of the central regulators of Dry Matter Intake (DMI) under certain circumstances such as when feeds with low digestibility are fed [5]. The dietary NDF, especially from forage, is a key contributor to reticulorumen fill. The greater NDF lowers the clearance rate of the rumen contents [6]. Hence, the dietary NDF can be a key controller of feed intake in early and peak lactation cows that have not peaked in DMI or with limited rumen fiber pool [2]. The NDF digestibility can significantly impact DMI [7]. As NDF digestibility increases, the depressing effect of NDF on DMI weakens. Allen [2] stated that DMI rose by 0.17kg per unit rise in in vitro or in situ NDF digestibility. At higher NDF digestibility, the NDF will have a smaller impact on rumen distension. Thus, factors affecting NDF digestibility will affect DMI.

Metabolic Constraints

Among the important metabolic constraints of appetite are rumen concentrations of volatile fatty acids [8,9]. Propionate injection into the portal vein has reduced feed intake in sheep [10,11]. Propionate rather than acetate seems to cause hypophagia [2]. Insulin secretion [12] and hepatic receptors [10] have been proposed to mediate the hypophagic effects of propionate. In addition to the hepatic chemoreceptors, hepatic thermoreceptors may also control feed intake. Di Bella et al. [13] heated the rat liver artificially and observed an increased chewing activity with reduced feed intake.

Feed intake is ultimately a psychological phenomenon integrating animal’s abilities to cope with changes in diet composition and metabolic demands [14]. Thus, one must consider that the rumen or blood VFA is only one of many factors involved in feed intake [15]. Illius and Jessop [16] suggested that imbalances in nutrient supply both in the rumen, postrumen, and hepatic levels can reduce feed intake. They proposed that maximizing acetate use for lipogenesis needs a synchronous glucose supply. Glucose fuels lipogenesis by providing ATP and cofactors such as NADPH needed for fatty acid elongation [16]. Thus, even the high production rate of acetate, if accompanied by adequate supply of other nutrients, May not necessarily down-regulate feed intake. The framework of Illius and Jessop [16] presumes that nutrient imbalances constrain feed intake via accumulation of excess metabolites such as acetate. Therefore, the animal targets a level of intake that minimizes nutrient imbalances. According to this framework, in the absence of adequate glucose, acetate will mount up and act as a hypophagic feedback.

β-hydroxybutyrate (BHBA) is another metabolite that can contribute to feed intake regulation. Subcutaneous administration of BHBA reduces feed intake in rats [17,18]. The satiety signals may arise partly from direct oxidation of BHBA. Consequently, reducing equivalents or NADH accumulate in the mitochondria and depress feed intake [19]. Unlike BHBA, subcutaneous administration of acetoacetate does not affect feed intake [17]. It seems, therefore, that the process of hepatic BHBA conversion to acetoacetate involving other metabolites and co-factors and not acetoacetate per se influences satiety. The role of BHBA on feed intake regulation needs further research in ruminants.

Mayer [20] was the first to suggest that blood glucose controls feed intake. Mayer [20] indicated that the hypothalamus takes up glucose and thereby monitors and controls peripheral blood glucose. According to the Mayer’s glucostatic theory, the hypothalamus controls blood glucose by controlling feed intake. Early trials with intra-ruminal, intra-venous, or intra-cerebroventricular glucose infusion in sheep [21], goats [22,23] and cattle [24] demonstrated no effects of glucose on feed intake. Blood glucose and its diurnal fluctuations are considerably lower in ruminants than in nonruminants [25]. Thus, blood glucose does not seem to be as significant in controlling feed intake in ruminants as it is in non-ruminants. This is not surprising, because due to the extensive rumen fermentation of dietary carbohydrates, VFA and not glucose are the main digestion end-products absorbed across the gut in ruminants [26]. When highstarch diets containing corn and sorghum grains are fed, however, the amount of intact or partially hydrolyzed starch escaping the rumen may increase [27]. The intestinal starch and the resulting glucose may affect feed intake. The role of the absorbed glucose across small intestine on feed intake regulation requires has not been elucidated.

Implications

Physical and metabolic constraints on feed intake in ruminants were reviewed. Optimal feeding strategies and feeding systems must be adopted to regulate feed intake such that rumen and intermediary metabolism can be optimized. Research is needed to elucidate how to optimize feeding strategies and feeding systems to improve feed intake in high-producing ruminants.

Acknowledgment

Thanks to the Iranian Ministry of Science Research and Technology, and National Elite Foundation.

References

1. Allen MS. Relationship between fermentation acid production in the rumen and the requirement for physically effective fiber. J. Dairy Sci. 1997; 80: 1447-1462.
2. Allen MS. Effects of diet on short-term regulation of feed intake by lactating dairy cattle. J. Dairy Sci. 2000; 83: 1598-1624.
3. Forbes JM. Voluntary feed intake and diet selection. In Quantitative Aspects of Ruminant Digestion and Metabolism. Eds: Dijkstra J, Forbes JM and France J. CABI Pub. Wallingford, UK. 2005; 60-626.
4. Mertens DR. Creating a system for meeting the fiber requirements of dairy cows. J. Dairy Sci. 1997; 80: 1463-1481.
5. Allen MS, Bradford BJ, Harvatine KJ. The cow as a model to study food intake regulation. Annu. Rev. Nutr. 2005; 25: 523-547.
6. Rayburn EB, Fox DG. Variation in neutral detergent fiber intake of Holstein cows. J. Dairy Sci. 1993; 76: 544-554.
7. Oba M, Allen MS. Evaluation of the importance of the digestibility of neutral detergent fiber from forage: Effects on dry matter intake and milk yield of dairy cows. J. Dairy Sci. 1999; 82: 589-596.
8. de Jong A. Short- and long-term effects of eating on blood composition in free-feeding goats. J. Agric. Sci. 1981; 96: 659-668.
9. de Jong A, Steffens AB, de Ruiter L. Effects of portal volatile fatty acids infusions on meal patterns and blood composition in goats. Physiol. & Behav. 1981; 27: 683-689.
10. Anil MH, Forbes JM. Feeding in sheep during intra portal infusions of short-chain fatty acids and the effect of liver denervation. J. Physiol. 1980; 298: 407-414.
11. Farningham DAH, Whyte CC. The role of propionate and acetate in the control of food intake in sheep. Br. J. Nutr. 1993; 70: 37-46.
12. Grovum WL. Mechanisms explaining the effects of short chain fatty acids on feed intake in ruminants-osmotic pressure, insulin and glucagon. Pages 173-197 in Ruminant Physiology: Digestion, Metabolism, Growth and Reproduction, Englehardt WV, Leonhard-Marek S, Breves G, Geisecke D, ed. Ferdinand Enke Verlag, Stuttgart, Germany. 1995; 81: 1396-1402.
13. Di Bella L, Tarozzi G, Rossi MT, Scalera G. Effect of liver temperature increase on food intake. Physiol Behav. 1981; 26: 45-51.
14. Provenza FD, Villalba JJ, Cheney CD, Werner SJ. Self-organization of foraging behaviour: from simplicity to complexity without goals. Nut. Res. Rev. 1998; 11: 199-222.
15. Forbes JM. Voluntary Food Intake and Diet Selection in Farm Animals. CABI Int. Wallingford, UK. 1995; 532.
16. Illius AW, Jessop NS. Metabolic constraints on voluntary intake in ruminants. J. Anim. Sci. 1996; 74: 3052-3062.
17. Langhans W, Wiesenreitter F, Scharrer E. Different effects of subcutaneous D, L-3-hydroxybutyrate and acetoacetate injections on food intake in rats. Physiol. Behav. 1983; 31: 483-486.
18. Scharrer E, Langhans W. Mechanisms for the effect of body fat on food intake. In: Forbes JM and GR Hervey (Eds). The control of body fat content. Smith-Gordon, London. 1990; 63-86.
19. Langhans W, Damaske U, Scharrer E. Different metabolites might reduce food intake by the mitochondrial generation of reducing equivalents. Appetite. 1985; 6: 143-152.
20. Mayer J. Glucostatic regulation of food intake. New Eng. J. Med. 1953; 249: 13-16.
21. Seoane JR, Baile CA. Effects of intravenous injections of 2-deoxy-d-glucose, glucose, and xylose on feeding behavior of sheep. Physiol. Behav. 1972; 9: 423-428.
22. Baile CA, Mayer J. 1968. Effects of insulin-induced hypoglycemia and hypoacetonemia on eating behavior in goats. J. Dairy Sci. 1968; 51: 1495-1499.
23. Baile CA, Mahoney AW. Hypothalamic function in ruminant food intake regulation. Proc. Int. Con. Nutr. 1967; 2: 67-72.
24. Dowden DR, Jacobsen DR. Inhibition of appetite in dairy cattle by certain intermediate metabolites. Nature. 1960; 188: 148-149.
25. Bergman, EN. The pools of cellular nutrients. Glucose. In Dynamic Biochemistry of Animal Production. World Animal Science, A3. Ed. P. M. Riis. Elsev. Sci. Pub. Co. Amsterdam, the Netherlands. 1983; 173-196.
26. Sutton JD. Carbohydrate digestion and glucose supply in the gut of the ruminant. Proc. Nut. Soc. 1971; 30: 243-248.
27. Nocek JE, Tamminga S. Site of digestion of starch in the gastrointestinal tract of dairy cows and its effect on milk yield and composition. J. Dairy Sci. 1991; 74: 3598-3629.