Introduction
Animal nutrition is an essential discipline within veterinary medicine, as adequate feed intake constitutes the main determinant of nutritional status, health, welfare, and productivity of animals. The study of voluntary feed intake and the mechanisms that regulate it allows understanding how animals adjust their feed consumption according to their physiological needs and environmental conditions (McDonald et al.; NRC).
Feed intake does not depend exclusively on feed availability, but is regulated by complex physiological, metabolic, environmental, and behavioral mechanisms. These mechanisms explain the variations observed among species, individuals, and production systems, and are fundamental for diet formulation and proper nutritional management (Church).
Importance of feed intake in animal health and production
Feed intake is the direct link between nutrition and the biological response of the animal, as it determines the supply of energy and nutrients required for maintenance, growth, reproduction, and production. Insufficient intake limits the availability of metabolic substrates, favoring weight loss, reduced productive performance, and increased susceptibility to disease, whereas excessive intake may lead to metabolic disorders such as obesity, ruminal acidosis, or endocrine alterations. In production animals, voluntary intake directly conditions feed efficiency, milk, meat, or egg production, and system profitability, making it a central parameter in animal nutrition (McDonald et al.; NRC).
General concepts related to intake
Voluntary feed intake is defined as the amount of feed that an animal freely consumes when it has ad libitum access. This parameter reflects the integration of physiological, metabolic, and sensory signals, and is widely used to evaluate feeding behavior and diet adequacy (McDonald et al.).
Appetite represents the physiological drive to initiate intake, whereas satiety corresponds to the inhibition of consumption once immediate needs have been met. The postprandial period is the interval following ingestion, during which digestive, metabolic, and hormonal signals are activated that influence the next meal (NRC; Church).
Gustatory appeal and palatability integrate sensory characteristics such as taste, odor, and texture, influencing feed acceptance. Forced feeding and restricted feeding are strategies used in clinical, experimental, or productive situations to control intake when it is not adequately regulated naturally (McDonald et al.).
Factors that modify voluntary feed intake
Environmental temperature is one of the most influential factors on intake. Under excessive heat conditions, animals reduce intake to decrease metabolic heat production, whereas in cold environments intake usually increases to meet higher energy requirements. Humidity and ventilation also play a role, especially in intensive systems (NRC; Church).
Diet composition, energy density, fiber content, physical form, and palatability condition the amount consumed. For example, diets with high fiber content limit intake due to ruminal fill, whereas highly energy-dense diets may reduce total intake (McDonald et al.).
Age, body weight, physiological status, and health status directly influence intake. Animals in growth, gestation, or lactation present higher requirements and, therefore, higher voluntary intake. Systemic or digestive diseases usually reduce feed consumption (NRC).
Social behavior, hierarchy, stress, and management conditions influence intake patterns. In group systems, subordinate animals may consume less feed due to competition, affecting their productive performance (Church).
Theories on the regulation of voluntary intake post-ingestion
This theory proposes that intake is regulated to maintain body thermal balance. Animals adjust their consumption according to environmental temperature, reducing intake in warm climates and increasing it in cold climates (McDonald et al.).
It states that blood glucose levels act as regulatory signals of appetite. Decreases in glycemia stimulate intake, whereas postprandial increases contribute to satiety (NRC).
This theory suggests that body fat reserves send signals to the central nervous system to regulate long-term intake, contributing to the maintenance of body weight (McDonald et al.).
Hormones such as leptin, ghrelin, insulin, and cholecystokinin integrate peripheral and central signals that modulate appetite and satiety. In addition, the hedonic effect considers that pleasure associated with food can stimulate intake independently of energy needs (Church; NRC).
Influence of voluntary intake on productive response
The relationship between voluntary intake and productive response varies among species. In ruminants, intake conditions milk and meat production, whereas in monogastrics it directly influences daily weight gain and feed efficiency. In poultry, small variations in intake can have significant effects on egg or meat production, highlighting the importance of precise nutritional management (McDonald et al.; NRC).
Parameters used in the prediction of voluntary feed intake
Metabolic body weight, expressed as body weight raised to the 0.75 power, is used to estimate energy requirements and predict voluntary intake, allowing comparisons among animals of different sizes (NRC).
Productivity, such as weight gain or milk production, is a key parameter for adjusting diets and estimating required intake. Animals with higher productive levels have greater nutritional requirements and, therefore, higher intake (McDonald et al.).
Conclusion
Voluntary feed intake is a central axis of animal nutrition and is regulated by multiple physiological and environmental mechanisms. Its proper understanding allows optimization of animal health, welfare, and productivity in different production systems (McDonald et al.; NRC).
References
McDonald, P., et al. Animal Nutrition. Pearson.
National Research Council (NRC). Nutrient Requirements of Domestic Animals.
Church, D. C. The Ruminant Animal: Digestive Physiology and Nutrition.