Controlling Feed Intake in Farm Animals

Metabolic Level: Substances within the body signal the animal to stop feeding.
Digestive System: Physical capacity of the tract determines if more food can be consumed.
External Factors: Environmental variables such as climate influence ingestion.

Central Nervous System (CNS): The hypothalamus, located beneath the cerebrum, acts as the satiety centre. Feeding continues unless the hypothalamus receives signals to stop.
Short-term Regulation (Chemostatic Theories):
- Plasma Glucose: Glucoreceptors in the hypothalamus monitor levels; falling blood glucose triggers feeding.
- Cholecystokinin (CCK): A hormone released when amino acids and fatty acids reach the duodenum. Intravenous injection of CCK has been shown to reduce intake in chickens.
Long-term Regulation (Lipostatic Theory):
- Body Weight Preservation: Motivation to return to an ideal weight after starvation or forced feeding.
- Leptin: Secreted by white adipose tissue. In pigs, intracerebroventricular injection of leptin reduces intake by up to $90\%$ .
- Sow Data: Pregnant sows with back fat over $25\,mm$ have serum leptin of $5\,ng/ml$ and intakes $20\%$ lower than sows with back fat less than $20\,mm$ (leptin $3\,ng/ml$ ).
Palatability: The readiness of food selection based on smell, touch, and taste. Studied by Baumont (1996) in sheep and Kienzle et al. (1998) in dogs.
Energy and Choice Feeding:
- Chicks adjust food intake to maintain constant energy intake even when diet energy content is reduced.
- Pigs and poultry can regulate intake from variety of foods to meet specific requirements (e.g., crude protein).

Short-term Regulation (Chemostatic): Limited impact compared to monogastrics. Intake is more likely regulated by the absorption of volatile fatty acids (acetate and propionate) rather than glucose.
Rumen Capacity: The primary limiting factor. Increased digestibility leads to increased food consumption.
Physical Form: Grinding and pelleting roughage-based diets increases dry matter intake as they are degraded more rapidly in the rumen.

Lactation: Increases energy requirements significantly, leading to higher gross energy intake compared to non-lactating cows.
Grazing Swards: Intake can be restricted by bite size when animals graze swards close to the ground.
Environmental Temperature: Intake declines at high temperatures as the energy required for maintenance decreases.
Nutrient Deficiencies: A lack of rumen degradable protein decreases diet digestibility and subsequent intake.