Palatability and feeding behaviour in ruminants. A review

Palatability usually designates those characteristics of a feed that invoke a sensory response in the animal, and is considered to be the corollary of the animal's appetite for the feed. When only one feed is given to animals fed indoor, palatability can be evaluated by the eating rate at the beginning of the meal. When several feeds are studied, preference tests are most often used to assess palatability. Intake measurements are critical since postingestive effects are partly confounded with palatability, but experimental procedures allow these two variables to be separated. Behavioural measurements assess motivation for a feed rather than intake. Operant conditioning procedures show how animals maintain their choice for a preferred feed as it becomes increasingly difficult to obtain. Physical characteristics of the feed (particle size, resistance to fracture, dry matter content, height and density of sward, etc) contribute to the sensory response invoked by the animal. They influence ease of prehension and ease of mastication and animals generally prefer the physical form of the feeds they can eat faster. Taste and odour are recognized as of importance in feed palatability; however, effects of the primary tastes depend on the experimental procedures used. Nevertheless, there is some evidence that sheep will develop a liking for the taste of monosodium glutamate and for the odour of butyric acid and a dislike for acetic acid. Most of the palatability studies are short term with time scales of minutes or hours. In the long term (several days or weeks), feed preferences seem generally to be associated with digestive modifications. Animals use their senses to learn to associate the postingestive effects of the feed with its sensory characteristics. Ruminants generally develop preferences for feeds that will provide a high satiety level rapidly. Thus, palatability measured as the sensory response invoked by the feed integrates its nutritive value. However, for a given nutritive value, sensory properties of the feed per se can stimulate or depress hedonic feeding behaviour. The role of hedonic behaviour on intake may be of particular importance in choice situations and for low producing animals. In a first approach, hedonic Value of the feed can be assessed by the difference between the observed intake and the predicted intake as affected by the nutritive value.