What Is Palatability and How Is It Created?

Most people assume that plant palatability depends on flavors that are inherently good or bad. That may be true in some cases, but an animal’s response to a flavor depends primarily on feedback. Flavor only allows animals to distinguish among plants. Whether a flavor is preferred or disliked depends on the nutrient and toxin content of the plant, the nutritional needs of the animal, the animal’s experiences with the food, and its ability to digest the plant. When nutrients are eaten in correct amounts, animals experience comfort or "satiety" and a liking for the flavor of the plant, so palatability increases. Conversely, when animals over-ingest nutritious or toxic plants, or plants containing inadequate nutrients, they experience discomfort and form a disliking for the flavor of the plant, so palatability decreases.

Many weedy species contain moderate to high levels of potentially toxic plant compounds. Over-ingesting toxins like terpenes, tannins, nitrates, alkaloids, and cyanogenic glycosides decreases palatability. However, ruminants rarely over-ingest toxins; rapid postingestive feedback causes nausea and limits the amount they can eat. If toxin concentrations decline, intake of the plant increases. Still, an animal’s ability to distinguish between safe and harmful plants sometimes fails, leading to deaths from toxic plants.

Animals typically avoid plants that are novel, low in nutrients, or high in toxins. Not all plants targeted for control under grazing prescriptions are toxic. But, caution is required. While some toxins may simply cause aversions to a plant, others have the potential to cause production loss, illness, or even death. Animals can learn to avoid a plant only if the toxin causes nausea. They cannot easily learn to avoid plants that cause neurological problems, respiratory failure, birth defects, or chronic liver disease.

It is a good idea to research the nutritional and toxic properties of plants that animals will graze, although information on the chemical content of many weed species is limited. Some excellent references are: 1) Natural Toxicants in Feeds, Forages and Poisonous Plants, by Peter Cheeke, 2) Toxic Plants of North America, by George Burrows and Ronald Tyrl, and 3) A Guide to Plant Poisoning of Animals in North America, by Anthony Knight and Richard Walter.

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