Much of the diet mixing and toxin work at USU has focused on three toxins: terpenes, tannins and oxalates. Terpenes are commonly found in sagebrush and juniper. Tannins are prevalent in many shrubs, about 80% of all woody plants contain tannins. Finally, oxalates are found in many weedy species such as pigweed, lambs-quarter, dock, greasewood and halogeton.

Experience affects intake. Experience eating toxins increases the likelihood that animals will eat substantially more high-toxin foods both voluntarily and when forced. When two groups of sheep had free access to ground alfalfa and a 50:50 alfalfa-barley ration, sheep experienced with high-toxin foods (three ground rations containing either oxalate, terpene or tannin) ate 11 times more of the high-toxin foods compared with sheep with no experience with toxins. When ground alfalfa and alfalfa-barley were severely limited, sheep experienced with high-toxin foods ate 2.5 times more of the high-toxin foods compared with sheep with no experience with toxins. Throughout the study, sheep showed no ill effects from eating diets high in toxins.

Toxin-Nutrient Interactions. The experiments below summarize the results of several studies examining how nutrients and toxins interact. These studies will enable managers to create supplements that enable animals to eat foods high in toxins and to use animals to modify plant communities to increase the productivity of rangelands and pastures.

Diet quality affects preference for toxins. Sheep offered three foods containing tannins, oxalate or terpenes ate increasing amounts of the food high in oxalate as the nutrient (energy and protein) quality of the diet declined. They increased intake of the food high in terpenes when the amount of nutrients (energy and protein) in the diet increased. Thus, the concentrations of nutrients in the diet affects which toxin and the amount of toxin lambs will eat.

Diet quality affects consumption of terpenes. Lambs ate more terpene-containing food when the food contained moderate (9 or 15%) amounts of protein instead of low (6%) or high (21%) amounts. They ate the same amount of food regardless of its protein content when food did not contained terpenes. Lambs ate greater amounts of food when it contained moderate amounts of energy (2.55 or 3.3 Mcal/kg) and no terpenes. However, when terpenens were added to the diets, lambs ate greater amounts of food with the highest concentration of energy (3.53 Mcal/kg). Thus, the ratio of protein/energy in the diet may be a more important factor in controlling intake than the absolute concentration of a single nutrient in terpene-containing plants. In addition, higher the concentration of energy in the plant, provide the balance of protein to energy is adequate, the greater the amount of the plant can be eaten.

Lambs preferred to eat foods low in terpenes regardless of the energy or protein content of the terpene-containing food or the amount of experience lambs had eating foods with terpenes. While lambs fed diets higher in energy or protein ate more terpenes when forced, they did not when foods with similar nutrient quality and without terpenes were available. These results contrasted an earlier study by Villalba and Provenza where lambs readily ate foods that contained toxins (oxalate, tannin or terpenes) even when alternative high-quality, low-toxin foods were available. Allowing lambs to mix their diets with foods varying in nutrient and toxin content and structure may have enabled lambs to eat more foods high in toxins.

When lambs were given a choice between a diet containing terpenes and alfalfa pellets, lambs were more likely to eat a terpene-containing diet as long as the diet contained more energy than alfalfa pellets and at least 9% protein. Thus, whether or not lambs will eat a plant that contains terpenes also depends on the nutritional value of the surrounding plants.

Terpenes Affect the Degradabilty of Energy Selected. The degradability (how quickly something breaks down in the rumen) of the energy in the diet affects preference for foods. Lambs preferred a barley-based diet (faster degradability) to a beet pulp-based diets (slower degradability) when neither diet contained terpenes. But they preferred the beet pulp-based diet to the barley-based diet when both foods contained terpenes. Beet pulp-based diets were also preferred when animals ate sagebrush. Both diets contained the same amount of energy and protein only the degradability of the energy in the two diets differed. Thus, the degradability of the energy in plants eaten by herbivores influences intake and preference for not only sagebrush, but also for other plant species or supplements eaten in conjunction with sagebrush.

Availability of Alternatives. Managers can affect the likelihood that herbivores will eat unpalatable plants on rangelands by limiting alternative foods. Sheep in good body condition are more likely to eat greater amounts of high toxin foods, especially those containing terpenes, as the amount of low-toxin food becomes more limiting. In fact, when the amount of low-toxin food was severely limited, sheep inexperienced with toxins (oxalate, terpenes and tannins) ate more food and gained more weight than when the amount of low-toxin food was only moderately limited. These results do not support the notion that sheep are more likely to eat foods high in toxins if they have no alternative foods. During the trials lambs were fed limited amounts high quality foods each day in conjunction with foods high in toxins. Providing lambs with limited amounts of nutrients each day is essential for animals to eat and detoxify plants high in toxins. These results do support the idea that animals must be pressed into trying new foods especially those with strong flavors.

First Impressions Matter. Initial experience with toxins and the nutrient content of diets interact to affect preference and intake of toxins. Sheep ate diets containing tannins and terpenes more readily when they are first exposed to the toxins in diets high in nutrients compared with sheep that are first exposed to them in diets low in nutrients. The order foods are offered to herbivores when they are first exposed to toxins can affect preference and intake of a high-toxin foods. When sheep were fed an alfalfa-grain ration immediately before and after they ate a novel poor-quality high-tannin food, they had a higher preference for the high-tannin food and spent less time searching for an alfalfa-grain ration during pen trials, compared with lambs fed the alfalfa-grain ration and high-tannin diet at different times of the day.

In summary, animals can eat substantially more toxins than once believed. Furthermore, experience and context are important forces driving food preference and intake. Managers may be able to use context and experience as tools to influence the composition of plant communities and increase animal productivity.

Personnel:

Fred Provenza
Professor, Behavior
Utah State University

Juan Villalba
Research Scientist, Behavior/Nutrition
Utah State University

Luthando Dziba
Graduate Student
Utah State University

Jocelyn Knight
Graduate Student
Utah State University

Greg Mantz
Graduate Student
Utah State University

Ryan Shaw
Graduate Student
Utah State University