Diet Mixing

Diet Mixing: Teaching Animals to Eat Unpalatable Plants

Ray Banister, a rancher in eastern Montana, uses a grazing management style he calls boom-bust management. It involves short, intensive periods of grazing followed by two growing seasons of rest. When Ray changed from rotational to boom-bust grazing, his Hereford cattle were no longer allowed to eat only the most palatable plants. Instead, they were forced to eat all of the plants.
 
Ray monitors the least palatable plant species – shrubs such as snowberry and sagebrush and various weeds including leafy spurge – to decide when to move his cattle to a new pasture. Cattle are allowed to move only after they eat most of the unpalatable species. Ray’s boom-bust management style reduces the competitive advantage unpalatable plants normally have over more palatable species.
 
It took Ray’s cows three years to adapt to boom-bust grazing, during which the weaning weights of calves plunged from over 500 pounds to 350 pounds, but then rebounded to over 500 pounds.
 
Today under boom-bust management, cattle eat unpalatable species like snowberry and sagebrush as soon as they enter a new pasture. The cows evidently have learned how to mix their diets in ways that better enable them to eat both palatable and unpalatable species. Eating palatable plants high in nutrients likely lessens the negative effects of toxins in unpalatable species.
 
Research assistant professor, Juan Villalba, graduate student, Ryan Shaw, and professor, Fred Provenza at Utah State University are conducting an ongoing project to determine: 1) if animals can learn to mix their diets to eat plant species high in toxins, 2) how nutrients influence ingestion of toxins and 3) ways to help animals learn to mix their diets to ease the transition from eating plants low in toxins to eating combinations of plants low and high in toxins.
Results:

Thus far, researchers at USU have conducted six studies on diet mixing. Listed below are the major findings of the research. Click here to read more details about each study.
 

  1. Sheep with experience eating toxins increases the likelihood that they will eat substantially more high-toxin foods both voluntarily and when forced than inexperienced sheep.
  2. Sheep prefer to eat different types of toxins depending on the nutritional composition of the diet.
  3. When foods contain terpenes lambs prefer foods with moderate amounts of protein (9-15%) and high levels of energy (3.5 Mcal/kg).
  4. Lambs preferred foods low in terpenes regardless of the amount of experience lambs had eating foods with terpenes unless the low-terpene food was lower in nutrients than the high terpene food.
  5. Lambs prefer diets with energy that is degraded slowly in the rumen when eating diets high in terpenes but prefer diets with energy that is degraded quickly when eating diets without terpenes.
  6. Managers can affect the likelihood that herbivores will eat unpalatable plants on rangelands by limiting alternative foods.
  7. Initial experience with toxins and the nutrient content of diets interact to affect preference for and intake of toxins.

 
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.