Date of Award:


Document Type:


Degree Name:

Doctor of Philosophy (PhD)


Animal, Dairy, and Veterinary Sciences


John C. Malechek


An experiment was designed to study the effects of forage availability, season and intensity of grazing, and distribution and behavior of sheep on their forage preferences. Three grazing periods of 15 days each were used to study the effect of season. Each grazing period included a heavy and a moderate stocking intensity. Pastures were divided by a grid into 30.48 m x 30.48 m compartments. Each compartment was sampled for botanical composition of available herbage prior to and after grazing. Esophageally fistulated sheep were allowed to graze freely and positions of individual sheep with respect t o compartments were recorded at 5-minute intervals during the daily forage sample collection period. Immediately following collection of fistula samples, daily measurements were taken on leaf area index and height for all plant species. Estimates of aerbage yield and forage utilization were derived from heightleaf area measurements by regressions. Botanical composition of the diet was determined through microscopic analysis of plant cuticle fragments on dried, ground esophageal samples.

Analysis of the dietary data indicated that season had no effect on the botanical composition of diets of sheep. However, grazing intensity significantly (P < 0. 20) affected diets of sheep. Significant differences (P < 0 . 01) were also found in proportion of plant species that comprised the diet at any particular time. Individual sheep were significantly (P < 0.01) different in their forage preferences . There were no significant changes in botanical composition of the compartments due to grazing. However. bare ground increased significantly (P < 0.10) more under heavy stocking than under moderate stocking.

Herbage yield was found to be highly correlated with leaf area index and height (r2 = 0.85) in the ungrazed control pasture. Forage yield in the grazed pastures was a l so correl ated with leaf area index and height (r2 = 0.79) . Utilization was estimated as the difference between the two parameters .

Sheep were observed to graze more heavily around the periphery of shrubs than in the interspaces. The heavily grazed areas around shrubs were found to be significantly larger in heavily stocked pastures (P < 0.10) . Observations of grazing behavior showed that sheep tended to orient themselves toward conspicuous objects . In so doing, they grazed a strip leading from one conspicuous object to another (ex. shrubs) .

In an experiment designed to determine the role of such conspicuous objects in animal distribution and feeding behavior, sheep distribution, in relation to randomly-placed cardboard boxes. was found to be non-random and significantly (P < 0.05) related to the position of the boxes. It was also found that sheep grazed the herbaceous species to a certain height below which the plants became inaccessible to grazing.

Micro-associations of plant species greatly influenced preferences. Sagebrush (Artemisia tridentata) plants exerted a negative effect on use of adjacent bitterbrush (Purshia tridentata) plants. This negative effect was determined by the distance between the two shrubs. This "critical distance" was found to be 56.1 ± 23.7 cm and was not affected either by season or stocking intensity .

An equation was developed by multiple regression to predict diets of grazing sheep. This equation explained 52% of the variation in botanical composition of the diet. Visual orientation of individual sheep, while grazing, modified to a large extent their forage preferences.