| A series of studies were conducted to investigate the physiological responses of cattle to multiple stressors. These stressors included heat stress, water restriction, and intake of endophyte-infected tall fescue. Many of the symptoms of these stressors are similar including reduced feed intake, reduced growth rate, hyperthermia, and in extreme cases death. However little is known about the interaction between two stressors.;In the first experiment, Eight Angus steers were maintained for 5 days at thermoneutrality (TN; 19-21°C) in the Brody Environmental Center followed by 14 days of cyclic heat stress (HS; 26-36°C). Water was removed starting on Day 5 of heat stress (dehydration phase). After 3 days, water was returned starting the rehydration phase. Steers showed no lasting effects of dehydration, with the thermal status of the animal returning to normal after 48 hours of rehydration. Unexpectedly, core body temperature remained relatively unchanged despite dehydration, demonstrating their ability to adapt to changing conditions.;In the second experiment, we utilized both a short-term, controlled exposure and a longer-term field exposure to determine adaptive changes. Results from this experiment showed only a few signs of adaptation. With exception of feed intake, animals in the two groups that switched (E-/E+ and E+/E-) treatments responded to the current diet rather than previous exposure, suggesting no adaptation to the toxin. Feed intake was lower for all treatments during the final chamber run which could signify adaptation to heat stress. Sweat rate showed the greatest change between chamber tests, as well as within chamber runs with a reduction after several days in the heat.;In the third experiment, twenty-three Angus steers were subject to both a controlled heat challenge and a field exposure. However, unlike above, animals were placed on pasture prior to a chamber test to adapt to the summer conditions. Steers were given a controlled stress challenge during the peak of summer when they would have the most heat acclimation. In addition to studying adaption of animals to heat and fescue toxicosis, the objective of this experiment was compared the field and chamber exposures to determine how similar the responses under each situation in order to be able to transfer information from one source into another.;In the final experiment, animals were again subjected to both field and chamber exposures. It has been suggested that cattle are more sensitive to endophyte-infected fescue at the end of summer when they have potentially lost some of their heat acclimation. Therefore, this experiment was conducted at the end of summer as temperatures were cooling off. Surprisingly, T rum showed no differences between groups during TN or HS. This is likely due to a decline in feed intake and heat production associated with consumption of endophyte infected seed. While E+ animals did show a large response under TN conditions, their response to HS was similar to experiments 2 and 3 suggesting they are not more sensitive to endophyte-infected fescue at the end of summer.;Results of these experiments showed little evidence that repeated exposure to the endophytic toxins gives animals a tolerance to the endophytic toxins. Feed intake, rectal temperature, sweat rate and skin temperature responded similarly for E+ animals regardless of previous exposure suggesting a lack of adaptation. Sweat rate however, did show signs of adaptation in the E- animals being reduced between the start to the end of summer. Sweat rate also showed a decrease after several days in the heat. This reduction occurred even though rectal temperature and respiration rate were still elevated, suggesting that reduction of sweat rate, and possibly water loss, is more important than a reduction of body temperature during heat stress. (Abstract shortened by UMI.). |
