Water restriction and heat effects on thyroid activity in ad libitium and force-fed cattle exposed to 180C and 320C

Seven non-lactating Holstein cows were maintained in a climatic laboratory for two weeks at 180C and 50% relative humidity, followed by two weeks at 320C and 50% relative humidity. Water was provided ad libitum in the first week and restricted 50% of the ad libitum level in the second week. Thyroid activity, determined by the 125I-triiodothyronine uptake with coated charcoal, decreased 10.9% (P<0.05) because of water restriction at 180C. Water restriction at 320C did not depress thyroid activity because environmental heat had already decreased thyroid activity to a low level. High ambient temperature depressed thyroid activity 14.7% in ad libitum cows (P<0.01) and a further 1.8% in water-restricted cows. Similar effects of water restriction and heat were obtained when the same experiment was repeated on four non-lactating rumen-fistulated cows, in which the refused feed during the water restriction periods was put into the rumen to maintain the feed levels for each temperature exposure. This indicated that forced feeding did not prevent the adverse effect of water restriction or heat on thyroid activity. (author)