Effects of induced hypothyroidism reproductive performance on weight gains, lactation, and of primiparous Brahman cows

Primiparous, spring-calving Brahman cows (BW = 425.0 +/- 13.8 kg, body cond ition score [BCS] = 5.0 +/- .2 units; SEM) were used to study the effects o f thyroid manipulation on weight gain, milk production, and reproduction. N ine cows served as controls, Nine cows were induced to become hypothyroid b y daily ingestion of 4 mg/kg BW of 6-n-propyl-2-thiouracil (PTU). Cows were stratified to treatment 1 d after calving based on season of birth, BW, BC S, calf sex, and calf sire; The treatment period lasted for 84 d and was fo llowed by a 56-d posttreatment period. Cow BW, BCS, and calf weight were re corded twice weekly. Milk production was estimated at 14, 28, 56, 84, 98, 1 12, and 140 d after calving. Weekly blood samples were obtained for analysi s of triiodothyronine (T-3), thyroxine (T-4), and progesterone (P-4) Estrus was monitored twice daily with the aid of a fertile bull equipped with a c hin ball marker. Hypothyroidism was effectively induced in all PTU cows dur ing the treatment period. The PTU cows gained more (P = .002) weight (54.6 +/- 7.6 kg) and tended (P = .06) to increase body condition (.61 +/- .17 un its) more than control cows (15.7 +/- 7.6 kg; .11 +/- .17 units) during the treatment period. Control calves gained at a faster rate (.85 +/- .04 kg/d ; P < .01) than PTU calves (.70 +/- .04 kg/d) during the treatment period. Milk production was lower (P < .05) in PTU cows on d 56 and 84. During post treatment all trends were reversed, and BW, ECS, calf weight, and milk prod uction were similar between the two groups by d 140. Reproductive performan ce was not affected by induction of hypothyroidism. In conclusion, inductio n of hypothyroidism was successful in increasing cow weight and BCS gains a nd suppressing milk production during the treatment period, but these chang es were not successful in improving reproductive performance-of primiparous Brahman cows.