Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 5, 1997
Publication Date: N/A
Interpretive Summary: The use of oilseeds in the diet is an important option for producers to increase the energy density of cattle diets in addition to supplying dietary protein. However, the literature suggests that certain dietary fats may not support performance at a level as great as that suggested by the estimated energy content of that fat. More specifically, oils and fatty acids have been shown in the rat and ruminant to increase metabolic rate suggesting an effect on the thyroid axis. This study investigated the effect in both nonimplanted and estrogen-implanted young beef steers of feeding a diet supplemented with roasted soybeans vs. soybean meal on performance and on the sensitivity of thyroid stimulating hormone (TSH) release following a challenge injection of thyrotropin releasing hormone+growth hormone releasing hormone. Implanting steers with an estrogenic growth promoter increased the release of TSH and the subsequent production of thyroxine by the thyroid. Roasted soybeans did not effect TSH but reduced plasma T3 concentration. These results suggest that part of the effect of oilseeds may be an effect on periferal T4 metabolism.
Technical Abstract: This study investigated the effect of feeding a roasted soybean supplemented diet to either nonimplanted or estrogen implanted young beef steers on thyroid status in response to thyrotropin releasing hormone (TRH)+growth hormone releasing hormone (GHRH) challenge injections. Twenty individually fed crossbred steers (initial BW 255 plus-minus 5 kg) were used in a 2 x 2 factorial of - or + dietary roasted soybeans (RSB) and - or + Synovex-S ear implant (SYN). After a 35-d growth period, steers were challenged (iv injection) with three levels of a combination of TRH+GHRH (.1+.01, 1.0+.1, 2.5+.25 ug/kg BW, respectively). After 3 wk, steers were reimplanted and a second 5-wk growth period was conducted followed by a single challenge of each steer of the 1.0+.1 TRH+GHRH dose. For Period 1, baseline thyroid stimulating hormone (TSH) concentrations were 36.7, 34.5, 60.8, and 32.5 ng/mL (SYN, P < .07; RSB, P < .01; SYN*RSB, P < .03; SEM=6.0) for -SYN,-RSB; -SYN,+RSB; +SYN,-RSB; +SYN,+RSB, respectively. Area under response curve for TSH was 66.4, 51.3, 91.4, and 64.4 (ng/ML)*min (SYN, P < .12; RSB, P < .08; SEM=12.0). Similar differences among treatments were obtained for TSH peak height and thyroxine (T4) differences among treatments reflected the effect of treatments on TSH. For triiodothyronine (T3), RSB depressed baseline concentrations (2.24, 1.91, 2.19, and 2.09 ng/mL; RSB, P < ..01; SYN*RSB, P < .19; SEM=.09). Similar treatment differences was seen in Period 2. The results are interpreted to indicate that SYN increases TSH release from the adenohypophysis and the primary effect of RSB is reduced plasma concentrations of T3 possibly through an effect on peripheral T4 deiodination.