Estrogenic activity and growth stimulation in steers fed varying protein levels.

Sixty-four steer calves were individually fed in a 181-day experiment to determine their gain, feed efficiency, plasma urea-N and seminal vesicle response following implantation with 30 mg of either stilbene, estriol, estrone, 16-estradiol, c~-estradiol or diethylstilbestrot (DES). Half of the steers were fed a constant level and the remaining half were fed a declining level of supplemental protein providing 1.5, 1 and .5 times that of the constant level of supplemental protein. Gain and dry matter efficiency were stimulated by /3-estradiol (24 and 13%, respectively) and DES (12 and 9%, respectively), whereas the other compounds did not result in any stimulation. The plasma urea-N response of these steers paralleled the gain and efficiency response. Thus, it is concluded that marked estrogenic potency of compounds is required for gain and efficiency stimulation in cattle. On the other hand, the seminal vesicle gland was stimulated 62 and 33% as much by estriol and estrone, respectively, as observed with DES or ~-estradiol. Thus it appears that the seminal vesicle gland can be stimulated in size by compounds that do not elicit a gain response in steers. Performance of these steers was not altered by feeding a declining level compared to a constant level of supplemental protein. Plasma urea-N levels indicate that 7 to 8 rag/100 ml may be the minimum level compatible with maximum feedlot performance. (