Response to reimplanting beef steers with estradiol benzoate and progesterone: performance, implant absorption...

This study determined the influence of the estrogenic ear implant Synovex-S on feedlot performance, tissue deposition, and thyroid status of growing-finishing beef steers implanted either once or reimplanted. The pattern of implant absorption was also determined. Two 112-d feeding trials were used with 48 Hereford steers per trial. Each trial was a randomized block design with eight groups (lots) of six steers each assigned to four treatments (two lots/treatment). Treatments were 1) no implant or control, 2) implanted on d 0 and reimplanted at 60 d on trial, 3) implanted at 30 d on trial, and 4) implanted on d 0 only. These implant treatments resulted in withdrawal periods before slaughter of approximately 60, 90 and 120 d for Treatments 2, 3, and 4, respectively. All steers were given ad libitum access to water and a 60% concentrate diet. Group intakes were determined daily, BW weekly, estimated body composition every 28 d, plasma thyroid hormone concentrations at 112 d and at slaughter, and carcass measurements and liver tissue deiodinase at slaughter. Approximately 25% of the original implant dose remained in the ear 60 d after implanting and this residual amount was absorbed linearly at the rate of approximately .15% of the original dose per day. Implant treatments increased (P less than .05) DMI, BW gain, feed conversion, and empty body gains for water and protein. Carcass measurements suggested a nonsignificant trend (P greater than .10) for leaner carcasses for implanted steers. An immediate shift toward greater protein and less fat deposition occurred within 28 d after initial implanting (Treatment 3).(ABSTRACT TRUNCATED AT 250 WORDS)