Growth and Endocrine Responses of Lambs to Zeranol Implants: Effects of Preimplant Growth Rate and Breed of Sire

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Abstract
A 46-day 2 × 2 × 2 trial was conducted to evaluate the effects of preimplant growth rate, lamb sire breed (Finnsheep or Suffolk), and zeranol4 (12 mg) on growth, endocrine, carcass and histological traits. Comparison of Sullfolk-sired (Sx) and Finnsheep-sired (Fx) lambs indicated that Sx lambs grew at a more rapid rate from birth to slaughter and produced heavier carcasses with more muscle and less fat deposition than Fx lambs (P<.01). Sx lambs had heavier endocrine glands (adrenal, thyroid, and pituitary; P<.01) although only the pituitary was larger on an equal weight basis (P<.05). Fx lambs had increased lipid storage in the adrenal cortex (P<.01), while Sx lambs had larger pancreatic islets (P<.01). Within sire breed, lambs were classed as slow-growing (Sg) or fast-growing (Fg) based on gain from birth to the start of the trial at a mean age of 90 days. Preimplant growth rate differences were not maintained during the trial although Fg lambs had heavier final weights (P<.01). Fg lamb carcasses had more subcutaneous fat and a correspondingly lower cutability (P<.01). Zeranol implantation increased 46-day weight gains, although carcass traits were unaffected. Endocrine changes associated with implantation included increased (P<.05) pituitary gland weights and decreased (P<.05) serum thyroxine levels 42 days after implantation. Zeranol also increased (P<.01) thickness of the urethra lining and prostatic transformation. None of the two-way interactions was significant, indicating that the main effects were additive. Copyright © 1979. American Society of Animal Science . Copyright 1979 by American Society of Animal Science.