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Growth and Endocrine Responses of Lambs to Zeranol Implants: Effects of Preimplant Growth Rate and Breed of Sire¹

The Pennsylvania State University, University Park 16802

Abstract

A 46-day 2 x 2 x 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.

¹ Approved by the Director of the Pennsylvania Agr. Exp. Sta. as Paper No. 5369 in the Journal Series. Technical assistance of J. L. Watkins and R. J. Kuzemchak is acknowledged.

² Department of Veterinary Science.

³ Department of Dairy and Animal Science.