Determinants of the annual pattern of reproduction in mature male Merino and Suffolk sheep: responses to a nutritional stimulus in the breeding and non-breeding seasons
Maria J. Hötzel A B , Stephen W. Walkden-Brown A C , James S. Fisher A D and Graeme B. Martin A E FA School of Animal Biology, The University of Western Australia, Crawley, WA 6008, Australia.
B Present address: Laboratório de Etologia Aplicada, Departamento de Zootecnia e Desenvolvimento Rural, Universidade Federal de Santa Catarina, Rodovia Admar Gonzaga, 1346, Itacorubi, Florianópolis, SC 88.034-001, Brazil.
C Present address: Animal Science, School of Rural Science and Agriculture, University of New England, Armidale, NSW 2351, Australia.
D Present address: Centre for Cropping Systems, Agriculture Western Australia, PO Box 483, Northam, WA 6401, Australia.
E CSIRO Division of Animal Production, Private Bag, PO Wembley, WA 6014, Australia.
F To whom correspondence should be addressed. email: gmartin@agric.uwa.edu.au
Reproduction, Fertility and Development 15(1) 1-9 https://doi.org/10.1071/RD02024
Submitted: 12 March 2002 Accepted: 28 November 2002 Published: 28 November 2002
Abstract
This study was designed to test whether an acute improvement in diet would increase gonadotrophin secretion and testicular growth in strongly photoperiod-responsive Suffolk rams and weakly photoperiod-responsive Merino rams in both the breeding (February–March) and the non-breeding (July–August) seasons. Mature rams (n = 5 or 6) of these breeds were fed a maintenance diet (0.9 kg chaff + 100 g lupin grain) or the same diet supplemented with 1.5 kg lupin grain for 42 days in each season. Lupin grain is a rich source of both energy and protein. Testosterone, luteinizing hormone (LH) and follicle stimulating hormone (FSH) were measured in plasma from blood sampled every 20 min for 24 h on Days −1, 12 and 35 relative to the change in feeding. In rams supplemented with lupins, body mass increased in both breeds in both seasons (P < 0.001). Scrotal circumference and LH pulse frequency increased with lupin supplementation in both seasons (P < 0.003) in Merinos, but only during the breeding season (P < 0.003) in Suffolks. Plasma FSH concentrations were affected by diet only during the breeding season, being elevated on Day 12 in lupin-supplemented rams of both breeds (P < 0.05). It was concluded that Merino rams exhibit reproductive responses to improved nutrition irrespective of time of the year, whereas Suffolk rams respond to nutrition only when the hypothalamic reproductive centres are not inhibited by photoperiod. Thus, Suffolks do respond to nutrition, just as Merinos do, but only when photoperiod allows. This difference between breeds appears to be a result of differences in the neuroendocrine pathways that control pulsatile gonadotrophin-releasing hormone secretion.
Extra keywords: breed
Acknowledgments
This study could not have been contemplated without the generous assistance of the students and staff of the Animal Science Group (University of Western Australia), particularly Mrs M. A. Blackberry and Mr L. Cranfield. We are also grateful to the National Institute of Diabetes, Digestive & Kidney Disease and Dr A. F. Parlow (Pituitary Hormones and Antisera Center, Harbour-UCLA Medical Center, 1000 Carson Street, Torrance, CA, USA) for the reagents used for the gonadotrophin assays. We acknowledge Dr R. I. Cox (CSIRO Division of Animal Production, Prospect, NSW, Australia) for the donation of testosterone-3-CMO-HSA. This work was funded by the Australian Research Council; the CSIRO Division of Animal Production; a scholarship from The Australian Wool Corporation (J. S. Fisher); the EHB Lefroy Fellowship (S. W. Walkden-Brown); and the Brazilian Research Council (CNPq) (M. J. Hötzel).
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