Conference Paper

Investigacion Australiana en reproducción de caprinos.

Conference: XIV Reunión Nacional de Caprinocultura, At Programa de Ganadería, Colegio de Postgraduados, Montecillo, Mexico

ABSTRACT Entonces, la expresión final de la actividad reproductiva en animales adaptados a ambientes no templados depende de la interacción entre de estos tres factores ambientales más importantes (fotoperíodo, factores sociales, nutricion), y por lo menos de dos mecanismos de control endócrino, uno que parece ser esencialmente metabólico más que reproductivo. Esto no es realmente sorprendente, considerando el rol crítico que el aporte de nutrientes o reservas juegan en determinar el éxito de la estrategia reproductiva.
Estamos ahora en una posición muy fuerte para empezar a trabajar nuevas estrategias que puedan sacar provecho de toda esta información. Esto nos debe llevar a nueva sistemas para el manejo de la reproducción, particularmente para rebaños manejados extensivamente.

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    ABSTRACT: The effects of season, diet and exposure to oestrous females on LH and testosterone secretion were examined in mature cashmere bucks to determine whether there is a seasonal cycle of LH and testosterone secretion, and whether this can be modulated by long-term differential nutrition and exposure to oestrous females. Three-year-old bucks were individually housed under natural photoperiod at 29 degrees S 153 degrees E and fed diets of high (crude protein 17.6%, metabolizable energy 8.3 MJ kg-1) or low (crude protein 6.9%, metabolizable energy 6.6% MJ kg-1) quality for 16 months ad libitum (n = 6 per treatment). Blood samples were collected to determine pulsatile LH and testosterone secretion immediately before experimental feeding, one month later, and every second month thereafter. Samples were collected for an 8 h period on successive days with the bucks isolated on the first day and each exposed to a single oestrous doe for the duration of the second day. In the absence of oestrous females, bucks exhibited a circannual pattern of secretion for both hormones with pulse frequency and mean concentrations highest in late summer and autumn and lowest in late winter and spring. Testosterone pulse amplitude followed a similar pattern, but LH pulse amplitude was highest in spring and lowest in autumn, indicating a seasonal shift in the relationship between the two hormones. Exposure to oestrous does increased LH and testosterone secretion depending on both season and diet. Responses were evident during summer, autumn and early winter, with bucks on a high quality diet exhibiting an earlier and more prolonged period of responsiveness than did bucks on a low quality diet, peaking in February compared with June. The magnitude of the LH and testosterone response was also significantly greater in bucks on a high quality diet. Weight loss during autumn appeared to reduce responsiveness in both treatments. These results demonstrate that there is a seasonal cycle in LH and testosterone secretion in mature cashmere bucks, and that nutrition and oestrous females are powerful modulators of the secretion of these hormones in a seasonally dependent way.
    J Reprod Fertil 04/1994; 100(2):521-31. DOI:10.1530/jrf.0.1000521
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    ABSTRACT: Australian cashmere goat bucks exhibit marked seasonal changes in gonadotrophin secretion and growth. We investigated the mechanisms underlying these changes by testing the effects of castration and gonadal steroid replacement in grazing bucks at 29˚S, 153˚E over a period of 2 years. Three year old bucks were either left intact (n=6), castrated (n=6), or castrated and treated with testosterone propionate (n=5) or oestradiol-17b (n=5). Oestradiol-17b (E2) was provided by 2 subcutaneous Silastic® implants while testosterone propionate (TP) was provided by implanting four 23.5 mg pellets subcutaneously every 12 weeks. Intact bucks exhibited marked circannual cycles of plasma LH, FSH, prolactin and testosterone concentration, body mass, paired testicular mass and male odour. Castration resulted in long term increases in LH and FSH concentration and abolition of the circannual pattern of change in these hormones without affecting prolactin concentrations. It also reduced the magnitude of seasonal changes in body mass. Treatment of castrates with TP, which produced peripheral concentrations of testosterone in the lower physiological range (mean 1.2±0.08 mg L-1), had no effect on plasma concentrations of LH, FSH or prolactin. On the other hand treatment with E2, which produced E2 concentrations in the mid physiological range (mean 12.3±1.08 ng L-1), re-imposed a circannual pattern in plasma LH and FSH, albeit significantly different from that of intact bucks. It had no effect on prolactin concentrations. Treatment with both steroids resulted in growth cycles of intermediate magnitude between those of intact bucks and castrates. We conclude that the circannual cycle of gonadotrophin concentrations in goat bucks is dependent upon the presence of the testis, and suggest that circulating metabolites of testosterone, including oestradiol, are more important than testosterone itself in the maintenance of this cycle. We also suggest that the seasonal growth cycle observed in these bucks is driven by factors other than prevailing nutrition, probably photoperiod, and is mediated by both steroid-dependent and independent mechanisms.
    Perspectives in Comparative Endocrinology, Edited by Davey K.G., Peter R.E., Tobe S.S, 01/1994: pages 574-585.; National Research Council of Canada, Ottawa.
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    ABSTRACT: Changes in the nutrition of mature rams and goat bucks lead to profound responses in testicular size and therefore the rate of production of spermatozoa. These effects are largely due to changes in the size of the seminiferous tubules and in the efficiency of spermatogenesis. With the exception of severe undernutrition, the effects on spermatogenic function are not accompanied by similar changes in endocrine function of the testes, as measured by the production of testosterone or inhibin. In rams, moderate changes in nutrition affect gonadotrophin secretion for only a few weeks, whereas testicular growth is affected for several months. In mature male goats during the non-breeding season, nutrition-induced testicular growth does not seem to be associated with a gonadotrophin response. Such observations have led us to develop the hypothesis that nutrition-driven testicular growth is at least partly independent of changes in gonadotrophin secretion. The energetic components of the diet, rather than the protein content, seem to be responsible for affecting gonadotrophin secretion in rams. The volatile fatty acids, and not glucose, are the active factors, although intracerebral insulin may also play a role. Where these substrates act and whether they are also involved in the gonadotrophin-independent pathways requires testing. In conclusion, nutritional signals exert powerful effects on the reproductive system of mature male ruminants, and the responses are partly independent of changes in gonadotrophin secretion. In the gonads, the gametogenic tissue responds rapidly to changes in nutrition, but the endocrine compartments are less affected. Variations in the expression of the nutritional responses among sexes, breeds and species probably reflect variations in the role of this environmental factor as a modulator of reproductive function.
    Journal of reproduction and fertility. Supplement 02/1995; 49:437-49.

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