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  • Title: Seasonal variation in live weight, testes size, testosterone, LH secretion, melatonin and thyroxine in Merino and Corriedale rams in a subtropical climate.
    Author: Pérez Clariget R, Forsberg M, Rodriguez-Martinez H.
    Journal: Acta Vet Scand; 1998; 39(1):35-47. PubMed ID: 9592944.
    Abstract:
    In the present investigation we studied the seasonal changes in live weight and testes and pituitary activity in Merino and Corriedale rams in a subtropical climate. Testes activity was measured as scrotal circumference (SC), plasma concentration of testosterone (T) and release of testosterone after exogenous GnRH injection. LH pulsatility and pituitary LH responsiveness to exogenous GnRH was measured as an index of pituitary activity. In addition, we wanted to characterize the seasonal pattern of thyroxine (T4) secretion and the 24 h secretory pattern of melatonin (M) at the winter and summer solstices in the 2 breeds. Nine Corriedale and 7 Merino adult (4-6 years) rams were kept on native pasture and managed in one group. Twice a month live weight (LW) and scrotal circumference (SC) were measured. To monitor plasma concentration of testosterone (T), and thyroxine (T4), 5 animals of each breed were bled every month except during autumn (March-May), when blood samples were collected with 15 day intervals and in spring (October) with 10 day intervals. To monitor pulsatile LH secretion, 3 rams of each breed were bled at 15 min intervals for 6 h at the winter and summer solstices and spring and autumn equinoxes. Pituitary LH and testicular testosterone response to GnRH injection was performed bimonthly from 2 animals of each breed. No effect of breed was found on any of the variables investigated. An interaction between breed and sampling date was found in LW (p < 0.001) and total T response after GnRH challenge (p < 0.001). Sampling date had a significant effect (p < 0.001) on all the variables studied. In both breeds SC decreased during autumn and increased during spring with minimum T concentrations in late autumn and maximum in mid-summer/early autumn. The lowest (p < 0.05) number of LH pulses were observed in winter (June) and the highest (p < 0.05) in early autumn (March). The highest LH and testosterone response to GnRH challenge was observed in autumn (April) (p < 0.05) in both breeds. Baseline concentrations of M were similar in both breeds at the winter and summer solstices and high concentrations were observed during the dark period on both occasions. In both breeds thyroxine was high at the end of winter/spring (February-April) and low from the end of summer to mid-autumn (August-November). The results show that Corriedale and Merino rams under subtropical conditions have annual reproductive cycles. There was an association (p < 0.001) between changes in LW and SC in the 2 breeds and this relationship was stronger in Merino rams (R2 = 0.68) than in Corriedale (R2 = 0.33) which indicates that nutritional factors may have more influence on the reproductive cycle of Merino than Corriedale rams. En undersökning genomfördes för att studera de säsongmässiga förändringarna i levande vikt och endokrin aktivitet i testiklar och hypofys hos Merino-(n = 7) och Corriedale- (n = 9) baggar under subtropiska förhållanden. Som ett mått på testikelaktiviteten studerades testikelomfång, plasma koncentrationen av testosteron och testosteronutsöndring efter injektion av GnRH. LH pulsatilitet och plasmakoncentration av LH efter injektion av GnRH användes som ett mått på hypofysaktiviteten. Dessutom mättes de säsongsmässiga förändringarna i tyroxin och dygnsförändringarna i plasmakoncentrationen av melatonin vid vinter- och sommarsolstånden. Den statistiska analysen visade inga rasskillnader mellan Merino och Corriedale vad avser de parametrar som studerades. Det fanns en interaktion mellan ras och provtagningstidpunkt i levande vikt (p<0.001) och testosteronsekretion efter GnRH injektion. Hos båda raserna minskade testikelomfånget under hösten och Ökade under våren (p<0.05). De lägsta testosteronnivåerna uppmättes under sen höst och de högsta under sommar/tidig höst (p<0.05). LH pusatiliteten var lägst under vintern och högst under tidig höst (p<0.05). Testosteron och LH sekretionen efter GnRH injektion var högst under hösten hos båda raserna (p<0.05). De högsta koncentrationerna av melatonin registrerades under den mörka delen av dygnet hos både Merino och Corriedale både vid vinteroch sommarsolståndet. Tyroxin var högst hos båda raserna vid slutet av vintern/början av våren och lågt under resten av året. Resultaten visat att såväl Merino- som Corriedalebaggar har en säsongsmässig fortplantningscykel under subtropiska förhållanden. Det fanns ett starkare samband mellan förändringarna i levande vikt och testikelomfang (p<0.001) hos Merino (R2 = 0.68) än hos Corriedale (R2 = 0.33), vilket kan tyda på att nutritionella faktorer kan ha ett större inflytande på fortplantningscykeln hos Merino än Corriedale.
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