[en] To evaluate the efficacy of melatonin implants on increasing the production of lambs, a meta-analysis was performed based on the results of 139 experiments carried out in Spain and published in 56 scientific publications or conference proceedings. To assess the effects on fertility, the relative risk (RR) was calculated and, in the case of prolificacy and fecundity, the analysis was based on raw mean difference (D) and the standardized difference between means (d) and their corresponding confidence intervals. In addition, the experiments were stratified by the productive aptitude of the flocks, breed, time of year, and age. For fertility, the mean RR was 1.29, which indicated that the treatment with melatonin increased significantly (by 29%) the probability of pregnancy. For prolificacy, overall D was 0.08 (d = 0.9089); i.e., the groups treated with melatonin increased their litter size by 0.08 lambs/lambing relative to the control groups. Ultimately, treatment with melatonin increased fecundity significantly (0.25 extra lambs/ewe treated) (d = 6.3188), and this trend was evident in all of the stratified datasets. In all cases, there was significant (p &lt; 0.001) heterogeneity and bias. In conclusion, in Spain, melatonin implants had a significant effect on the probability that a ewe became pregnant, increased the number of lambs born per lambing, which ultimately increased the number of lambs born per treated ewe. In addition to confirm the results of previous descriptive reviews in the scientific literature, the metaanalysis made use of a statistical tool that synthesizes and estimates more precisely the effect of this hormone on sheep reproduction.

[es] Se ha realizado un meta-an&aacute;lisis sobre la eficacia del uso de implantes de melatonina en ovino en Espa&ntilde;a, analizando los resultados de 139 experiencias publicadas en 56 publicaciones o comunicaciones. Para la fertilidad se han calculado los riesgos relativos (RR) y para la prolificidad y fecundidad se han calculado la diferencia no estandarizada o bruta entre medias (D) y la diferencia estandarizada entre medias (d). Los resultados se han estratificado por aptitud productiva de los reba&ntilde;os, raza, &eacute;poca del a&ntilde;o y edad. Se han calculado los estad&iacute;sticos de heterogeneidad y el posible sesgo entre publicaciones. Para la fertilidad, el RR medio fue 1,29, lo que significa que la melatonina increment&oacute; de manera significativa las posibilidades de parto un 29%. Para la prolificidad, la D global fue 0,08 (d = 0,9089), es decir, los grupos tratados incrementaron su prolificidad 0,08 corderos/parto. El tratamiento con melatonina aument&oacute; de manera significativa la fecundidad, con 0,25 corderos extra producidos/oveja tratada (d = 6,3188). En todos los casos, la heterogeneidad y el sesgo calculados fueron significativos (p &lt; 0,001). En conclusi&oacute;n, el metaan&aacute;lisis realizado ha revelado un efecto significativo de la melatonina en Espa&ntilde;a sobre la posibilidad de que una oveja quede gestante, adem&aacute;s de elevar el n&uacute;mero de corderos nacidos por parto, lo que al final incrementa el n&uacute;mero de corderos nacidos por oveja tratada. Esto no ha hecho sino confirmar lo revisado previamente en la literatura de manera narrativa, mediante una herramienta estad&iacute;stica, que sintetiza y estima de manera m&aacute;s precisa el efecto de esta hormona.