Effect of age and of reproductive status on reproductive indices in horse mares carrying mule pregnancies

Abstract

Mules (Equus mulus) are interspecific hybrids derived from crossing horse (Equus caballus) mares with jackasses (Equus asinus). In Italy, the production of mules is very important for the special environmental areas, where they are used as working animals, instead of the forbidden agricultural machines. Although many horse mares carry mule pregnancy successfully at term, low fertility of mares used for producing mules was reported, but limited data about reproductive efficiency in mares carrying mule pregnancy, especially when age and reproductive status are concerned, are available. Therefore, the present study aimed to assess the effect of age and of reproductive status on some reproductive indices of horse mares carrying mule pregnancy, in the final purpose to better clarify factors affecting reproductive performances in the valuable production of mules as working animals under special environmental areas in Italy. The study, performed on 96 draft mares inseminated with Martina Franca donkey semen, showed a satisfactory 89.6% 14 days PO pregnancy rate, lower pregnancy losses (19.8%) than those previously reported for mule pregnancies, leading to satisfactory foaling rates (71.9%), similar to those reported for intraspecific horse pregnancies. A clear, significant detrimental effect of mare's age on pregnancy losses, foaling rate, and on number of estrous cycles needed to achieve pregnancy, was found. Reproductive status also affected the reproductive indices, with significantly higher pregnancy losses and lower foaling rate in barren mares, reopening the question about the possible lower reproduction efficiency in horse mares carrying mule pregnancy. The present study results, although mostly descriptive, add useful information about some factors affecting reproductive performances in the valuable production of mules as working animals under special environmental areas in Italy.

Female age and parity in horses: how and why does it matter?

“…Similar observations on reduced fertility in maiden mares older than 8 have also been reported by others (Samper et al 2002). In terms of embryo development, while some did not find any effect of parity (Platt 1973;Chevalier-Clément 1989;Gibbs and Davison 1992;Vidament et al 1997;Morris and Allen 2002;Langlois and Blouin 2004;Hanlon et al 2012;Panzani et al 2016), others showed that mares that did not previously foal are more productive with lower embryo and fetal mortality compared to multiparous mares (Baker et al 1993;Brück et al 1993; Barbacini et al 1999;Hemberg et al 2004;Nath et al 2010;Hanlon et al 2012;Carluccio et al 2020).…”

“…In terms of age, studies with large animal samples mostly observed that old mares have longer gestations (Satué et al 2011;Langlois and Blouin 2012;Reilas et al 2014;Ewert et al 2018;Rodrigues et al 2020). The studies that indicate a similar gestation length have usually smaller sample numbers (Winter et al 2001(Winter et al , 2007Davies Morel et al 2002;Sevinga et al 2004;Heidler et al 2004;Aoki et al 2013;Bene et al 2014;Reilas et al 2014;Meirelles et al 2017;Malinska et al 2019;Popova 2020;Carluccio et al 2020;Milojevic et al 2020). Only three studies reported a shortening of gestation length with increasing maternal age (Bos and van der Mey 1980;Valera et al 2006;Kuhl et al 2015), but these study did not consider any very old mare nor had very high sample size of the same breed.…”

“…The age of oocyte donors, however, does not appear to affect blastocyst production (Altermatt et al 2009) nor gestation rate after oocyte transfer (Carnevale et al 2005). In addition, several studies did not observe any difference in mare fertility related to maternal age (McCue et al 2009;Uliani et al 2010;Avci et al 2016;Pérez-Marín et al 2017;Malheiros de Souza et al 2020;Carluccio et al 2020). These discrepancies on the effect of mare age on fertility could be due to the difficulty to define at what age a mare will experience reproductive aging.…”

Nulliparity affects the expression of a limited number of genes and pathways in Day 8 equine embryos

“…The few existing studies that consider maternal parity on fertility effects are controversial. While some found that parity did not affect fertility [21–27], others reported that mares that have never foaled have reduced embryo and fetal mortality compared to mares that previously foaled [27–33]. Confounding effects of maternal parity and age is probably the source of those discrepancies.…”