Simple SummaryThe mechanism which ensures a mare recognises the presence of a pregnancy and receives an embryonic signal to prevent her returning to oestrus is known as maternal recognition of pregnancy (MRP) and occurs approximately 12 days after ovulation. We do not fully understand the MRP signal in the mare or the mechanisms involved in how it prevents lysis of the corpus luteum, the progesterone secreting structure on the maternal ovaries. This experiment aimed to determine if the timing of the MRP signal was specific to individual mares by examining when luteostasis (maintenance) of the corpus luteum reliably occurred in individuals following embryo reduction. Singleton (n = 150) and synchronous twin pregnancies (n = 9) were reduced in 10 individuals (5–29 reductions/mare) at pre-determined time points within days 10–14 of pregnancy. Individual mares showed a significant variation in when they consistently entered a period of luteostasis following embryo reduction (272–344 h post-ovulation). The presence of twins, the size of the embryonic vesicle, the interovulatory period and mare age did not factor into the timing of MRP in the individuals. The factors and mechanisms underlying the individuality in the timing of MRP were not determined and warrant further study.AbstractThe present experiment aimed at determining whether the timing of the maternal recognition of pregnancy (MRP) was specific to individual mares by determining when luteostasis, a failure to return to oestrus, reliably occurred in individuals following embryo reduction. Singleton (n = 150) and synchronous twin pregnancies (n = 9) were reduced in 10 individuals (5–29 reductions/mare) at pre-determined time points within days 10 (n = 20), 11 (n = 65), 12 (n = 47), 13 (n = 12) or 14 (n = 15) of pregnancy. Prior to embryo reduction, the vesicle diameter was measured in 71% (106/150) of the singleton pregnancies. The interovulatory interval (IOI) was recorded on 78 occasions in seven of the mares in either non-pregnant cycles (n = 37) or those in which luteolysis followed embryo reduction (n = 41). The earliest time post-ovulation at which the embryo reduction resulted in luteostasis in an individual was 252 h (mid-Day 10). Consistency in luteostasis following embryo reduction showed individual variation between mares (272–344 h). Binary logistic regression analysis showed an individual mare effect (p< 0.001) and an effect of the interval post-ovulation at which embryo reduction was undertaken (p< 0.001). However, there was no significant effect of vesicle diameter at the time of embryo reduction (p= 0.099), nor a singleton or twin pregnancy (p= 0.993), on the dependent of luteolysis or luteostasis. The median IOI between individual mares varied significantly (p< 0.05) but was not correlated to the timing of MRP. The timing of MRP varied between the mares but was repeatable in each individual. The factors and mechanisms underlying the individuality in the timing of MRP were not determined and warrant further study.