Gestation lengths of Finnhorse and Standardbred mares in Finland: effects of breeding season and reproductive status

The aim of this study was to investigate factors influencing the gestation length (GL) in Finnhorses (FH) and in American Standardbreds (SB), with particular emphasis on the effect of the mare's reproductive status. Data including 15 breeding seasons (1991-2005) was obtained from Suomen Hippos, the Finnish trotting and breeding association. From the records, 8952 pregnancies in 3649 SB mares and 4579 pregnancies in 2149 FH mares fulfilled the inclusion criteria: artificial insemination (Al) using either fresh or transported semen, a viable foal, and a GL of 300-390 days (last Al-parturition). The effects of breed, mare age and reproductive status (maiden, barren, rested or foaled) at the beginning of the mating season, parity, month and year of mating, stallion station, and sex of the foal on GL were estimated using linear mixed models. All models were fitted using the residual maximum likelihood estimation method. The GL in foaled mares was significantly shorter than in maiden, barren or rested mares (SB: P<0.0001; FH: P<0.01). In maiden, barren and rested mares, the GLs were similar. The mating month had a significant effect on the GL (P<0.0001). The longest and shortest gestations were found in mares mated in April and August, respectively. The SB mares mated in February-March had shorter pregnancies than mares mated in April (P<0.01). The FH mares carried their foals significantly longer (adjusted mean 334.8 days, 95% Cl 334.2-335.5) than SB mares (adjusted mean 331.7 days, 95% CI 331.2-332.3 days) (P<0.0001). In both breeds, primiparous mares had longer gestations than multiparous mares (P<0.0001). The GL between different age groups did not differ in the SB, but young FH mares had 2.3 days shorter gestation than very old mares (P<0.05). Colt foals were carried 1.8 days longer than fillies in both breeds (P<0.0001). In conclusion, the relationship between mating date and gestation length in Standardbred mares was positive from March to April and negative from April to August. A short mean GL of the SB mares in Finland supports the influence of latitude on GL. Reproductive status of the mare at the time of Al had a significant effect on the GL. This finding suggests that the uterine environment in early pregnancy may affect the length of gestation.