Sexual behaviour and fertility in Icelandhorse herds

Abstract

During recent years horsebreeders in Iceland observed decreasing fertility of their mares. In order to gain knowledge about factors influencing fertility of Icelandhorses on pasture several aspects were monitored. In the years 1996-1998 close to 100 breeding herds comprising over 2000 mares (from 9 to 39 per herd) with one stallion per herd (age from 2 to 26 years) were monitored repeatedly for 8-9 weeks, starting at least 50 days after the stallion joined the mares. Monitoring comprised weekly serum sampling and determination of pregnancy by means of measuring equine choriogonadotropin (eCG) levels by enzyme linked immunosorbent assay (ELISA). An overall pregnancy rate of 76% was found provided the stallion was allowed to serve the mares for at least 10 weeks. Two other herds were monitored more closely by observing them for one and two heat periods continuously for 24h per day (made possible by sufficiently lucid Icelandic summer nights) by placing a camper in the middle of the pasture and by unequivocal marking of each horse. Some of the mares received clinical, gynaecological, and ultrasonic investigation in order to determine precisely the beginning of their heat and their date of ovulation. Herd A comprised 23 fertile mares plus an 9 years old stallion. Herd B was made up of 10 fertile cycling mares plus 3 years old stallion. These herds received continuous observation for 42 and 28 days, respectively. The stallions served each mare of their herd with 6.0 and 5.7 mounting per heat which lasted in average 4.4 and 4.7 days, respectively. The result of their matings were determined by the subsequent foaling rates of 87% and 90% respectively. These foaling rates were clearly above the average pregnancy rate determined in the overall eCG-study. Of major influence on the remarkably different mating success (between 36% and 96%) among the herds studied was the mare to stallion ratio per heat period. The age of the stallion and a composition of the mares in the herds is of great importance too. In order to obtain optimal foaling rates with Icelandhorses on pasture these results suggest herds of not more than 15 fertile cycling mares per stallion for one heat cycle of preferably, a maximum of 20 fertile cycling mares per stallion staying in the herd for 6 weeks (two heat cycles) at least.