Nonpharmacological suppression of oestrus in the mare.

Abstract: The maintenance of luteal function is essential for the establishment of pregnancy in all mammals. While the lifespan of the corpus luteum (CL) is independent of the presence of a conceptus in carnivores (Lawson and Findlay 1977), prolongation of luteal function in polyoestrous animals requires the presence of the conceptus to defuse and suppress the cyclical luteolytic mechanism in the nonpregnant state (Short 1969). In the horse, the conceptus remains spherical and unattached (Van Niekerk and Allen 1975) after it enters the uterus between 144 and 168 h after ovulation (Battut et al. 1997) and it migrates continually throughout the entire uterine lumen for the next 10 days (Ginther 1983a; Leith and Ginther 1984), liberating its antiluteolytic signal onto the endometrium (McDowell et al. 1985; Sharp et al. 1989; Stout et al. 2000). On or around Day 16 after ovulation, the conceptus becomes ‘fixed’ at the base of one uterine horn by a marked increase in uterine tonicity (van Niekerk 1965; Ginther 1983b). By this time, the cyclical luteolytic mechanism has been suppressed (Hershman and Douglas 1979) and the CL persists in the ovaries and continues to secrete progesterone for at least the next 100 days of gestation (Squires et al. 1982). Spontaneous prolongation of luteal function occurs commonly in nonpregnant mares during the breeding season (Stabenfeldt et al. 1974), due either to the occurrence of a late dioestrous ovulation so that the new CL is nonresponsive to the Day 14–16 cyclical release of PGF2
(Vernon et al. 1979), or to embryonic death occurring soon after completion of the ‘maternal recognition of pregnancy’ process (Ginther 1992; Niswender and Nett 1993). Experimental removal of conceptuses from the uterus after Day 16 post ovulation has been shown to result in prolongation of the interovulatory interval, maintenance of uterine tonicity and a continuation of progesterone secretion (Ginther 1992), and hysterectomy of pregnant mares on Day 24 of gestation was found to prolong the lifespan of the CL for an average of 30 days (Kooistra and Ginther 1976). Excessive display of oestrous behaviour by some fillies while in training for racing or other competitive disciplines can seriously disrupt their training and competition schedules to the point that the affected animals must be retired to stud or other duties. Although, in theory, it is possible to suppress such behaviour by the administration of one of a number of available progestagens (e.g allyl trenbolone; Regumate1) such treatment is expensive and is banned as a ‘nonnormal nutrient’ by administrative authorities. Insertion of a 35 mm glass ball into the uterine lumen within 24 h after ovulation was reported to prolong luteal function in the mare, but the therapy was efficacious in only 40% of mares treated (Nie et al. 2001). Hence, there remains a need for a nonpharmacological method of suppressing unwanted oestrous behaviour in the competing filly. This study was undertaken to determine whether the natural prolongation of luteal function achieved by the luteostatic functions of the conceptus might be exploited for this purpose.