SERUM PROGESTERONE LEVELS IN MARES IN WINTER AND DURING TRANSITIONAL PERIODS

Weekly blood samples were collected for serum progesterone determinati on from 68 mares during the period from September 1990 to June 1991. A total of 78% (53/68) of the mares became anoestrous: 88% (15/17) of m ares with foals and 75% (38/51) of dry mares (p = 0.323). Six mares of the 15 which continued cycling showed persistent corpus luteum for 77 days on average (range 42-106 days). Anoestrus began between Septembe r and February, most frequently in December, and ended in January to J une, most frequently in March. Mares with foal at foot became anoestro us in 2 separate groups, in September and in December. Anoestrus ended most frequently in February in foaled mares. The mares with foals dif fered significantly from the other mares both at onset (p = 0.0084) an d end (p = 0.0218) of anoestrus. However, the effect of the stable was also significant at onset (p = 0.0386) and end (p = 0.0170) of anoest rus. Since all mares with foals were kept in one stable and dry mares in 4 different stables, the effect of stable and possible differing li ghting conditions cannot be separated from the foal effect. The length of anoestrus was on average 101 days for lactating mares and 95 days for dry mares (p = 0.6691). In foaled mares its length was on average 159 days in mares which had entered anoestrus as early as in September -October and 62 days in mares entering anoestrus in December (p = 0.00 1). The effects of breed and age were not significant in any of the pa rameters.