Serum Progesterone Levels in Mares in Winter and During Transitional Periods

Abstract

Weekly blood samples were collected for serum progesterone determination 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 mares 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 September and February, most frequently in December, and ended in January to June, most frequently in March. Mares with foal at foot became anoestrous in 2 separate groups, in September and in December. Anoestrus ended most frequently in February in foaled mares. The mares with foals differed significantly from the other mares both at onset (p = 0.0084) and 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 anoestrus. Since all mares with foals were kept in one stable and dry mares in 4 different stables, the effect of stable and possible differing lighting 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 = 0.001). The effects of breed and age were not significant in any of the parameters.

Sammanfattning

Progesteronkoncentrationen i blodserum uppmättes en gång per vecka under perioden september-juni hos 68 ston. Av samtliga ston inträdde 78 % (53/68) i anöstrus: 88 % (15/17) av stona som hade föl vid sιdan och 75 % (38/51) av icke lakterande (p = 0.323). Sex av 15 ston, som inte gιck in i vinteranöstrus, hade en persιsterande corpus luteum under i genomsnitt 77 dagar (variation 42–106 d). Anöstrus påbörjade under perioden september-februarι, oftast i december och upphörde mellanjanuari och junι, oftast i mars. Fölston inledde sin anöstrusperiod antingen i september eller i december. Anöstrus avslutades oftast i februarι hos ston som fölat. Gruppen fölston skilde sιg signifikant från övriga ston både beträffande inledning och avslutning av anöstrus (p = 0.0084 och 0.0218). Emellertιd visade sig effekten av stallet vara signifikant (p = 0.0386 respektιve 0.0170). Eftersom fölstona hölls i ett stall och övrιga ston var fördelade på 4 olika stall kunde effekten av stall och möjligtvis olika ljusförhållanden inte skiljas från effekten av föl vid sidan. Anöstrus varade ι medeltal 101 dagar hos digivande ston och 195 dagar hos ston slutat laktera. Hos fölston som inträtt i anöstrus i september-october varade anöstrus i genomsnitt 159 dagar och hos ston som inträtt i anöstrus ι december 62 dagar. Stonas ras och ålder hade ingen signifikant effekt på de analyserade parametrarna.