Variables that influence the gestation length and parturition-conception interval on a New Zealand Thoroughbred studfarm
Rijssen, Andrea van
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There are many variables of maternal, paternal, foetal and environmental origin that influence the gestation length and the parturition-conception-interval (pci) of a mare. The aim of this study was to examine some of these variables in a commercial Thoroughbred stud farm in New Zealand. It is important to know the effect of these variables to assist the studmaster to work as efficiently as possible in the reproduction of the horses of his clients and improve the prediction of parturition day. The following parameters on the studfarm New Market Lodge (NML) were analyzed to detect their influence on the gestation length and on the pci: sex of the foal (colt vs. filly), stallion used, year of gestation (1992 until 2007), parity of the mare (1-18 parities), reproductive status of the mare (foaling vs. empty vs. maiden), residence of the mare (NML or not), mating month (August until January) and the mare herself. The breeding records of NML, a Thoroughbred stud farm in New Zealand, from the breeding seasons of 1993 until 2007 were used. The data consisted of 875 serving periods which resulted in 627 viable foals from 333 different mares. The stud farm was managed by the owner who was also the stud veterinarian. All the mares were served naturally by one of the five resident stallions. All the data were tested using a statistics program called Minitab®. The mean gestation length of the 627 viable foals was 351.97 ± 10.35 days (range 309 to 398days). The gestation length was significantly influenced by sex of the foal (colts 353.3days; fillies 351.4days), the reproductive status of the mare (foaling 349.3days; maiden 352.4days; empty 355.8days), the residence of the mare (NML 353.3days; other 350.4days), the mating month (September 357,0days; October 354,0days; November 350,1days; December 346.0days) and the mare herself. The mean of 406 pci’s of 237 mares was 32.0 ± 16 days (range 8 -117 days). The interval was significantly influenced by the mating month (the pci’s of October (30.7days) and November (30.2days) were significantly shorter than December (38.7days) and the mare herself. With the knowledge of the sex of the foal, (which can be determined by ultrasonography at about two months pregnancy) the reproductive status of the mare, the residence of the mare, the mating month and the previous gestation lengths of the mare, it is possible to make a better prediction of the expected parturition day. A good estimation of the parturition date is important because of the need for regrouping of the mares, who live outside during the whole gestation period, at the end of the gestation period on expected foaling date. The influences on the parturition to new fertilisation interval are limited to mating month and the mare herself. This study maybe helpful for a better prediction of parturition date and can be useful to plan an efficient reproduction strategy. Further research on more farms is necessary to assess whether the influences identified on this studfarm are consistent and repeatable.