Body condition score of mares in relation to reproduction parameters and sex ratios
Drift, M.M.G. van der
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Thoroughbred breeders in New Zealand are under continuous pressure from the racing industry. For breeders it is important to produce early born colts, as buyers believe early colts are more likely to be able to race successfully as two year olds, when potential earnings are high. In order to do so, breeders believe that it is important to have mares on a rising plane of nutrition after foaling. The hypothesis of this research project is: mares with a high body condition score (BCS) will have (1) shorter gestation lengths, (2) a shorter interval from parturition till first ovulation, (3) will need fewer oestrous cycles till conception and (4) will produce more colts than fillies compared to mares with low BCS. We went to two stud farms in Palmerston North, New Zealand: Goodwood Stud and Wellfield Lodge. In total we used 69 mares. We measured their BCS at the moment of foaling and conception. We retrieved data about these mares from both stud farms and the New Zealand Thoroughbred Racing studbook, including the dates of service, dates of parturition and sex of the foals. These data gave us the possibility to test our hypothesis. We found no significant relationship between the BCS at time of conception in 2009 and the gestation lengths (n=24). On the other hand, we did find a significant relationship (P=0.001) between the moment of covering in the season and the gestation length. The gestation length shortens when length of the photoperiod increases at the moment of covering. We found a significant (P=0.003) linear relationship between BCS at time of conception and the interval from parturition to date of first service (n=45). We also found a significant (P=0.002) linear relationship between BCS at time of first service and the interval from parturition to date of first service (n=45). The result was contradictory to our hypothesis: a high BCS at first service of conception will lead to a prolonged interval between parturition to first service. We found no significant correlation between the BCS at time of conception and the interval between parturition and conception (n=54). Like the gestation length, this interval is under influence of photoperiod as well. We found a significant (P=0.001) relationship between the moment in season of conception and the length of the interval from parturition to conception (n=56). We found a significant (P=0.004) difference between the mean BCS of mares that gave birth to a colt (n=14) and the mean BCS of mares that gave birth to a filly (n=10). The mean BCS of mares that gave birth to a filly was 5.63 ± 0.29. The mean BCS of mares that gave birth to a colt was 6,84 ± 0,25. In order to produce colts, the mare should have a BCS between 6.3-7.3. In order to produce fillies, the mare should have a BCS between 5.0 and 6.2. We conclude that the ideal BCS of a mare at the moment of covering is around 6.5. This is the BCS at which mares will produce more colts than fillies. Breeders should not attempt to have their mares on a higher BCS at the moment of covering than 6.5-7, because the interval from parturition to first date of service will prolong when mares gain weight. Breeders should not attempt to have their mares on a BCS under 6.2 since their mares will then produce more fillies than colts. Then the intervals from parturition to first date of service will decrease, but this will only be a few days per point BCS the mare loses. It is more important to breed colts. In order to reach short gestation lengths, breeders should breed late in the season. In order to reduce the interval from parturition to last date of service, breeders should breed early in the season. The BCS has no significant influence on these variables.