Reproductive performance measures among Thoroughbred mares in central Kentucky, during the 2004 mating season.

REASON FOR PERFORMING STUDY: To improve efficiency at the farm level, a better understanding of how farm management factors impact reproductive performance is important. OBJECTIVE: To assess reproductive efficiency and effectiveness among Thoroughbred mares in central Kentucky. METHODS: A cohort of 1011 mares on 13 farms in central Kentucky was followed during the 2004 mating and 2005 foaling season. Information on farm level practices was collected via interviews with farm managers. Reproductive records were collected for each mare mated to obtain information on mare characteristics. The influence of mare age and status (maiden, foaling, barren) on Days 15 and 40 post mating pregnancy rates, foaling rates and total effective length of the mating season were assessed. The influence of stallion book size on reproductive performance measures was also examined. RESULTS: Per season pregnancy rates on Days 15 and 40 post mating and live foal rate were 92.1, 89.3 and 783%, respectively. Per cycle rates for the same time periods were 64.0, 583 and 50.8%. There were no significant associations between stallion book size and reproductive performance outcomes. The mean +/- s.d. interval from the beginning of the mating season to the last mating of the mare was 36.5 +/- 26.1 days. CONCLUSIONS: Mare age had a significant impact on efficiency of becoming pregnant, maintaining pregnancy and producing a live foal. Overall, fertility did not decrease among stallions with the largest book sizes. Total interval length of the mating season can be reduced if managers ensure maiden and barren mares are mated at the beginning of the season and foaling mares are mated at the earliest oestrus after acceptable uterine involution has been achieved. POTENTIAL RELEVANCE: Measures identified in the study can be used by owners, farm managers and veterinarians to improve mare reproductive performance and identify parameters to assist with the implementation of effective culling practices.

Impact of reproductive efficiency over time and mare financial value on economic returns among Thoroughbred mares in central Kentucky.

REASON FOR PERFORMING STUDY: There have been no studies reporting the impact of reproductive efficiency and mare financial value on economic returns. OBJECTIVE: To explore the economic consequences of differences in reproductive efficiency over time in the Thoroughbred mare. METHODS: Complete production records for 1176 mares were obtained. Production history and drift in foaling date were calculated. Multiple logistic regression was used to identify factors influencing the probability of producing a registered foal in 2005. The 'net present value' and 'internal rate of return' were calculated for economic scenarios involving different initial mare financial values, levels of reproductive efficiency, and durations of investment. RESULTS: Among mares that did not produce a foal every year (63%), the mean time before failing to produce a registered foal was 3.4 years. The majority of mares drifted later in their foaling dates in subsequent foaling seasons. Increasing mare age, foaling after 1st April, needing to be mated multiple times during the season, and producing a lower number of foals in continuous sequence during previous years decreased the probability of producing a registered foal. Over a 7 year investment period, live foals must be produced in all but one year to yield a positive financial return. Profitability was highest among mares of greatest financial value. CONCLUSIONS: Mares are long-term investments due to the extended period before there is a return on the investment. Improving our understanding of mare, stallion and management factors that affect the likelihood of producing a live foal are critical to ensuring a positive financial return. Additional work is needed to test the robustness of the study's conclusions when the cost and revenue assumptions are varied. POTENTIAL RELEVANCE: This information can assist in assessing mare profitability and developing management strategies to maximise profitability.

Three autosomal chromosome translocations associated with repeated early embryonic loss (REEL) in the domestic horse (Equus caballus).

Repeated early embryonic loss (REEL) represents a considerable economic loss to the horse industry. Mares that experience REEL may be overlooked as potential carriers of a chromosome abnormality. Here we report three different autosomal translocations in Thoroughbred mares presented for chromosome analysis because of REEL. The karyotypes were 64,XX,t(1;21), 64,XX,t(16;22), and 64,XX,t(4;13), respectively. In order to confirm the chromosomes involved in the translocations, to map the breakpoints, and to determine if the translocations were reciprocal, genes surrounding the breakpoints were identified using existing maps and from the newly assembled horse genome sequence. Bacterial artificial chromosomes containing the genes of interest were identified and mapped to the translocation chromosomes by fluorescence in situ hybridization (FISH). FISH confirmed that the t(16;22) and t(4;13) translocations were reciprocal, while the t(1;21) was not. The breakpoints on horse chromosomes 1 and 16 appear to be the same or near breakpoints previously identified in translocations. These breakpoints are at the fusion boundary of human chromosomes 10 and 15 on horse chromosome 1 and at human chromosome 3p and 3q on horse chromosome 16. These sites may represent ancient breakpoints reused during equid evolution. Overall, chromosome abnormalities may have a greater influence on mare fertility than previously known. Thus, it is important to karyotype subfertile mares exhibiting REEL.

Hormone profiles of mares affected by the mare reproductive loss syndrome.

While searching for the cause of the Mare Reproductive Loss syndrome (MRLS), we postulated that 1 of 3 tissues in 40-120 D pregnant mares was the likely primary target of the noxious factor that caused early abortions: The corpora lutea (CL), the endometrium or the fetus and/or its membranes. At this stage of gestation, progesterone (P4) is solely produced by luteal tissue, eCG by endometrial cups in the endometrium and oestrogens by the feto-placental unit. We determined whether concentrations of P4, eCG and/or total conjugated oestrogens (CE) would indicate which tissue was targeted during the MRLS. P4, eCG and CE were measured in single serum samples collected from 216 mares, 60-110 D after ovulation during the 2001 MRLS outbreak. All mares had previously been confirmed pregnant by ultrasonography. The following data was obtained from each mare: Interval from ovulation, pregnancy status and normalcy of fetal fluids at the time of sampling, and pregnancy status 3 weeks after sampling and at term. There were no meaningful differences in hormone concentrations between pregnant mares that had normal and excessively echogenic fetal fluids at the time of sampling. CE were lower (p < 0.05) in mares that aborted after sample collection than in mares the carried to term. In 8 mares from which multiple samples were obtained, CE consistently decreased prior to any decreases in P4 or eCG. Arguments are presented that lead to the hypothesis that the fetal trophoblast was the primary target of the MRLS agent.

Ultrasonography as an adjunct to pregnancy assessments in the mare.

Ultrasonographic detection of pregnancy was performed in 952 Thoroughbred mares. Characteristic images were obtained between 12 and 55 days after breeding. Twins in the same uterine horn as well as twins in opposite horns were imaged, and for the first few months of gestation appeared similar to single pregnancies. Growth curves as a function of time were developed, utilizing horizontal diameters of the blastocysts versus breeding dates. Comparisons between horizontal and vertical diameters demonstrated no appreciable differences when observed as a group. Growth curves derived from twins did not differ significantly from those for single pregnancies during the period we observed them. Ultrasonographic evaluations were valuable in substantiating mechanical destruction of one of the twin blastocysts immediately following the maneuver. Uterine artifacts such as endometrial and lymphatic cysts also were visualized and portrayed. Although these structures represented a source of false-positive results, they usually could be properly identified on the basis of their morphometric and growth characteristics.