This study identified factors that influence reproductive efficiency in the Thoroughbred.

SummaryReason for performing study: Aged mares with endometrosis suffer higher rates of pregnancy loss than young mares, due to poor placental development. Reduced uterine blood supply may be one contributory factor. Objectives: To measure uterine artery (UA) blood flow and other Doppler indices throughout pregnancy and compare placental and foal development in young mares and aged mares. Methods: Thoroughbred mares were grouped according to age and endometrial biopsy score: 1) 6 young mares (mean age 7.3 years, Category I); 2) 6 aged mares (mean age 18.3 years, Category II). Vascular pathology was nil or mild except in one aged mare with moderate perivasculitis. Both UA were scanned fortnightly throughout pregnancy. Total blood flow volume (BFV, ml/min/kg bwt), peak systolic velocity (PSV, cm/s) and resistance index (RI) were determined by pulsed wave, Doppler ultrasound and UA diameter using B-mode. Mixed-effects regression analyses were used to relate vascular parameters with different predictive variables, whilst accounting for the multiple repeated measurements taken from individual horses through the duration of their pregnancies. Results: PSV, RI and total BFV were best predicted by stage of pregnancy (P<0.001; r 2 >78%). The UA diameter was also associated with stage of pregnancy (P<0.001; r 2 = 87%) and was significantly greater in the gravid horn (P<0.001). There was a tendency for lower total BFV in older mares (P<0.05) and they delivered lighter foals than young mares (P<0.05). Gross placental morphometry was similar, but microscopic surface density of the microvilli was lower (P<0.02), in aged than young mares. Conclusions and potential relevance: Increased uterine blood flow and decreased vascular resistance reflect fetal growth and development of the placental microcirculation. Older mares have poorer placental microvillus development and lighter foals with reduced UA blood flow. Poor uterine blood flow may be an important contributory factor for pregnancy loss in aged mares.

The effects of feeding a diet restricted in energy on the endocrine mechanisms regulating LH secretion in prepubertal heifers were examined. On Day 0, thirty heifers were assigned to be either ovariectomized (OVX; n = 10), ovariectomized and administered an estradiol implant (OVXE; n = 10), or to remain ovary-intact (INT, n = 10). Five heifers each were then assigned to be fed either a control (C) or a two-phase, low-to-high energy (L), dietary treatment. The C diet was formulated to support weight gains of 1.1 kg/day throughout the experiment (Day 0-197). Heifers receiving the L treatment were provided a diet restricted in energy (33% of C diet) from Day 0-127 followed by a diet similar to that fed to heifers receiving the C treatment from Day 128-197. Secretion of LH increased rapidly following ovariectomy in C-OVX and L-OVX heifers. During the period of energy restriction, LH pulse frequency was reduced, and mean LH concentration, pulse amplitude, and pituitary response to LHRH were greater in L-OVX than in C-OVX heifers. In L-OVXE and L-INT heifers, secretion of LH was low and unchanged during the period of dietary energy restriction. In contemporaneous treatment groups fed the C diet (C-OVXE and C-INT), frequency of LH pulses increased gradually during this period and C-INT heifers attained puberty on Day 121 +/- 18. Upon switching heifers in the L-OVX, L-OVXE, and L-INT treatments to the C diet, all characteristics of LH secretion changed markedly within 14 days to levels similar to those detected in the C treatments.(ABSTRACT TRUNCATED AT 250 WORDS)

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