Relationships between stallion age, book size, number of matings (covers), breeding soundness examination findings, and fertility parameters in 15 Thoroughbred stallions (34 stallion years)

Abstract

Pre-season breeding soundness examinations were performed on 15 stallions at one breeding farm in central Kentucky during the years 2005–2007 (34 stallion years). Pregnancy rates per cycle, seasonal pregnancy rates, and pregnancy loss rates were determined from breeding and foaling reports. Spearman rank order correlations or Pearson’s product moment correlations between semen quality in second ejaculates and fertility parameters were calculated, as well as between fertility parameters and stallion age, stud fee, testicular size, mare age, book size, and number of covers required to service the mare book. Stallion age was negatively correlated to both pregnancy rate per cycle (r = -0.462; P < 0.01) and seasonal pregnancy rate (r = -0.412; P < 0.02). Seasonal pregnancy rate was positively correlated to both book size (r = 0.584; P < 0.001) and number of covers required to service the mare book (r = 0.552; P < 0.001). Pregnancy rate per cycle was linearly correlated to percent (r = 0.495; P < 0.01) and total number (r = 0.396; P = 0.02) of progressively motile sperm, and percent (r = 0.484; P < 0.01) and total number (r = 0.437; P = 0.01) of morphologically normal sperm, in second ejaculates. Pregnancy rate per cycle and mean-αt determined in the sperm chromatin structure assay of second ejaculates were negatively correlated (r = -0.398; P < 0.02). These data support breeding soundness examination findings as beneficial for assessing potential fertility of Thoroughbred stallions prior to the onset of the breeding season.