Prediction of breeding soundness in pre-, peri- and postpubertal colts

Prediction of future fertility is important for stallion breeders as rearing of colts for breeding purposes is expensive. Thus, the study examined the testicular development and hormonal profiles in a group of pre-, peri- and postpubertal Warmblood colts (n=46) and evaluated the fertility of selected stallions (n=12) at 2.5 years of age, aiming to find parameters for predicting the breeding soundness at the youngest possible age. Exams were performed at 177±23, 604±23 and 894±24 days of age and included body condition scoring and weighing, examination of the testes (palpation, ultrasound, volume determination) and blood sampling (before and after stimulation with hCG (5000 I.U., i.v.). Hormone analysis included determination of AMH (basal) and testosterone (at 0, 2, 24 and 72 h after stimulation). Stallions with abnormal (n=6) and normal (n=6) were subjected to spermatological examination. During the examination period, palpatory findings were inconsistent in individual colts and testicular volume increased gradually with age. Body mass was 455±38 kg for peripubertal (BCS: 5.17±1.02) and 526±40 kg for postpubertal stallions (BCS: 5.26±0.94). Stallions (n=12) were grouped according to their total testicular volume (small vs. normal) and palpatory findings (abnormal vs. normal) at 2.5 years of age (937±21 days). No significant differences were observed for the total number of morphologically normal, progressively motile spermatozoa for different stallion groups, but more progressively motile spermatozoa (PMS) were observed after storage for 24 h in a centrifuged extended semen sample (p<0.05). Flowcytometric analysis revealed a lower percentage of membrane intact spermatozoa after incubation with Ca-ionophore for 120 min in stallions with small testes, while the percentage of acrosome reacted spermatozoa was higher in this group. A positive correlation of BCS and PMS was found (extended sample: r=0.765, centrifuged sample: 0.699, p<0.05). Analysis of AMH plasma concentrations showed no differences between abnormal or small testes irrespective of age group. AMH concentration was significantly higher in peripubertal colts as compared to younger and older animals. hCG stimulation affected testosterone in all age groups and resulted in the strongest increase in postpubertal stallions 72 h after administration. Based on the evaluated parameters in pre- and peripubertal colts, no prediction of sperm quality at 2.5 years of age was possible. Moreover, inconsistency in palpatory findings (except for cryptorchism) and testicular size for individual colts at 0.5 and 1.5 years of age renders early prediction of breeding soundness in stallions impossible. The results demonstrate, however, an influence of the BCS on sperm quality in postpubertal colts.