Does Size Matter? Testicular Volume and Its Predictive Ability of Sperm Production in Rams

SIMPLE SUMMARY: Establishing the most suitable tool and the most accurate testicular volume determination formula could be essential in sperm production prediction. Traditionally, the caliper has been used to perform testicular measurements. In recent years, B-mode ultrasound has also been used as a measurement tool, providing, in addition to greater reproducibility as demonstrated in this work, other advantages such as being a non-invasive and non-ionizing technique that allows for the examination of the organ to see pathological processes. In the ovine species, the demand for seminal doses varies according to the genetic value of the donor males. The frequency of semen collection is high in males of great demand and genetic value. Testicular volume has a different influence on sperm production depending on the frequency of semen collection and the season of the year. This study demonstrated the effect of these factors on donor males to improve their reproductive performance. ABSTRACT: Over the years, testicular volume has been used to evaluate the reproductive capacity of rams and the effects of different factors related to reproductive performance. The aim of this study was to determine the most suitable tool and formula to calculate testicular volume under field conditions to guarantee a more accurate determination of sperm production. First, testicles from 25 rams (n = 50) were measured in vivo and postmortem using calipers and ultrasonography during the breeding season (BS). The accurate testicular volume (ATV) was calculated through water displacement. In addition, the sexual status of donor rams was evaluated during a period of four years in a reproduction center, and the three most crucial groups in terms of genetic value and seminal collections were studied in the second part of this experiment: ER-NBS (Elite rams during the non-breeding season), ER-BS-S (Elite rams with a standard frequency of seminal collection), and ER-BS-O (Elite rams with a high frequency of seminal collection). The total testicular volume (TTV), testosterone (T), and total spermatozoa obtained from two consecutive ejaculates in the same day (SPERM) were measured, and the relationship between SPERM and TTV and T was analyzed to predict SPERM. Although all published formulas revealed statistically significant differences (p ≤ 0.05) from the ATV, our proposed formula (ItraULE) (Testicular volume = L × W × D × 0.61) did not show significant differences. In the second part of the study, in the ER as a model donor ram for its high genetic value and high demand from farmers, TTV and T showed strong positive correlations with SPERM (r = 0.587, p = 0.007 NBS; r = 0.684, p = 0.001 BS-S; r = 0.773, p < 0.0001 BS-O). Moreover, formulas were established to predict SPERM in these practical scenarios. In conclusion, the use of ultrasonography and a new formula adapted to rams could improve the prediction of SPERM considering crucial factors such as season and semen collection frequency.