Testicular volumetry and prediction of daily sperm output in stallions by orchidometry and two- and three-dimensional sonography.

Accurate determination of the testes volume and prediction of the daily sperm output (DSO) is valuable information for reproductive management of a stallion. The aim of this study was to compare different methods for measuring the testes volume, including caliper, 2D and 3D ultrasound. Special emphasis was on feasibility of 3D volume analysis. First, 22 castrated testes were measured and derived volumes were compared with volumes determined via volume displacement in a graded cylinder with saline solution. Then, during the breeding season, testes sizes of 52 stallions were measured in vivo and analyzed. With the derived volumes, predicted DSO (pDSO) values were calculated which were compared with actual values (aDSO) determined from semen evaluation. Analyses of castrated testes revealed a discrepancy between volume assessments via the caliper and ultrasound methods and actual volumes as found via volume displacement. The smallest difference was found for 3D volume analysis, followed by caliper and 2D ultrasound. Testicular volumes of breeding stallions were highest if determined via 3D ultrasound, followed by measurements using 2D ultrasound and caliper. Correlation between the total testicular volume (TTV) and aDSO was high with volume assessment via ultrasound (2D: r = 0.639, p < 0.001, and 3D: r = 0.604, p < 0.001), and moderate for using caliper (r = 0.46, p < 0.01). Linear regression analyses of TTV and aDSO values revealed that changes in aDSO in part could be explained by differences in testes volume: 32% and 27% in case of 3D and 2D ultrasound, and 12% with caliper. pDSO values that were predicted from testicular measurements correlated best with aDSO values from semen collection protocols in case of using 3D ultrasound (r = 0.56, p < 0.001), followed by 2D ultrasound (r = 0.52; p < 0.001) and caliper (r = 0.34, p = 0.01). In conclusion, 3D ultrasound can be performed on equine testes for more accurate volume predictions, which in turn may increase precision when determining the breeding potential of a stallion.