Prediction of daily sperm output in stallions

Ten sexually inexperienced Thoroughbred stallions, ranging in age from 4 to 7 yr, were used to determine the relationship between available extra-gonadal reserves (AEGR) and daily sperm output (DSO). Ejaculates were obtained during the months of June through December. Each stallion was ejaculated at 0700, 0800, and 1700 h on Day 1 and 0700, 1200, and 1700 h on Days 2 and 3. A single ejaculate was collected at 0700 h on Days 4 through 7. DSO was calculated by averaging the total spermatozoa obtained on Days 5, 6, and 7. A minimum of 14 d was allotted to each stallion between trials to allow replenishment of AEGR. Weekly trials were classified as 1) primary: ejaculates taken from sexually inexperienced stallions; 2) normal: all weekly ejaculated in which the total spermatozoa in the second ejaculate (0800 h on Day 1) divided by the total spermatozoa in the first ejaculate (0700 h on Day 1) yielded percentages >20 and <70%; and 3) all: all weekly trials. Regression analysis yielded an equation of estimated DSO=0.18 (total spermatozoa in first ejaculate) + 0.93. Measured DSO, mean of total spermatozoa collected on Days 5, 6, and 7, divided by total spermatozoa in the first ejaculate of normal weekly trials averaged 27.5+/-1.9%. When 27.5% was multiplied by the total spermatozoa found in the first ejaculate (0700 h on Day 1) in primary, normal, and all trials, correlation coefficients between measured and estimated DSOs of 0.95, 0.95, and 0.92, respectively, were obtained. In conclusion, there appears to be a relatively stable relationship between AEGR and DSO in Thoroughbred stallions aged 4 to 7 yr. This relationship allowed a famrly accurate (82%) estimation of DSO when the total number of spermatozoa found in the first ejaculate of sexually rested stallions is multiplied by 27.5%.