THE INFLUENCE OF AGE AND BREED ON STALLION SEMEN

This study reports on the variation in semen quality and in spermatozo al and behavioral characteristics of 168 stallions representing 9 bree ds and ranging in age from 2 to 26 yr. Semen samples were collected in to an artificial vagina and the number of mounts and urethral pulsatio ns per semen sample were recorded. Semen characteristics were examined for total volume, gel-free volume, gel volume, color score, mass acti vity, nonmotile spermatozoa, dead spermatozoa, semen density, spermato zoa concentration, total number of spermatozoa and semen pH. Morpholog ical characteristics of the spermatozoa included abnormal heads, abnor mal mid-pieces, abaxial mid-pieces, protoplasmic droplets and abnormal tails. Sources of variation were evaluated and the overall means calc ulated by least-squares analyses of variance for nonorthogonal data. T he significance of breed effects and between stallion variability were estimated using mixed-model procedures. All semen characteristics wit h the exception of color and urethral pulsations had significant varia tion due to age. Semen quality (gel-free volume, sperm concentration, total sperm numbers and sperm abnormalities) was poorest in stallions under 3 yr of age and over 11 yr. Significant breed variation was appa rent in most characteristics except for pH, semen color, abnormal midp ieces and urethral pulsations. It is recommended that both the age and breed of stallion be taken into consideration when evaluating stallio n semen.