Semen or germplasm is the complete-discharge of the male genital tract occurring at the time of ejaculation by the male. It is a white, opaque, creamy fluid, occasionally yellowish green due to the pigment carotene. Semen consists of cellular part spermatozoa or sperms and the fluid parts, known as seminal plasma.
Spermatozoa is a male germ cell apportioned into three regions: head, middle piece and tail. The shape of the head of the sperm in the bull, ram, boar and rabbit is a blunt ovoid. In fowl, the sperm head appears as elongated cylinder. In the bull, the spermatozoa measuring 80 microns in length resembles an agile tadpole. The head is a blunt ovoid structure known as acrosane.
The liquid portion of semen i.e. seminal plasma is nothing but the secretions of accessory sex glands such as the prostrate, seminal vesicles and cowper's glands. Seminal plasma present an ideal medium for the viability and mobility of the male germ cells.
The volume of single ejaculate in the bull ranges from 2-10 mI. Variations in quantity may be due to breed differences and also age, frequency of service, season and nutritional status of the individual. Table-l gives volume, density and sperm count of some species of animals.
Once semen is collected, it is examined to judge its suitability for inseminations. There can be two types of examinations. First, a routine examination of every sampIe of semen collected, and another method is a more detailed examination done at much longer intervals to study the suitability of a male for artificial insemination before initiating it to A.I. and at half-yearly or yearly intervals to evaluate if the male is continuing to produce sufficient viable sperms or not.
In the routine examination, the colour, volume, mass activity (initial gross motility), individual motility after dilution, sperm density and percentage mortility are ideally examined. In a more detailed examination hydrogen ion concentration, dead and live count of spermatozoa, morphological study of spermatozoa to judge abnormalities, resistance to cold shock or to sodium chloride, etc. may be carried out in addition to those listed under routine test. Besides, the activity of spermatozoa is judged by tests like methylene blue or resajurine reduction test, fructolytic index, oxygen utilization test and ascorbic acid concentration test.