Motility is an assessment of movement of the spermatozoa and can be measured objectively by a Computer Assisted Sperm Analyser (CASA) or assessed subjectively at the microscope by a trained technician. There are two parameters for motility, quantity of motile spermatozoa (% motile) and quality of motility (score or % progressive motile). Motility must be measured in optimal conditions. Whilst boar semen should be stored at 17°C, reducing both metabolism of the cell and therefore motility, to assess motility a small sample of diluted ejaculate (1-3ml) should be kept at 37°C for 15-20 minutes. This temperature mimics the environment of the sow's reproductive tract post insemination.

Many external components affect motility, as demonstrated by simply adding or removing constituents to the diluent, and motility can change dramatically in the female tract due to the molecular changes necessary for successful fertilisation (the capacitation process). Therefore sub-optimal motility under the microscope may produce different patterns when entered into the sow. The key message is, therefore, that laboratory measures of motility correlate poorly with semen fertility.

Eosin-nigrosin stain of boar semen showing "typical" malformations. A - spermatozoon with a bent tail. B - spermatozoon with a coiled tail.

SYBR14/Propidium dual stain of boar semen showing live spermatozoa in green, dead cells fluoresce red; damaged spermatozoa show red and green.

Malformation rate is a basic check that should be completed frequently. Picture 1 shows an eosin-nigrosin stain of boar semen with some "typical" malformations. Letter A shows a spermatozoon with a bent tail, B shows a coiled tail. Another typical malformation of boar spermatozoa is the presence of a cytoplasmic droplet (proximal or distal) on the midpiece of the spermatozoa. A sign of stress it can affect semen quality from 2 to 6 weeks after the animal was subjected to stress.

Viability of spermatozoa is a valuable quality parameter which is measured by permeability of the plasma membrane to specific stains. The most common staining procedure has been eosin/nigrosin being easy to use, requiring only a basic microscope. Its drawback is that assessment of viability can be extremely subjective due to the varying intensity of background coloration. Fluorescent stains have also been developed, making measuring viability easier and more accurate.

The initial investment is higher, with such stains only being visible using a UV microscope. Dual staining with SYBR14/Propidium has been commercially available for several years. Picture 2 shows that this test stains live spermatozoa in green whilst dead cells fluoresce red; damaged spermatozoa display dual staining i.e. red and green. This removes most of the subjectivity linked to the eosin staining procedure.

The concluding message is that using a single trait to assess semen quality is insufficient, especially if that trait is motility. All AI centres should also include malformation rate or viability, to confirm that all semen is fit for purpose, and producers should ensure their suppliers are scrutinising their semen with the necessary rigour prior to purchase.

Appeared in International Pig Topics

May 2006