Morphology (anatomy) of the sperm has been shown to be one of the most important indicators of fertility in the bull and a very useful selection tool for improving herd fertility.
What is semen morphology?
Sperm morphology refers to the shape, size and structure of the sperm. Sperm quantities and its ability to move forward and reach the egg (motility) is assessed crush side by the vet at the time of a bull test. A sample of the semen is placed into a small vial of preservative which kills, preserves and dilutes the sperm and other cells. The sample is later examined for morphology using a much higher power specialised microscope in a laboratory. The sperm are counted and 100 sperm are classed as either normal or abnormal, with the abnormal sperm being categorised into differing abnormalities.
Why is sperm morphology important?
Morphology results can predict the sperm’s ability to get a calf from the egg, and importantly, can pick up defects which may initially start to fertilise the egg but then fail to result in an ongoing pregnancy. Normal morphology has been demonstrated to be heritable, relatively repeatable (doesn’t change much year to year), can be used to predict the number of calves the bull can produce and can even predict the fertility of his male and female offspring. The heifer’s age at puberty and her time between calving and cycling again, can also be influenced by the morphology of her sire. The selection of bulls with high normal sperm morphology counts results in increased calving rates, tighter calving periods, reduced empty cows, increased weaning weights and faster rates of genetic gain. The stocking rate of bulls to females may also be lowered.
Bulls which do not qualify by morphology standards may still result in a large number of calves on the ground, but these calves may have come at a high cost. There may be an unacceptable high rate of empty or dry cows, and the cows may have lost one or two embryos during the mating season prior to maintaining pregnancy. The delays in achieving pregnancy result in smaller calves at weaning, and increased pressure on the cow to achieve pregnancy the following year as she has a shorter recovery time post calving.
What about variations in sperm morphology results?
Semen is sensitive to extreme temperatures, stress and diet, so morphology results can also reflect recent illnesses, transport stress, lameness and high grain diets, all of which may have temporary or permanent effects on fertility. As a result, repeat tests may be required on bulls with defects on the initial exam. Some bulls are more sensitive to these stressors than others and their morphology and fertility can be affected more easily than others in the same mob exposed to the same stressors. Overall, morphology in mature bulls is largely repeatable, so the majority of bulls will have similar results on repeat exam.
Variations in results due to poor semen collection and handling are extremely rare and identifiable. VBBSE accredited veterinarians are trained to avoid these complications.
What about young bulls?
Young bulls may occasionally have defect sperm which reflect their sexual immaturity. In this case, the defective sperm count often progress to become normal as the bull matures sexually. Part of the testicle responsible for making important changes to the sperm necessary for fertilisation, may not mature to function effectively until some bulls are 15 months old. When it is not yet functioning normally, proximal droplets and other defects are seen on the sperm. Sperm with these droplets fail to bind to the egg and when numbers of droplets are high, the normal sperm (without droplets) also fail to bind to the egg. These defects should be diminished by 18 months of age or may reflect a slower maturing bull or another reason that their testicles aren’t functioning properly. Bulls which are sexually mature at a younger age are more profitable. They are useable earlier and their male and female offspring mature younger, increasing production rates.
How can producers benefit from morphology testing?
Only purchase bulls that have passed a morphology test. Ask to see the actual morphology test results and select bulls with higher percentage normal. Bulls which fail to be sellable at the morphology stage are an expense to the breeder, but the benefits of this step to the purchaser are considerable for their production and herd fertility. Breeders selling morphology tested bulls should be proud of their high standards which will be reflected in the long term quality of their product and in their reputation.
For more information contact your BULLCHECK ™ Australian Cattle Veterinarian