By Joseph Dalton
Herd managers, veterinarians and consultants monitor production data, health events, and reproductive protocol compliance regularly; however, little time is spent evaluating semen storage and handling. Unfortunately, this is a critical oversight, as every successful AI program begins with proper semen handling.
The use of sexed semen has become common. It is important to remember, however, that sexed semen is a different product than conventional semen. To achieve 90% purity of a specific sex, sperm are treated with a fluorescent dye and X and Y chromosome bearing sperm are sorted with a flow cytometer/cell sorter based on intensity of fluorescence following exposure to a laser.
At North American AI studs, on-site sperm sorting services are currently provided by Sexing Technologies (Navasota, TX; www.sexingtechnologies.com).
There is ample data in dairy heifers describing an average conception rate to sexed semen of 70 to 80% of the conception rate to conventional semen used at first service.
The specific reasons contributing to lower fertility following AI with sexed semen, as compared to conventional semen, are currently unknown. Nevertheless, given the potential negative effect of the procedures necessary to sort sperm, it is clear that it is important to handle sexed semen with great care to optimize fertility.
All frozen semen must be stored, thawed and handled properly to maintain sperm viability and offer the greatest opportunity to obtain optimal fertility. Commercial AI studs, through stringent collection, processing and quality control, provide a highly fertile product to their customers. When semen is purchased and transferred to a farm or professional AI technician’s liquid nitrogen tank, the maintenance of male fertility is in the hands of the producer, farm employees, and AI technicians.
The liquid nitrogen tank consists of a “tank within a tank,” with insulation under vacuum between the inner and outer tanks. Liquid nitrogen tanks should be stored in a clean, dry area, preferably on a wood stand to avoid possible corrosion (due to contact with wet or damp concrete). Also, the liquid nitrogen tank should be securely fastened during transportation to avoid tipping the tank over and damaging the tank, which usually results in the premature loss of liquid nitrogen.
A detailed inventory of semen should be easily accessible, so that straws may be located and removed from the tank quickly to avoid exposure of semen to ambient temperature.
When removing a straw from a liquid nitrogen tank, it is imperative that the technician keep the canister, cane and unused semen straws as low as possible in the neck of the tank.
A best management practice is to keep all unused straws below the frost-line in the neck of the tank. Keep in mind that although the temperature of liquid nitrogen is -320°F, there is a temperature gradient in the neck of the tank.
For example, a tank with a neck tube that measures 6 inches long may have a temperature of -103°F in the middle of the neck (3 inches below the top), while the temperature at 1 inch below the top may be +5°F.
Why is the temperature in the neck of the tank important? Because sperm injury (as judged by sperm motility) occurs at temperatures as low as -110°F. Furthermore, injury to sperm cannot be corrected by returning semen to the liquid nitrogen.
As would be expected, the temperature in the neck of the tank becomes warmer as the liquid nitrogen level in the tank decreases. Therefore, another best management practice is to monitor the liquid nitrogen level in your tank regularly, and never let the tank go dry.
More sensitive to errors
Sexed semen for commercial use is currently packaged in 0.25-mL straws with each straw containing 2.1 million sperm. Although 0.25-mL straws containing sexed semen may be handled similarly to 0.5-mL straws, the smaller diameter makes them more sensitive to semen handling errors. Recent research from ABS Global demonstrates the decline in sperm motility over time when sexed semen is not handled properly (Figure 1).
As shown in Figure 1, providing thermal protection for sexed semen at normal body temperature (98.6°F) results in the greatest maintenance (least decline) of progressive motility, as compared with sexed semen held at 108°F (heat shock) or 40°F (cold shock), both of which result in sharp declines in progressive motility over time.
To maximize the potential fertility in each straw of sexed semen, extreme caution must be exercised during semen handling. Conception rates will most likely be maximized when AI personnel:
n Accurately identify heifers in estrus.
n Follow the AI stud’s recommendations for thawing semen.
n Maintain thermal protection of straws during AI gun assembly and transport to the heifer.
n Use appropriate hygienic procedures.
n Deposit semen in the uterus of the heifer as soon as possible (within 5 to10 minutes after thawing).
Frozen semen must be stored, thawed and handled properly to maintain fertility and offer the greatest opportunity to obtain optimal conception rates.
In a recent report of commercial data compiled by Select Sires, Holstein heifer herds that reported ≥ 50 services to sexed and conventional semen had an average conception rate to sexed semen (for all services) of 45% (range 27 to 70%) compared to 56% (range 34 to 83%) for conventional semen.
The range in fertility achieved following the use of sexed (and conventional) semen is quite large and may be due to many factors, including semen storage and handling errors. Handle sexed semen with care – and consider evaluating semen handling procedures regularly as every successful AI program begins with proper semen handling.
■ To contact Joseph Dalton at University of Idaho, Caldwell Research and Extension Center, call 208-880-1050 or e-mail him at, email@example.com