A nondescript, well-used tank in the farm office might be holding one of the most valuable aspects of the dairy herd: frozen semen. If semen isn’t handled and stored properly from the time it arrives on the farm to being deposited in a cow, the time invested in selecting the right bulls for a herd may be wasted.

Dr. Joe Dalton, professor and Extension dairy specialist at the University of Idaho, discussed proper handling of frozen semen. He described the liquid storage vessel as a tank within a tank – between the two tanks is a vacuum space and insulation. “The inner shell holds the liquid nitrogen,” he said. “The canisters hang down and hold canes of semen.”

On most farms, the best place to store a liquid nitrogen tank is in an office with the appropriate ambient temperature. Tanks should not be stored in the milk house where temperatures and humidity can be high.

Although the tank is durable, the neck tube at the top is fragile, and anyone handling the unit should be careful not to drop the tank. Tanks should be secured when moved in a vehicle. The appropriate stopper should be kept in the neck tank to stop liquid nitrogen from evaporating.

It’s important to monitor liquid nitrogen levels across time. Dalton suggested farmers check levels every few days by placing a dipstick with measurements through the neck to the bottom of the tank to determine the frost line.

“Maintain a minimum of about 10 inches of liquid nitrogen in the tank,” said Dalton. “Make sure to fill the tank at regular intervals, which can be done by working with the AI supplier.”

He explained the inner environment of the tank, and what happens relative to the ambient environment and maintaining frozen semen. “There’s temperature variation within the neck of the tank,” he said. “The temperature at the bottom of the neck is close to the liquid nitrogen temperature: about -292º to -313º Fahrenheit. Toward the outside environment, the temperature rises. At the top of the tank, the temperature is above freezing, which shows the effect of the ambient environment relative to temps within the neck.”

Dairy farmers should know the holding capacity of the tank. Tanks are rated by the number of weeks they can hold liquid nitrogen. Farmers considering purchasing a tank should have the tank filled with liquid nitrogen and understand the loss of liquid nitrogen across time. The tank might lose nitrogen rapidly over the first few days versus an appropriate decline over time.

Dalton cautioned against purchasing a used liquid nitrogen tank. A new tank with adequate capacity for the farm maintains semen at the correct temperature, and the value of pregnancies to the business will bring the expected return on investment.

If a tank isn’t damaged and maintained with sufficient liquid nitrogen, frozen semen can be stored for decades. “When we remove [a straw] and thaw appropriately, the expectation and reality is that sperm thaw and come back to a metabolic state of progressive motility and have the ability to fertilize,” said Dalton, adding that the same is true for properly stored frozen embryos.

Research proves sperm injury occurs at -110º. “We also know injury to sperm cannot be corrected by returning semen to liquid nitrogen,” he said. “Sperm do not have a mechanism that allows them to repair damage to membranes. Always work below the frost line within the tank to maintain frozen sperm at an optimal temperature.”

Maintaining genetics in a tank

The liquid nitrogen tank should remain open only as long as it takes to remove the correct straws. Photo by Sally Colby

Canes should be kept below the frost line, even if one of the canes is being used. If necessary, access lower straws with tweezers to avoid damaging other straws.

Every successful AI program begins with proper semen handling. Dalton listed four factors that influence the quality and viability of stored semen: time, temperature, hygiene and skill. He suggested keeping an inventory of canister contents on the side of the tank or digitally to avoid having to search for certain straws once the tank is open.

Timing and temperature are critical to the AI process. “If we have 0.5 ml straws, thawed appropriately at 96º to 98º, if those are held for 15 minutes at that temperature, there’s no difference in mean progressive sperm motility,” said Dalton. “But if a 0.5 ml straw is thawed appropriately then held for 15 minutes at room temperature, about 70º, there’s a decrease in … motility.”

If a thawing unit is not used, semen should be thawed in 95º – 98º water for about 45 seconds. Use a thermometer that’s been checked for accuracy to ensure water temperature is correct. Dalton urged inseminators to keep the loaded AI gun close to their body, within their coveralls. Battery pack systems allow the inseminator to carry AI guns maintained at the appropriate temperature.

Thawing a large number of straws at one time isn’t a good practice. Dalton said to generate pregnancies, it’s important to follow the correct procedure. “When we control time, temperature, hygiene and skill, then deposit semen within 10 to 15 minutes, we can thaw more than one straw at a time,” he said. “Know your comfort zone.”

Be aware of how long it takes to load guns and don’t allow straws to touch during thawing. “We know from research that if straws touch when thawing, sperm is damaged and results in reduced fertility,” said Dalton. “If you’re thawing multiple straws, have multiple thaw baths and gently swirl the straw when dropping in the water so they stay apart.”

Although sexed semen is a good option for some breedings, it isn’t the same as conventional semen and must be handled appropriately to generate pregnancies.

“Sexed semen packaged in 0.25 ml straws is very sensitive to semen handling errors,” said Dalton. “To guard against those errors, we shouldn’t use the 10- to 15-minute rule for thawing and breeding – we need to be in the five- to eight-minute range with sexed semen. Thaw no more straws than you can breed cows within five to eight minutes, and the timing begins as soon as the first straw hits the thaw bath. Include loading the gun and walking to the cows.”

Dalton urged inseminators to develop correct procedure in a system that makes sense to them. “The worst thing an inseminator can do is to be scatterbrained, wasting time looking for something,” he said. “A lot of forward thinking will make the process go smoothly and will make us able to retain the fertility that’s inherent in the straws we purchased.”

by Sally Colby