Taking care of bovine semen tanks and semen

Although semen costs are only a small fraction of overall dairy farm expenses, the absolute value of the semen inventory at any given time may represent a significant sum of money. Semen is half the investment in a dairy farm, without which the next generation will not be born and hence no milk. It is important to take good care of the semen as well as the tank in which it is stored.

Liquid nitrogen is an atmospheric gas in liquidized form.  It is a cryogenic fluid that can cause almost instant freezing when it comes in contact with living tissue.  A liquid nitrogen tank keeps its materials well below the freezing point of water.  Contact with skin would cause instant frostbite.

Semen is stored in Liquid Nitrogen Tanks (LN2) and the temperature is -195 deg C at the bottom of the tank and there is a gradient from top to bottom.

Although tanks are designed to be sturdy and dependable, they are more fragile than they appear. They require regular care and attention to ensure they function properly for many years. Failure to do so can lead to lost inventory, or reduced pregnancy rates because semen quality is compromised.

A - LID / B - NECK / D - LOCKING TAB / F - CANISTER NUMBERING  (color coded)

Few tips to take care of the LN2 Tank 

1. Store the semen tank in a dry, well-lighted and well-ventilated area but out of direct sunlight. Tank must not be put in a place where there is excessive water or chemical use. Observe the tank daily. Once a tank “goes bad” the nitrogen is lost very rapidly. Develop a plan to have an alternative semen tank available in case your tank is damaged. 

2. Keep the tank elevated above the concrete floor or other wet and poorly ventilated surfaces. Corrosion of the outer shell shortens the functional life of the tank and possibly causes tank failure.

3. Avoid excessive movement or abuse of the tank. Any stress cracks (which normally appear on the neck), must be taken seriously.

4. Frost is one of the key external warning signs prior to tank failure. A properly functioning tank will not develop frost on the outside. Frost buildup around the outside top of your tank, particularly around the neck, indicates a vacuum loss - which results in rapid volatilization of the liquid nitrogen. Semen quality can be compromised under these circumstances. Keep in mind that this frosting usually happens quickly and doesn't last too long because the liquid nitrogen has escaped.

5. Routinely monitor nitrogen levels and keep a record of nitrogen usage. Remember even new tanks can have defects and fail. The temperature in the neck of the tank becomes warmer as the liquid nitrogen level in the tank decreases.  Never let the tank go dry.

6. The tank's cork is handled frequently every time semen is retrieved, it should be inspected monthly to make sure it is doing its job. A poor quality cork increases the boil-off rate by 20 percent to 30 percent. That's because insulation properties decrease for these corks and more heat gets into your tank. Exposing an open tank to the environment and wind can multiply the boil-off rate by up to 8 times the normal amount.

One person should be made responsible for the tank, create a checklist of things to do and check along with a timetable. This will help in plugging any loopholes and minimize damage to the investment.

Retrieval techniques also help in limiting the damage to the semen

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.

Location on the Neck Tube	Range in Temperature in Deg C
TOP	+2.2 to +12.2 Deg C
1 inch from TOP	-15 to - 22.2 Deg C
2 inch from TOP	- 40 to – 46 Deg C
3 inch from TOP	- 75 to – 82 Deg C
4 inch from TOP	- 100 to – 120 Deg C
5 inch from TOP	- 140 to – 160 Deg C
6 Inch from TOP	- 180 to – 197 Deg C
Source: Pennsylvania University newsletter – IRM-11	Typical Temperatures in Field Semen Tanks in USA

Keep in mind that although the temperature of liquid nitrogen is -195 degrees C, there is a temperature gradient in the neck of the tank. For example, a tank with a neck tube that measures six inches long may have a temperature of -75 degrees C in the middle of the neck (3 inches below the top), while the temperature at 1 inch below the top may be + 2.2 degrees C. Reports have shown that sperm injury (as judged by sperm motility) occurs at temperatures as low as -78.8 degrees C. Furthermore, injury to sperm cannot be corrected by returning semen to the liquid nitrogen.

Use tweezers/angled forceps to retrieve the straws and keep the canisters as low as possible and do not use the fingers as they might get damaged due to frostbite. If the technician is unable to retrieve the straw in 10 seconds, the canister should be lowered and allowed to re-cool for 20 seconds before trying again. More time in a warm zone leads to partial thawing and can compromises the semen quality.

More about handling frozen semen in the next blog

Amit Sachdev

India consultant

World Wide Sires

Email: indiawws@gmail.com