Paul Larson’s Jersey produce 65 to 70 percent heifer calves thanks to gender selected semen.
Having enough heifers coming into the milking string is a top concern for a farmer trying to put milk in the tank.
Cows leave the herd for a multitude of reasons — poor milk production, failure to become pregnant, injury, sickness, even death — and farmers need young stock in the pipeline to replace them.
And, for the first time since cattle were domesticated, having a heifer calf no longer need be a matter of chance.
For the past several years, we have used a new technology that significantly improves our chances of having female, or heifer, calves born from certain breedings.
Sexed, or gender-selected, semen technology has been available commercially for about 15 years. Without gender selected semen, a herd runs — you guessed it — about 50 percent heifer calves and 50 percent bull calves. But with gender-selected semen, we have about a 90 percent chance the calf born will be a heifer.
In our herd of Jerseys, we calve 65 to 70 percent heifer calves. When we breed a heifer at about a year of age, we often use gender selected semen. We use gender-selected semen on only our top milk-producing cows. The combination increases our heifer calf births by about 20 percentage points over the use of conventional frozen semen only.
Maybe you remember learning about chromosomes in biology class. XX are female, XY male. The dam’s egg carries an X chromosome. The sire’s sperm cells are X or Y, and the single sperm that fertilizes the egg determines the sex of the embryo, hence the calf.
Sexing technology separates sperm cells based on the total DNA content of the sperm. The X chromosome has more DNA than the Y chromosome. For bull semen, X chromosome sperm have 3.8 percent more DNA than Y sperm. (In human sperm, the X sperm have just under 3 percent more DNA than the Y sperm. Is that why women are more complex than men?)
A machine called a flow cytometer can measure the amount of DNA in sperm. Sperm cells are marked with a fluorescent stain that adheres to the DNA. “Female” sperm (X chromosome) fluoresce more than “male” sperm.
Sperm go through the flow cytometer under pressure in a stream of very small droplets, single file. An electrical charge is placed on each droplet after the fluorescence has been read, and sperm are separated into groups by electrical charge. Sexing semen is done before it undergoes the process of freezing and storage in liquid nitrogen.
It’s a slow process requiring expensive technology. To make sexed semen affordable, fewer sperm cells are packaged in a straw (a dose for a single breeding) than for “regular” bull semen. A straw of gender-select semen contains about 2 million sperm cells. Non-gender-select semen contains about 10 million to 20 million sperm cells per straw.
While the physical process of running sperm cells through a flow cytometer damages some, no birth defects nor other reproductive failures have been linked to using sexed semen in cattle.
Unsexed semen that costs a farmer $15 to $40 per straw costs twice that if run through the sexing process. Sexed semen has about 75 percent of the fertility of unsexed frozen semen. So using gender-select semen is a sizable investment, and it will be almost three years before the resulting female produces milk. However, she will also produce her own calf at that time, one presumably a step ahead genetically as well.
There are advantages to sexed semen. Farmers can enhance the rate of genetic improvement and milk production. Farmers can grow female replacement numbers without purchasing extra animals — and the inherent diseases that can come with such purchases. Each farmer has to decide whether the technology is cost effective.
A number of companies were involved in developing semen sexing technology, and they gravitated to cattle and hogs — where there was the most potential economic return.
Source: LaCrosseTribune
