There are a couple of different ways to semen test a bull
Electro-ejaculation with a rectal probe, or a live jump using a steer and an AV
I have had some interesting experience with vets collection Bulls so here is some advice
First the bull should have it scrotal measurment taken, at 12 monthes of age the bull should have a bare minamum of a 32 cm circumfrence, this will affect the age of maturity of his daughters,
Second the secondary accessory sex glands should be palpated rectaly by someone that does this regularly
Third, before collection the bulls penis should be examined, for defects, such as scaring, hair ring, damage from breeding a female through the fence ect.
All these things can affect a bulls performance and pregnancy success
the bull should also be able to have and maintane a full errection in order to breed the cow
Next the Bull should be collected to evaluate VOLUME, CONCENTRATION, using as mass spectrometer, which will show the conc. of sperm in the ejaculate
Then a know conc. of semen needs to be diluted to look at
Motility: ativity of sperm (Blizard affect)
, Morphology (concentration of normal well formed V mal formed sperm)
, and Morbidity (dead V Live cells)
Pressence of WBC (white blood cells) skin cells, bacteria
Will indicate infection, other tests include STD's, the prime reason for buying and using VIRGIN bulls, is to prevent the spread of STD's to other cow herds which can leave you with a 0 % calf crop, late calvers etc.
Don't buy a bull that is older and has been used, unless he has been tested at a bull stud, rested for 60 days and has a clean bill of health for all Bovine STD's, Jones, Leucocsis, Bangs, TB etc.
It is the equivalent of shooting yourself in the foot
Why would you do that?