Probably more acurately stated Do some bull's straws of semen work better? As bull is one variable and packaging another.
Quick anwer- yes. Its a numbers game.
Longer answer-
As you know, bulls vary in their semen quality/quantity- which is why we use measurements of testicle size (a part of a piece of the puzzle- suggesting higher quantity). Some bulls make a higher percentage of good swimmers vs non-swimmers (motility). A semen analysis looks at all this and even shape- which are the biggest 3 factors affecting a bulls fertility although other things can be wrong with sperm that appear normal during evaluation. The chromatin (DNA) can be damaged, there can be antibodies etc... Meaning that just b/c a bull has a normal basic sperm analysis does NOT guarantee his sperm will get the job done.
There is also small variables in the way collection facilities and even individuals in the facilities handle the collection, divide samples into straws, and freeze the sample These factors may not be caused by the bull, but may be seen as differences in fertilization percentages. At least for that particular sample.
With timed AI'ing you are depositing semen when the cow may or maynot have starting showing signs of standing heat. Studies show that the variable of time of insemination after a cow shows heat will either increase or decrease your chances of success.
So taking all this into account, one bull may have higher statistics of pregnancy vs another if quality/quantity of the bulls sperm is significantly higher- allowing for more healthy sperm to be present (no matter which method of AI'ing you use).
In timed AI'ing, if you start out with one bull having higher numbers of- able to get the job done sperm deposited at time x. Then 48-72 hours later (for the latercyclining cows) he will likely still have the most quality sperm present- though neither will be near the orignal quantity. i.e. 90% kill off of 100 leaves 10. 90% kill off of 1000 leaves 100. Thus, your window of opportunity (time variable) is lengthened.
I don't know if some bulls semen may naturally remain viable longer in a hostile environment (female reproductive tract)?- I am sure that there have been studies performed in humans as reproduction is big business.
