When I decide on a bull I would like to flush to, I ask the owner to check out with the stud where he was collected and see what batch dates were the highest quality.In some cases, I check with the stud myself and the studs that are professional and honest, usually are willing to be upfront and tell me which batch dates are best for flushing to. The studs all keep records of the quality on all collection dates. Some bulls will have significant variations in semen quality, especially younger bulls or any bull that are stressed in any way. Heat, cold, changes in feed, etc etc can affect the semen quality, and usually these stresses affect young bulls that are still trying to grow harder than more mature bulls.
Most studs usually have fairly high minimum standards for semen quality before it can be kept for processing, but I have found that some studs are bad for processing some very poor semen. Seems they are only interested in getting the income from each straw they produce and could care less about the quality of the semen they produce.