I have often heard the argument that Jerseys have higher components. Problem is the percentages are higher but the actual pounds of BF and protein are almost always lower than a Holstein. I get paid for pounds of BF and pounds of protein.
We do get paid on components here.
The main argument for Jersey is when you calculate your profit on feed in - milk out, not on number of cows. I can run 4 Jerseys for every 3 Holstein Friesians. There's some increase in costs due to the extra number of individual cows but the feed input is linked to the total weight of livestock. On the same feed (grazed grass) I could have 4 Jerseys doing 350 kg MS or 3 Holstein Friesians doing 450 kg MS (I'll let you work it out). Jerseys are more efficient feed converters. The lower protein:fat ratio of Jersey is the main drawback under our payment system.
I like longevity. Somatic cells often are higher in older cows but it's not a rule - some cows have 'em, some seem to keep them low as they age. I've been told by other farmers that production starts to drop gradually after about age ten but heck, she owes you nothing at that age and probably still producing more than the 2 yr old you'd replace her with.
Productive life on average is held back by heifers that breed once and never again, in this country at any rate. I've almost got to the stage of rolling my eyes at any farmer who tells me what steps they are taking to help their milking heifers breed back. My heifers don't have any more trouble with weight maintenance or breeding back than the older cows. My policy: breed 'em to a Jersey bull for easy calving, grow them well from day one so they arrive in the herd at their full potential weight at 22 months old (I have a good grazier who does most of that for me) - that's it. There's still farmers running big bulls with heifers and pulling calves and wondering why half of them don't breed back, likewise the ones who think they can bring them into the herd at a low condition score and expect them to cycle on schedule.
So when you see it written that an average cow only does three or four seasons, remember that statistic is influenced by the many 2 yr olds who calve once and never again. Of course, some heifers do turn out otherwise unsuitable and get culled young also - temperament, bad udders or mastitis, low production, sickness, BVD PIs.
The oldest cows in herds I've worked with are in their early to mid-teens, after the age of ten there's not that many left. Under US conditions they don't seem to last as long (Should I refrain from knocking the Holstein breed which *is* more susceptible to many ailments than the less highly bred cattle or Jerseys?).