As a DHIA milk tester for 25 + years here's the basics; most dairies "test" once a month. The original way was to test 2 consecutive milkings but most now test one milking, and alternate morning and eve from month to month. This gives an overall average - computer generated - for the results for the cow for the year. About a week after the test the farmer gets a comprehensive report from the lab with butterfat, protein, somatic cell count, and also can test for MUN and now we have a milk preg test and a johnes test from the milk. We also record any info about the cow; calving date, dry off ,breedings, can put in things like hoof trimming,any health related stuff like mastitis treatments, the sex of the calf they are carrying if the vet does preg cks using ultrasound. Have several farms doing the milk preg test, pretty accurate at 28 days and saves a vet trip out at 100./hour or so and handling the cows. These records are essential for comparing your cows against each other, and making informed decisions of profitability of each cow. Now with all the electronics that are being used in the barns, alot of that is done through the viscousity of the milk so the milk tester is no longer nearly as necessary. And now with robots, they can "tell" you just about everything there is to know; guess they ought to @ 250,000 each!!!!! Still, the records are essential for registered cows as we provide an "unbiased" independent confirmation of a cows production. Our lab does not test for the A1/A2 which we have had requests for.
All that said, Holsteins run 3.0 to 4.0, all according to the genetics the farmer uses, I have one that is more old school and his holsteins regularly test in the high 3's and low 4's. Swiss in the 4's, guernseys in the 4's and low 5's, jerseys in the high 4's to high 5's. Don't know much about the ayshires but I hear they are in the 4's and milking shorthorns in the 4's. This is average and it is based on commercial conditions; ie, silage, grain etc. Any animal on grass will USUALLY test higher in BF and protein due to the way they digest the grass as opposed to more concentrated rations.
2 to 2 1/2 inches of cream on a gal jar is pretty decent, BF probably in the 4.5 to 5% range. I used to milk 4 by hand and make butter once a week and mix the skim back into the calf milk bottles and feed to the pigs etc. I have a 3 gal electric churn and would put the cream in and turn it on while doing other stuff too. I usually would have 2-3 gal cream from skimming by hand, a separator would get alot more. Hope to get back to that as soon as retirement happens....I think that before this is all over we are going to need more people who can do some stuff the old way; I am scared of where this country is going....
Also most beef breeds figure in the 4.5 - 7 % BF range from what little actual testing has been done. They tend to have higher fat less milk and if you ever pay attention to the calves, they get pretty chunky on a good cow so they are the ones that you want to keep heifers out of. Yeah, it has to do with the bull too, look at weaning weights especially to see where the heavier calves come from; better milkers and more fat in the milk. The biggest tallest calf is not always the best and thank goodness we are getting away from these huge beef cows that require so much input. There are bigger breeds and smaller breeds naturally, but these huge cows in any breed need alot of input in comparison to what they wean and are not as efficient.