|Xami the ram lamb, who's standing funny because he's peeing|
A highly fertile ram will get more ewes bred. He will also ensure that more of the eggs ovulated by each ewe he breeds gets fertilized, leading to more multiple births. A low-fertility ram can't do this.
In a July 1990 article in Sheep! magazine called "Is your ram a dud or a stud? Knowing the difference pays off" the authors talk about the difference in breeding fertility between rams. In the wild, all rams are in a genetic competition to see who can breed the most ewes. The highly-fertile ram produces far more offspring than his less-fertile brethren, and so his genes have a greater influence over the wild herd.
In domestication, most male lambs grow up to be dinner. The shepherd picks future herd sire based on many different factors, from birth number (twin or single) to coat color to horned/polled status.... We just assume that the pretty boys we pick out will be fertile. And sometimes we are disappointed.
The article speaks of a procedure called a serving capacity test. A ram is observed in a small pen with about three in-heat ewes. The number of times he services the ewes is counted. If a group of rams is given this test, the numbers generated have been shown to predict breeding efficiency.
In a small farm flock the article authors recommend exposing each of your rams to three unrelated in-heat ewes for 30 minutes for the test. Rams who do not breed at all should be given a second test on the next day.
Another test the article recommends for larger flocks is to test 4 or 5 rams at once with 4 in-heat ewes for 20 minutes. Each ram that services an ewe is marked and removed.
For today's even smaller farm flocks, there may be some difficulty in coming up with enough in-heat ewes that the shepherd is willing to let get bred for the purposes of a test. A vasectomized ram with a marking harness can be used to pick out ewes that are in heat. (Make sure your vasectomized ram is really sterile first!!!)
You may be only able to conduct a little testing. My suggestion is to use a test to choose between several rams of the same age. I wouldn't bother testing the ram lamb or yearling who is the least dominant of a group of same-age rams. He may not have the right personality to get the job done.
If you can't pull off a full serving capacity test, try this method of test-breeding. Pick your top two or three future-sire candidates, create three small breeding pens and fill them with three or four low-value but proven-fertile ewes, and leave each ram with a ewe group over a normal breeding session. Any ram who doesn't get all the ewes bred might be considered likely to be less fertile than you need. Using ram lambs, this may be a way to select for early sexual maturity without using an unproven ram lamb on your whole flock.