Everything else being equal …
Blocky head ~~ The head of the most fertile bulls will be half as wide as they are long.
The bull that keeps the hair rubbed off of his poll. ~~ this is an indicator of extra energy from Testosterone
Small in diameter tail ~~ a good indicator of butterfat in his daughters. There are other butterfat indicators but they get covered up if the animal is overfed. Overfeeding is covered very well in the book Factors Affecting Calf Crop (first written in 1946 and then updated). If you go there, start reading at chapter 6 unless you live in Florida.
In my opinion, fertile BULLS that create fertile, easy-keeping daughters are more important than growth in the steers for the feedlot if you want to make more profit per acre from your grass. 40% of your profit on the ranch is cows that have a calf every year. 30% of your profit is what does it cost to run that cow.
20% growth and 10% carcass traits.
The slick magazines talk growth and carcass traits ~~ because those traits make the feedlot system money.
Everything else seems to either get covered up or changed as a result of feeding grain, making it had to see the other fertility indicators a bull should have.
Feeding grain (including DDG’s) … Puts fat cells in the testicles and in the scrotum. Lowering volume and quality of semen production for the rest of the bulls life.