The way I look at it is its all about growing bulls in a responsible manner which I think can be described as a system that optimizes a bulls future breeding ability as opposed to the systems that compromise a bulls breeding ability.
Grassfarmer is right that a bulls genotype isn't altered by ration, but his ability to breed can be compromised by excessive or insufficient nutrition. An all forage ration of straw won,t do the job and an all they can eat forage ration of high dairy quality(150 Relative Feed Value or plus) can give you 3 lbs a day gain. Twenty lbs of corn or barley in a ration is a good way to market bulls and also a good way to compromise there future breeding performance. A good quality hay 100 rfv and 1/2% or even 1% body weight of grain won't hurt a bull and IMO shouldn't be viewed as irresponsible.
The bull Leanin H described from the poor bagged cow if he was used strictly as a terminal bull could be a great bull assuming he had all the necessary traits. If he was sold as a bull worthy of raising replacements then it's questionable that he will do fine.
To me evaluating a bull development system is only one aspect of the overall system or management a bull is a product of. To me the overall cowherd management system is of more importance than the bull development alone.
For maternal seedstock, cowherd management is my first assessment priority, length of breeding season, culling policy, degree of management and labor for calving, degree of energy supplements in the dormant season and corresponding BCS maintenance levels, cows can be thin because they are poor doers or they can be thin because they are being worked hard it is important to know the difference, degree of structural soundness of feet and udder, ect,ect. Then the bull development assessment follows.
Elwapo my preference like Grassfarmers is for the grass tested, forage raised maternal seedstock approach which we have been experimenting with since 1993 is a result of the value I perceive in identifying those bulls that are able to utilize and thrive on the primary feed stuff their daughters will be expected to make do with when they become producing range cows themselves. To me it simply isn't logical to test a maternal seed-stock bull under a development program that is entirely different than the environment his daughters will be expected to produce in. If cows spent their producing years in a feedlot being fed grain then it would make sense to test their sires that way other wise it makes about as much sense as testing physics students with an English exam.
The other aspect is that over the years I have noticed that cattle that do well on grain won't necessarily do well on grass but virtually all cattle that do well on grass will do well on grain. This is entirely anecdotal of course and if a study was done on this subject I may be proved wrong but if I were a betting man I know which way I would bet.
All this being said does that mean that there are no feed lot tested bulls of genetic merit or worth sold at these sales. No. A number of those students that pass the English test will do well in Physics just probably not the most accurate way to identify degree of aptitude for a physicist.
Nor does it mean that just because a bull is grass tested and forage raised that he is a bull of genetic merit or worth? But my guess is that if you compared population progeny adaptability or value in the range producing environment over a number of generations with all other variables being equal, the bulls that were tested and selected as the bulls of merit under the grass tested forage raised model would come out ahead.