The beef industry has long measured feed conversion in ways that are counterproductive for the low-cost beef producer.
People have been measuring conversion of grain to beef in the feedlot for a long time, trying to improve said conversion efficiency to be able to compete for high-priced grain with other animals such as pigs, poultry, fish.
By measuring pounds of beef produced per unit of grain they have been selecting indirectly for lean meat, since lean meat requires much less calories than fat to produce.
The implications for this way of measuring efficiency of conversion are huge because it is not scientifically viable to compare pounds of lean meat to pounds of fat or marbled meat.
Ergo, when we use bulls selected for good efficiency of conversion from grain to beef we are effectively selecting for hard-keeping cows in their progeny which will need grain or lax selective grazing to maintain good body condition, meaning adequate fat. As we know fat is needed for estrogen to be produced for the cow to cycle and conceive.
This problem could be easily solved if the measurement we used to select for efficiency was calories in to calories gained as beef or fat. Fat contains much more calories, which are needed when cows are under duress. This means they are not comparing apples to apples
This is why a cow that deposits excess energy as fat can survive longer periods of cold, drought and deprivation of feed than a cow that only carries lean meat. A pound of fat releases many more calories than a pound of lean meat for the cow to survive under difficult conditions.
The animal that deposits marbled fat has a larger store of energy to draw from if she gets sick and that makes for better survival, marbled fat is a readily available source of energy to the cow.
Remember that marbled fat is what gives more flavor to beef and is essential for a good eating experience.
So we need to ask ourselves, "Do we want to have easy-keeping cows and be able to make some profit or are we selecting for hard-keepers that will require pampering to survive when the going gets tough?"