RFI research doesn't show that intake can be held constant. One of the problems with rfi. Basically low (or good) RFI means an animal eats less than expected.
RFI does what it is supposed to do. It decreases intake for the observed gain and weight of the animal. It isn't a problem, it does what it does.
A couple of ways to tackle it is to lock down ADG at say 3 pounds a day, and then work on reducing the feed to do this.
You just defined RFI, with a slight modification on a specified gain. It will have the same long term effect as RFI. Gain can vary from year to year based on environment and feed quality, so it will be hard to manage.
Another approach is to lock down feed DMI at 21 pounds per day (7:1) and work on increasing gain.
This is will result in poorer carcass quality over a period of time because of restricted intake. Of course, the limitation as you specified earlier is that you can only use RFI to the point at which gain or carcass quality is compromised. As such, it is not a tool with which to make sustainable improvement.
The solution is probably some combination of these with all the other important traits, but there are some issues.
1. Is RFI on grain the same trait as RFI on forage? They are similar but not the same. This is important as more efficient cows may not make more efficient feeders and vice versa.
The correlation between RFI on forage and RFI on grain (0.65) is actually similar to the correlation between RFI and FC/FE. While the experts are content using RFI synonymously with "efficiency" since the genetic correlation is around 0.65, they forget to mention that the correlation with Intake is around 0.75. So, they are spending money to investigate the correlation between RFI Forage and RFI Grain, they call it "efficiency" and they ignore the fact that what they are really doing is decreasing gain?
2. Is RFI a proxy for appetite? For example, I can think of specific breeds of cattle that are very efficient (feed:gain) but can't/won't/don't eat much and thus take a long time to finish.
By definition, it changes intake. For the reason you mentioned above, the pig industry quit using RFI and decreased intake as a selection tool over 20 years ago. This was before the Aussies decided to select for RFI.
3. Particularly in the case of a cowherd that may undergo environmental stress, might a drop in appetite or intake affect the ability of the cow to consume enough nutrients for maintenance in the event of an environmental challenge (eg: cold, or drought). (we probably are a long way off of this, but it could conceivably happen)
You say we are probably a long way off. I would say, along with reproduction decreases that need to be investigated, and which may in fact be caused by changes in intake, and with changes in carcass quality, that this may be the thing that is most important to investigate and get to the bottom of. I would say this is an immediate need, not one that is probably a long way off. Less confuse the issue more. In arid environments, the question becomes, "Is appetite decreased to the point that cattle will forage less, and fail to remain productive?" In hot, humid environments the question becomes, "Will the cattle have trouble ridding their bodies of the excess heat of digestion that accompanies RFI, resulting in fetal losses?" How's that for confusion? While we are selecting for RFI, why don't we incorporate a geneotype x environment interaction to contend with?
4. Does RFI inadvertently select for some rumen bugs over others, or does it select for actual rumen bypass, or change rumen function in the long run (we know the abomasum and intestine is more efficient at getting nutrients out of high quality feeds than the rumen is)?
It looks like it does. Don't know if this is a concern yet. Rumen bugs change frequently as new feeds are introduced.
I think RFI is exciting and worthwhile, and we will probably look at it in our cowherd going forward as I could either run more cows or get more performance out of the same resource, but there are still a lot of questions.
You could run more cows, but getting more out of hte same resource would happen by selection for just RFI.
RFI is an indicator of efficiency, efficiency is an indicator or profit. When we select for RFI, we select for an indicator of an indicator of profit. When measuring RFI, we have the gain and the intake. Why don't we just put a dollar value on each and select for Net Profit when it is right in front of us? Net Profit is a sustainable selection strategy, RFI is not.
Badlands