By some happenstance or another, it appears that I have stumbled on feed trained RES.
Our ¼ acre forage, “HSB grow-out, HSB/CNBG, now HSB/CNBG/RES pond has been a challenge from the start to feed train.

Introduction of stocking large 8-9 inch CNBG last fall kicked off HSB feeding and now a feeding frenzy when feeder goes off.
ALL female CNBG are now being removed.

I have had success with foam pellet flies with 2-3# HSB that intimidate the 10-12 inchers, so I went with Cecil’s AQMX pellet to sink below the surface, and catch the smaller HSB for transfer to main pond.
Larger HSB are being released to control BG spawn.

When I let the pellet sink even deeper the RES jump all over the AQMX fly.
Feed trained RES?

Me thinks so!

Could it be the RES are so difficult to feed train because they are not as surface oriented as other species?

RES can be feed trained and probably easier than LMB. There is just not the commercial need , thus the $, to warrant the process. Not only have studies on the subject shown this but forum members have done so. We have some RES that through conditioning eat pellets right along with the BG and LMB but it is a small part of the RES population.

I started out my RES feed training experiment last year with modest goals. Out of 27 Redears I started with, I told myself to feel successful if half learned to take pellets (and if I managed to return most of the "failures" to the pond before they expired).

I lost one Redear to BG aggression (before those 2 smaller BG bullies were removed from the PBR), but the other 26 all learned to take pellets at the surface. And like Cecil mentions, I think just learning to eat at the surface is at least half the struggle.