From, (https://en.wikipedia.org/wiki/Behavioral_sink) (https://en.wikipedia.org/wiki/John_B._Calhoun) (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1644264/)(https://www.gwern.net/docs/sociology/1962-calhoun.pdf)
As I see, Calhoun does not say what they fed to the rodents. I suspect that they might be foods without all the essential nutrients in enough amounts.
I suspect that answer to this question is not published, but may be found by staff in the institutes where Calhoun worked.
There is a nutrient that can deсrease slowly over generations, omega-3 fatty acids: see google's translation from https://znatok-ne.livejournal.com/86130.html :
During the period of intensive brain formation during fetal development, the human fetus receives DHA from the mother's body. In this case, the placenta selectively absorbs maternal DHA and transfers this PUFA to the fetus. For example, the transfer rate of DHA across the placenta is three times higher than that of ARA (Lauritzen et al., 2001). Due to the intensive and selective transfer through the placenta, the content of DHA in the mother's blood plasma is reduced by half (Broadhurst et al., 2002). We found a similar phenomenon in fish: during the gestation of eggs, which have an extremely high level of DHA accumulation, the content of this acid in the muscles of the fish almost halved (Sushchik et al., 2007). During the period of breastfeeding, the mother's stores of DHA also continue to be depleted, as this PUFA passes into breast milk (Lauritzen et al., 2001). DHA from the blood is selectively absorbed precisely by the cells of the brain, as well as the nervous system and organs of vision (Bazan,. The cells of these organs are able to retain the "captured" DHA for an extremely long time, ensuring its constant concentration. For example, in order to reduce the content of DHA in the brain and retina in rodents need to be kept on a DHA-free diet for two generations (Bazan, 2009).It is believed that the same effective conservation of DHA is characteristic of the human brain (Lauritzen et al., 2001).However, according to some estimates, in the human brain as a result metabolism consumes 2-8% of DHA daily, and these losses must be replenished by the body (McNamara, Carlson, 2006).