Here's something to test those who still believe in commercial feed:
Sapkota et.al. 2007 What do we feed to food-production animals? A review of animal feed ingredients and their potential impacts on human health
https://doi.org/10.1289/ehp.9760
Selected highlights to give some idea of what's in this paper: The 1st para below is from the Abstract, the other excerpts are from the main body of the paper.
"Animal feeding practices in the United States have changed considerably over the past century. As large-scale, concentrated production methods have become the predominant model for animal husbandry, animal feeds have been modified to include ingredients ranging from rendered animals and animal waste to antibiotics and organoarsenicals. In this article we review current U.S. animal feeding practices and etiologic agents that have been detected in animal feed. Evidence that current feeding practices may lead to adverse human health impacts is also evaluated... Findings emphasize that current animal feeding practices can result in the presence of bacteria, antibiotic-resistant bacteria, prions, arsenicals, and dioxins in feed and animal-based food products. Despite a range of potential human health impacts that could ensue, there are significant data gaps that prevent comprehensive assessments of human health risks associated with animal feed. Limited data are collected at the federal or state level concerning the amounts of specific ingredients used in animal feed, and there are insufficient surveillance systems to monitor etiologic agents "from farm to fork."
...
Table 1 provides an overview of feed ingredients that are legally permitted and used in U.S. animal feed... we focus on feed ingredients listed in Table 1 that raise specific concerns for public health, including rendered animal products, animal waste, plant- and animal-based fats, antibiotics, and metals.
...
Because of current animal feeding practices, biological, chemical, and other etiologic agents have been detected in animal feeds (Table 2) (Hinton 2000; Orriss 1997). These agents include bacterial pathogens, antibiotic-resistant bacteria, prions, metals, mycotoxins, polychlorinated dibenzo-p-dioxins (PCDDs), polychlorinated dibenzofurans (PCDFs), and PCBs... Poultry meal and bone and meat meal (nonpoultry) samples represented the greatest number of feed ingredient samples containing bacteria resistant to five or more antibiotics (Hofacre et al. 2001).
...
in spite of the wide range of potential human health impacts that could result from animal feeding practices, there are little data collected at the federal or state level concerning the amounts of specific ingredients that are intentionally included in U.S. animal feed. In addition, almost no biological or chemical testing is conducted on complete U.S. animal feeds... "
https://pubmed.ncbi.nlm.nih.gov/17520050/ (full text open access)