I know you mean well but baking soda does not kill rats. Could it? In mice it can be lethal at 5 to 10% of their body weight. But that is simply impossible, doubly impossible using your receipe.
Let's consider a 200 pound human, would they eat ten to twenty pounds of pure sodium bicarbonate, AKA baking soda? No? Or twenty to forty pounds of cornmeal mixed with baking soda?
Then would a 20 gram mouse eat one to two grams of baking soda? No.
Would an eight ounce roof rat eat 10% of its body weight of baking soda?
Mixed with cornmeal as in your receipe, that would be 20% of its body weight?
Or say you weigh 120 pounds, would you eat 24 pounds of cornmeal and baking soda even with sugar in it?
Now, if you search for soda bicarbonate/baking soda as a rodentcide you won't find much in scientific studies. But, you will find baking soda used as an antidote to poisoned mice or for preventing lab mice from emptying their bowels after prolonged exercise done in studies
https://scholar.google.com/scholar?hl=en&as_sdt=0,37&q=rodentcide+sodium+bicarbonate&btnG=
Scientists also used baking soda as lavage, a solution to wash out the stomach of a test subject like a lab mouse or poisoned mammal in order to freeze the dose so to speak at certain blood levels.
Baking soda has been used to attempt to mask the taste of rodentcides like strychnine. It doesn't work for that either. Rats don't like the taste of it.
But say you do get a mouse to eat 20% of its body weight in cornmeal and baking soda. Guess what? Rodents are like most mammals and a lot of insects,
they do fart. There is a book about what critters fart and which ones don't. Search Google Books for "Does it Fart?" Sloths don't fart because it takes three weeks to digest a meal and it passes the gas through its bloodstream and out its lungs. Chickens don't or bats because the food goes through their digestive system so fast they don't build up an gas from digestion.
But RATS DO FART
https://pubmed.ncbi.nlm.nih.gov/845703/
https://thepetfaq.com/can-rats-fart/
https://petshun.com/article/can-rodents-fart
Peppermint spray doesn't work either short of being at levels that would choke you and your chickens to death. (pun not intended).
What does work with rats is one of the three methods that Howard E. told us about ten years ago; sanitation, exclusion, elimination.