It is not feasible for a farmer to buy feed during the lifetime of the animal. It’s the reason they have huge pastures for grazing during warm months. During winter they are usually fed hay.
All beef is grass fed period. Some are finished at the end with corn or other dense grains (your soy patties).
Here’s an article from a Meat Scientist.
https://meatscience.org/TheMeatWeEat/topics/raising-animals-...
Visit the Harrison Ranch (on I-5, south-east of SF), and see for yourself how they're treated.
Mad cow disease propagates from feeding cows the ground up bits of other infected cows.[0]
[0]https://www.fda.gov/animalveterinary/resourcesforyou/animalh...
[1]https://www.cdc.gov/prions/bse/bse-north-america.html
[2]https://www-m.cnn.com/2013/07/02/health/mad-cow-disease-fast...
This article recommends no more than 20% soy in cattle’s diet. The article is from last year. [1]
>Researchers have found that when the oil content of the ration exceeds 7 percent, it can be toxic to the microbes in the cattle’s rumen and decrease digestibility. Too much oil in cattle rations will lead to scours (diarrhea), cessation of rumen fermentation and, eventually, death.
“Because of these limitations, the recommended upper limit of feeding would be about 20 percent of the ration,” Hoppe says. “Practical feeding levels are probably more like 2 to 3 pounds per head per day. At this low rate of supplementation, soybeans provide an excellent source of protein and energy.”
[1]https://www.drovers.com/article/soybeans-may-be-viable-cattl...