Maybe 1 calorie of cow per 12 of corn, but dairy cows are much more efficient than that:

Cow eats 40.57 MCAl/day [1]

Produces 45.5 KGS of milk [1]

Milk 610 kcal/liter

Energy of cows milk per day 27.755 MCAL

Cow milk efficiency 68%

This feels high as a gut reaction, maybe calculation is missing energy to grow the cow. Still it shows how efficient they are!

Would love a source for your cow meat efficiency numbers, I'm currently exploring efficiency of fermentation methods to produce meat alternatives and that would be a helpful data point.

Source: [1] http://www.milkproduction.com/Library/Scientific-articles/Nu...