Where I am in CA it doesn't get much below freezing, so I have little real experience. 25 degrees here is cold. But as mentioned, a battery has way less power in cold temps, yet the engine oil is thicker and the trans fluid is thicker so it needs more power just when it has less.
I'd first make sure all the glow plugs are working. If so, a block heater set on a timer should do the trick. I have family in Fairbanks, AK. They typically have a battery blanket, a coolant heater, and a pan heater to warm the oil. Plug these in to a timer, and the engine thinks it's in Hawaii.
Of course fuel will gel if untreated at certain temps, and water in the fuel can freeze. But using winter or treated fuel should eliminate that.
I think at zero degrees with a tractor stored outdoors, you need at least a coolant heater. At 20 degrees, it ought to start fine. But hey, I'm from sunny CA......