This is a strange argument. Many of those plowed fields, at least in the US, are being used to grow livestock feed, not food for human consumption.
In that case let us make those fields meadows and pastures for livestock to be fed naturally, instead of growing corn, soy beans and other nonsense there, destroying the land and ecosystems, making them basically dead and depleted because of these monocultures.
The natural food of cow is grass anyway, not corn, soy or grains.
The majority of those calories are lost as body heat from the livestock; they never become meat or milk. That means more crops must be grown to feed the livestock, resulting in even more animal deaths, not less, when more meat and dairy is consumed, vs. plant foods.
I do not understand this argument. Cows eat grass on pastures and, in the other end, make the land fertile with the waste. Its a natural cycle.
It takes about 1.5 acre of land to feed a cow for a year. Such a cow can then feed a man for 1-2 years.
I do not know what is producing more heat, if a cow or all the activities needed for a field, but these calculations will be wildly guessed and dependent on many local conditions, IMO. And it cannot stand alone, if farming is destroying the ecosystem and poisoning air and waters and if its unhealthy diet for humans, then it does not matter if it produces less heat.
In any case, one of the problems is overpopulation of Earth. There are too many people and those people consume too many things because of the consumer society and move from place to place too much.