I made a remark at Climate Progress that the net shift from grassland to crops had been relatively tiny in the past five years, here's the clip, followed by a longer explanation. I expect to update this if necessary. There's enough to make the conclusion.
"Tiny" being about the percentages.
Here's how it goes starting with the five state chart in the article.
1.3 million acres / 640 acres = 2,031 square miles.
I compared that shift to the total farmland in the five states.
Nebraska’s farms and ranches utilize 45.5 million acres – 93% of the state’s total land area, so over 70,000 square miles in agriculture and ranching.
Iowa land area is 60% row crops, 30% ranch
Iowa has total 55,857 square miles,
so agriculture and ranching has over 50,270 square miles.
75,811 square miles of land
of which 90% is in crops or ranches, or approximately 68,229 square miles.
A recent report showed 16.55 million acres harvested in fall 2012, or 25,859 square miles in crops.
That number does not include the rangeland.
69,000 square miles of which about 90% is in farms or ranches.
79,626 square miles
20% of Minnesota's corn crop goes to ethanol.
By the time I saw just the numbers from Nebraska and Iowa, it meant that the 2,000 square mile shift in five years, spread out over the cropland and grazing land of five states was going to be well under 2%.
UPDATE: If we add up all the crop and range land together for the five states, its around 297 thousand square miles, of which a net shift within it of 2 thousand square miles over five years is 0.6% over five years.
Meanwhile... in 2004 the Amazon rainforest shrank by 10,000 square miles
More recent report is that the Amazon rainforest shrank 93,000 square miles in the past ten years that is a loss off of the rainforest's 2.4 million square miles. That's a rate of over 3.8% over ten years.
If the rainforest loss was averaged for five years instead of ten, that would be 1.9% over five years, quite a faster loss than the 0.6% net loss of grassland in the US over five years.
As it is, I wouldn't want to overlook that it takes somewhat less time to reestablish a grassland than it does a rainforest! Neither is a trivial project, as conventional crop farming can leave pesticide and herbicide residues that preclude the regrowth of a full grass land ecosystem
What could be said more carefully (based partly on the north central US map) is that parts of Iowa and the Dakotas changed over from grassland at rates that exceeded the rate of loss of Amazon rainforest. It is not correct to say that the whole five-state agricultural region experienced a shift at the rate of the rainforest.