|New book by Nicolette Hahn Niman: Defending Beef|
I have not read this book, but the Wall Street Journal just ran an article covering the main points. The article does not mention two things, which are global deforestation and biodiversity. Nor does the article compare grass to a forest in any of the metrics discussed. It does not mention the vast deforestation in tropical zones that has brought thousands of species to the brink of extinction, and surely has played a large part in the CO2/climate equation.
The article points out grass is better for the soil than 'conventionally tilled agricultural fields', and that may well be true. However for protection of the soil, or for carbon sequestration, a grass field can not compare to a forest. Trees have deep roots that can bring large amounts of water to leaves. Sequestering CO2 involves a reaction between H2O, sunlight and the CO2. Mangrove forests, where the roots are in the water, are among the best for storing carbon.
It seems obvious that a corn field or a wheat field can feed more people than a cow pasture, using less land and less water. The saved land and water can be used for a forest. As you can see, I think forests are the most important part of the ecological equation, and any land that not needed by humans should be returned to its original state (generally a forest). I am not an expert on agriculture, but certainly intuitively growing crops along with grass and trees (shade grown coffee for example) seems better for the soil and for the crop. One article studies mixing trees and different types of crops reduced greatly the need for pesticides, by creating a healthier ecosystem.