Drought myth - look at the food to reduce heat deaths

I found this very interesting - he says during a drought the roots have to go deeper looking for water - below the fertility area in the soil so the plants are dying from lack of nutrients rather than lack of water.

Then - and this pertains to rabbit keeping, he tells of a study where they fed rabbits different kinds of food during the very hot weather of the drought. In one group none of the rabbits died, even when highs were 112. In other groups the rabbits were dying off pretty badly - those fed grains as part of their ration, for instance.

http://www.soilandhealth.org/01aglibrary/droughtmyth.pdf

I've wondered of the real cause of heat related rabbit death was the temps messing up the flora in their rumen.
The change to more geen roughage would cause a bias in which bacteria multiplied, seemingly correcting the temperature bias.
Thanks!