This metabolic nutritional-respiratory disturbance has also been reported in other ruminants (red deer)[3] and on a wide variety of grasses, alfalfa, rape, kale, and turnip tops.
[4] 'Fog fever' results from feedlot economics and the biochemistry of the cattle stomach (rumen) being slow to adjust to green grazing.
The change in diet to vegetation rich in L-tryptophan causes a corresponding increase of the amino acid typically found in protein in the rumen.
If this is not possible, the new diet should be introduced slowly by grazing the cattle just a few hours each day and increasing gradually, over a period of a fortnight.
Drugs are available (monensin or lasalocid) which change the rumen biochemistry in preparation for high tryptophan levels, inhibiting the bacteria that convert L-tryptophan to 3-methylindole.