One definition of milk protein to fat inversion (using crude protein, not true protein) is when fat test is 0.4 point lower than milk crude protein (i.e., 2.8% fat and 3.2% protein). If over 15 percent of the cows are inverted, there may be a problem, i.e. acidosis, using this definition. Many high-producing cows will be lower in fat than protein by 0.1 to 0.2 point due to nutrient shortages and demands.
Mike Hutjens, Extension Dairy Specialist, University of Illinois - Illini DairyNET