A 92-pound calf — born with about 3.5 to 4.1 pounds of body fat — uses about 0.6 to 1.8 pounds of that fat while learning to live outside the womb during the first day or two of life. That only leaves it about 1 pound of fat to draw from when stressed, explains Mike Van Amburgh, dairy nutritionist at Cornell University.
Researchers have learned that without adequate fat stores, calves cannot mount an effective immune response. That’s because fat provides energy and other signals like leptin needed to stimulate an immune response. (Fat stores are especially important if the calf has gone off feed.) If you feed milk replacer at the traditional rate — 1 pound of 20:20 milk replacer powder per day — at temperatures less than 68°F, calves will be in a negative energy balance and will not develop fat stores to fight off a disease challenge during the first couple of weeks of life.