But calves still do not really gain weight until after weaning. The question is why.
When calves are born, their systems are similar to pigs. But when you compare the performance of calves to pigs, you see a huge difference. Baby pigs go from about 3 pounds to 12 or more pounds in three weeks while nursing — a quadrupling of birth weight. Calves do well to hold onto their birth weight or gain a few pounds. The difference appears to lie in the nutrients fed.
Skinny calves aren’t normal
Calves are born with limited fat stores. 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, explains Van Amburgh. That only leaves about 1 pound of fat to draw from when stressed.
If you feed 1 pound of 20:20 milk replacer powder per day, with no environmental stress, maintenance requirements will be met with little left over for growth. If, however, the temperature drops below 68 degrees F, the calves will be in a negative energy balance and won’t be able to develop the fat stores needed to fight off disease during the first few weeks of life.
This is important for two reasons, says Jim Reynolds, veterinarian at the University of California Veterinary Medicine Teaching and Research Facility. First, the immune system needs energy — provided by the calf’s fat stores — to respond to a disease challenge. And second, when it comes to fighting an infection, the immune system is more important for winning the battle than the medication.
“Calves in a negative energy balance, or with minimal fat stores, cannot adequately fuel the immune system,” says Reynolds. Contrary to what some producers believe, these skinny calves are not normal. Skinny calves mean more sick calves and more dead calves.
But once you get enough energy and protein into calves, you can get a good functional response from the immune system, which can help minimize the medications needed. And if an antimicrobial is needed, a well-fueled immune system enables a good response to medications and to vaccines.
Research conducted at Virginia Tech with Jersey calves helps quantify the difference. Jersey calves fed 1 pound of 20:20 milk replacer for five weeks had 3 percent body fat. By contrast, Jersey calves fed the same volume of whole milk for five weeks had 9 percent body fat. (Please see picture on page 26.) Because whole milk contains more nutrients (on a dry-matter basis whole milk is 25.4 percent protein and 30.8 percent fat; traditional milk replacer is 20 percent protein and fat), calves were able to replenish their fat stores, grow and even build an energy reserve. With three times more body fat, these calves have more energy reserves to fuel their immune systems and survive adverse weather conditions.