A dairy cow has a desired body fat level and will increase or decrease feed intake to meet this requirement, says Phil Garnsworthy, dairy science professor at the
Changes in BCS during early lactation highlight the role of body fat in controlling feed intake. The strong relationship between BCS at calving and change in BCS provides compelling evidence that cows have a target BCS in early lactation, he says. “Cows that are fatter than their target BCS mobilize body fat; those that are thinner than their target BCS gain body fat.” Of course, the rate at which a cow changes BCS toward its target is affected by diet composition as well as current BCS.
Recent reports indicate that average target BCS is lower now than it was in the 1980s, says Garnsworthy. “This is because cows selected for higher milk production over the past 30 years are genetically thinner, so they have a higher drive to mobilize body fat.”
Consequently, cows of high genetic merit are more likely to experience deeper and more prolonged negative energy balance in early lactation. “Negative energy balance is undesirable because it reduces reproductive performance and increases susceptibility to diseases,” he adds.
To reduce the impact of negative energy balance on cow health and performance, Garnsworthy suggests that BCS at calving should be no more than 0.5 BCS units above a cow’s target BCS. For example, cows with low genetic merit for milk yield (target BCS 2.5 to 3) should calve with BCS of 3 or less. Cows with high genetic merit for milk yield (target BCS 2 to 2.25) should calve with BCS of 2.75 or less.