Maureen Hanson
The heaviest heifers at calving aren’t necessarily the most successful herd replacements long-term, according to a recently published study by researchers at Pennsylvania State University.
Chad Dechow, Penn State Associate Professor of Dairy Cattle Genetics, lead the study, the results of which were published in an article in the Journal of Dairy Science. On an edition of the “Dairy Science Digest” podcast, Dechow described how his team compiled long-term performance data from more than 2,300 Holstein cows spanning 15 years, from 2001 to 2016.
The data came from cows in university herds at both Penn State and the University of Florida. Cows were stratified according to one of five at-calving weight groups. The lightest group (#1) averaged about 1,045 pounds immediately after first calving, while the heaviest heifers weighed about 1,350 pounds after freshening.
The heaviest quintile of heifers:
Produced the most milk, but not significantly more than the third and fourth weight groups (or top 60%).
Lost the most weight in the first month of lactation (about 3.65% of bodyweight).
Had the highest likelihood of culling in the 24 months after freshening (nearly a 50% higher chance on a given day).
“Collectively, we found that the heaviest heifers made a little more milk in the first lactation, but were at a much higher risk of culling,” said Dechow. “Their early lactation weight loss is problematic because it can lead to health consequences.”
Negative energy balance early in lactation has been shown to adversely affect reproduction, which could lead to removal from the herd. Animals that become early culls ultimately produce less lifetime milk, even if they start with relatively high production in their first lactation.
So, what is the ideal weight at which to freshen heifers? The researchers calculated “maturity rate” by dividing bodyweight at first calving by bodyweight at the start of third lactation. The average maturity rate was 77%. Based on the study’s performance data, they determined that heifers that reached a maturity rate of 73% to 77% -- or roughly, about 75% -- at first calving were in the best position to milk well in the first lactation, without sacrificing long-term milk yield or herd longevity.
In the future, Dechow would like to see the emphasis on first-calf heifer development focus more on stature and body condition versus bodyweight. He believes those metrics would help drive proper growth versus just accumulation of bodyfat.
“Don’t obsess about achieving a certain bodyweight by calving,” advised Dechow. “Getting them heavy for the sake of being heavy doesn’t seem to make much sense.”
dairyherd.com
