How Inadequate Heifer Weight Data is Hurting US Dairies’ Performance

To fully harness genetic potential and enhance future milk yield, it’s crucial to time the first breeding of maiden heifers based on their attainment of a specific percentage of their mature body weight, rather than simply relying on their age.

Research conducted in the United States indicates that lighter heifers at the time of calving tend to produce 4.5 kg less milk daily during the early stages of their lactation compared to their heavier counterparts.

This highlights the importance of monitoring heifer growth, according to Dr. Joao P Martins, who spoke at this year’s Dairy Calf and Heifer Association conference held in Colorado.

For further insights, check out: Expert Advice on Rearing Heifers from Weaning to Calving

Dr. Martins, an assistant professor in large animal reproduction at the University of Wisconsin-Madison, pointed out that rearing a heifer to calving centers costs approximately $2,355 (£1,779).

“The heifer rearing expense is significant, with 46% attributed to feed and 13% to labor,” he noted.

“However, only 36% of dairy heifer growers in the US actively record average daily gain and monitor body weight to assess the heifers’ growth progress.”

Dr. Martins suggests that by shortening the time to first breeding, there’s potential to enhance reproduction rates and lower overall rearing costs by reducing feed demands.

Since 2011, the age of heifers at first calving in the US has steadily declined, reaching a median age of 23.9 months by 2021.

Yet, Dr. Martins cautioned against overly hastening the age at first breeding, due to possible adverse effects on first-lactation milk production.

Mature Weight Benchmark

At the time of first insemination, a heifer should meet 55% of her mature body weight and achieve 90% of her mature height, he stated.

Before calving, she should weigh 94% of her target weight (decreasing to 85% afterward) and reach 95% of her height.

To determine a herd’s target mature weight, weigh third- and fourth-lactation cows 30 days into their lactation cycle, he added.

A comprehensive study of a 7,000-cow Holstein herd evaluated the reproductive performance of maiden heifers based on their body weight and subsequent lactation outcomes.

Heifers were weighed 30 days post-calving, with no prior weights taken.

The lightest heifers achieved calving at an average age of 674.6 days, weighing 74.7% of their mature body weight, with impressive conception rates of 72%.

In contrast, the heaviest heifers calved at 694.6 days, weighing 91.9% of their mature body weight, but their conception rate was only 46%.

However, Dr. Martins noted that early pregnancies came with a trade-off: milk production measured at both four and 12 weeks post-calving was 10 lbs/day (around 4.5 kg/day) lower.

Genetic Considerations

Dr. Tom Tlyutik, head of Ag Modeling and Training Systems, shared insights from historical data, noting that the average weight of US Holstein cows in 1984 during scientific trials was 535 kg, while by 2024, it had risen to 640 kg.

“This accelerated genetic advancement implies that our existing benchmarks for breeding and weight targets at first calving are continually evolving,” he remarked.

As genomics accelerates genetic improvements, Dr. Tlyutik emphasized that dairy producers might be unaware of the target mature weights and performance capabilities of the cows they aim to breed.

He proposed a pragmatic goal: to triple calf birth weight by the time calves reach 90 days of age.

Nevertheless, determining growth rate targets should take into account the non-uniform growth patterns observed in heifers during the rearing period.

Monthly Weight Monitoring

“Optimizing growth in the initial six to nine months is the most profitable approach, as growth efficiency decreases as the heifer matures,” he added.

“The desired calving weight should be reached six to eight weeks prior to calving, as pregnant heifers typically stop gaining weight in the final four to eight weeks of gestation.”

“It’s essential to obtain weights of your heifers monthly, and at a minimum during key intervals: at birth, weaning, six months, first service, eight weeks before calving, and at calving.”

For further details on the Dairy Calf and Heifer Association and information regarding their 2025 conference.