Therefore, the drive to lower the age at first calving makes good economic sense. Animals which calve younger stay in the herd longer and have a higher lifetime Net Merit. Lower age at first calving also results in higher milk yield and improved fertility, conception rates and daughter pregnancy rates. All of these factors contribute to a dairy farm’s overall profitability.

CanWest DHI annual herd benchmark reports show that from 2010 to 2018, the average age at first calving in Ontario has decreased from 26.4 months to 24.5 months (all breeds combined). Heifers today are calving for the first time almost two full months younger than they were eight years earlier. But there is still much room for improvement. Research from the USDA has found that to maximize production, the optimal age at first calving for Holstein heifers is 21 to 22 months, and the optimal age at first calving for Jersey heifers is 20 to 21 months. Calving younger than this target age tends to increase the rate of stillbirths, while calving at an older age reduces lifetime milk, fat and protein yields.

dairy cow breeding benchmarks

Canada-wide, the average dairy heifer is calving for the first time several months later than is considered optimal. To achieve the optimal age at first calving, Holstein heifers should be bred at 14 to 15 months old, and Jersey heifers should be bred at 12 to 13 months old.

Dairy heifers are generally thought to reach puberty when they weigh 40% to 50% of their adult bodyweight. Based on a mature Holstein weighing 710 kilograms (1,565 pounds), a Holstein heifer should weigh 357 kilograms (785 pounds) at first service. Ideally, a Holstein heifer will be 14 to 15 months old when she reaches 357 kilograms. 

A mature Jersey should weigh 455 kilograms (1,000 pounds), and therefore a Jersey heifer should weigh 250 kilograms (550 pounds) at first service. A Jersey heifer should be 12 to 13 months old when she reaches 250 kilograms.


The age at which a heifer reaches puberty is therefore dependent on heifer nutrition. A heifer fed a low-energy diet will gain weight more slowly and therefore reach puberty later, which ultimately pushes her age at first calving later as well. Carefully balance heifer rations and intentionally manage environmental factors such as parasites and disease to ensure an ideal rate of growth in anticipation of breeding Holstein heifers at 14 to 15 months and Jersey heifers at 12 to 13 months.

To reach the optimal weight by the optimal age, researchers at Université Laval determined the target average daily gain (ADG) for Holstein heifers younger than 15 months old is 848 grams (1.9 pounds) per day and, after 15 months old, the target ADG lowers to 747 grams (1.6 pounds) per day. For Jerseys, the target ADG before 15 months old is 747 grams (1.6 pounds) per day, and after 15 months is 486 grams (1.1 pounds) per day.

At the optimal age at first service, Jersey heifers will weigh 107 kilograms (235 pounds) less and are two months younger than their Holstein counterparts. In addition to their smaller stature, Jerseys reach puberty about eight weeks sooner than Holsteins. Also, Jerseys overall have fewer incidences of stillbirth or calving difficulties and therefore are more successful at calving before 21 months than Holsteins.

Lowering the age at first calving has been proven to improve herd profitability due to fewer unproductive days before a heifer begins to generate revenue for the farm. Average age at first calving is currently on a downward trend in Canada but, nationally, dairy heifers still tend to calve for the first time when they are older than the optimal age. Herd managers can ensure heifers are bred at the optimal age by managing heifer nutrition and monitoring growth rates.  end mark

PHOTO: Jerseys reach puberty about two months faster than Holsteins. Therefore, they should receive their first service at 12 to 13 months old in order to calve in between 20 and 21 months. Photo by Jacob Lucs, Jersey Canada.

Kathryn Roxburgh
  • Kathryn Roxburgh

  • General Manager
  • Jersey Canada