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AgridianDairy KB

Age at first calving

Definition

Section titled “Definition”

Age at first calving (AFC) is a heifer’s age in months on the day she first calves. Herd AFC is the average (or median) for heifers that calved for the first time in a period.

AFC is set by two things:

  • Growth: how fast the heifer reaches breeding size (about 55% of mature body weight) and calving size (about 85% of mature body weight after calving). See calf growth.
  • Reproduction: how soon after reaching breeding size she is bred and conceives. See heifer reproduction.

AFC matters economically because heifers produce nothing until they calve. Each extra month adds a month of feed, labor and housing per heifer, and raises the number of heifers needed on hand to supply the same replacements. Rearing cost per extra month is commonly quoted around $50–$100 per heifer, depending on feed prices; Hutchison et al. 2017 cite heifer costs as high as $2.50 per day (about $75 per month). Calving too young or too small is linked to lower first-lactation milk and more stillbirths: in Ettema & Santos 2004, heifers calving at ≤ 700 days (about 23 months) had lower first-lactation milk and component yields and the highest stillbirth incidence (19.8%).

Formula

Section titled “Formula”

Plain text:

AFC (months) = (first calving date − birth date, in days) / 30.4


Herd AFC = mean of AFC over heifers with a first calving in period P

LaTeX:

\mathrm{AFC} = \frac{t_{\text{first calving}} - t_{\text{birth}}}{30.4}
\qquad
\overline{\mathrm{AFC}}_P = \frac{1}{n}\sum_{i \in P} \mathrm{AFC}_i

Use 30.4 days per month (365.25 ÷ 12). State whether the figure is a mean or a median. The AFC distribution has a long right tail, so the mean sits above the median.

Distribution

Section titled “Distribution”

The average hides the tail. Also report:

  • the share calving under 21 months, at 21–24, at 24–27, and over 27 months;
  • the 10th and 90th percentiles.

A herd with a 24-month mean can still have 15% of heifers calving after 28 months, usually repeat breeders or heifers set back by disease.

How to pull it in DC305

Section titled “How to pull it in DC305”

First-lactation cows have LACT=1, a fresh date FDAT and a birth date BDAT. Some DC305 installs have an age-at-first-calving item; otherwise compute AFC from FDAT − BDAT.

LIST ID BDAT FDAT FOR LACT=1 FDAT>-365
SUM AGEFR FOR LACT=1 FDAT>-365

AGEFR (age at freshening) appears in the VAS and Select Sires analysis examples but is not in every setup. If the herd has no such item, compute AFC from FDAT − BDAT on an export.

For cows in second or later lactation, FDAT is the current lactation’s fresh date, not the first. To get AFC for older cows, use the first fresh event in the event history. Event names vary by herd; check the herd’s private dc305-vocab mapping.

Purchased heifers often have estimated or missing birth dates. Flag or exclude them.

Typical range

Section titled “Typical range”

Herd context: US Holsteins, freestall or dry-lot dairies, heifers raised on the dairy or at a contract grower.

Measure Typical US Target
Holstein mean AFC 23–25 months 22–24 months
Jersey mean AFC 22–24 months 21–23 months
Heifers calving after 27 months 5–15% < 5–10%

Published optima differ by outcome:

  • Ettema & Santos 2004 (1,905 California Holstein heifers): the highest economic return was for heifers calving at 23 to 24.5 months (701–750 days). Calving at ≤ 700 days cut first-lactation yields and reproduction; calving after 750 days brought no gain.
  • Hutchison et al. 2017 (more than 14 million US cows calving 1997–2015): the AFC that maximizes lifetime production while limiting stillbirths is 21–22 months for Holsteins and 20–21 months for Jerseys. Mean AFC fell by 2.4 months (Holstein) and 2.7 months (Jersey) after 2006.

Common mistakes

Section titled “Common mistakes”
  • Using current FDAT for multiparous cows. It is the latest calving, not the first.
  • Including purchased heifers with guessed birth dates.
  • Reporting only the mean. Show the distribution and the late tail.
  • Mixing breeds. Jerseys calve younger.
  • Chasing low AFC without size. Small heifers calved early give less milk and have more dystocia. Pair AFC with body weight or height at calving.
  • Over-reading months with few first calvings. In small herds use a rolling 12-month window.
  • Treating AFC as a calf-raiser metric only. It reflects calf health and growth, the heifer breeding program and heifer feeding together.
Section titled “Related”