Twinning rate
Definition
Section titled “Definition”The twinning rate is the share of calvings that produce two or more calves. Almost all dairy twins are fraternal (dizygotic), from a double ovulation. Double ovulation becomes more common as milk yield rises, so high-producing multiparous cows twin far more often than heifers (Fricke & Wiltbank 1999).
Twin calvings matter because they bring more stillbirths, dystocia, retained placenta, metritis, early culling and poorer fertility in the next lactation.
Freemartins
Section titled “Freemartins”A freemartin is a heifer twinned with a bull. Placental blood vessels fuse early in pregnancy, so the female is exposed to male hormones and cells and is usually infertile. Over 90% of heifers born co-twin to a bull are sterile (Merck Veterinary Manual, reviewed Aug 2026; the older figure of about 92% is widely repeated). Heifers twin to a heifer are normal. Freemartin status can be confirmed by a blood test (XX/XY chimerism) or a vaginal length check (Padula 2005 reviews PCR tests for Y-chromosome DNA).
Formula
Section titled “Formula”Plain text:
Twinning rate (%) = calvings with ≥ 2 calves / calvings with known calf count × 100
Freemartin risk (%) = heifer calves born co-twin to a bull / all heifer calves born × 100LaTeX:
\mathrm{TR} = \frac{N_{\text{calvings with}\geq 2\text{ calves}}}{N_{\text{calvings}}} \times 100How to pull it in DC305
Section titled “How to pull it in DC305”Twins are recorded on the FRESH event, usually as two calf codes in the
remark (sex and status for each calf) or in calf items, and sometimes as a
separate twin event or code. Formats vary by herd; check the herd’s private
dc305-vocab mapping.
EVENTS\B FRESH ID LACT FDAT FOR FDAT>-365 FRESH events with calf codes in the remark, including dams that leftEVENTS\3 calf tableName the event after EVENTS to select it; \S prompts for a date window
instead of the FDAT filter. EVENTS covers live animals only by default, so
\B keeps dams sold or dead since calving.
- Count a calving as a twin calving when it has two calf records for the same dam on the same date.
- Classify twin pairs as female/female, male/male or mixed sex. Flag the heifer in a mixed pair as a probable freemartin.
- Split by lactation (1, 2, 3+).
Typical range
Section titled “Typical range”Herd context: US Holsteins, freestall or dry-lot.
| Group | Typical twinning rate |
|---|---|
| First-calf heifers | about 1% |
| Cows (2nd+ lactation) | 4–9% |
| All calvings | 3–6% |
For reference, Minnesota DHIA records for 1996–2004 (2.3 million calvings) showed 1.2% twins for heifers, 5.8% for cows and 4.2% overall, rising from 3.4% in 1996 to 4.8% in 2004 (Silva del Río et al. 2007). Twin calvings had one or both calves dead in 28.2% of cases, vs 7.2% for singles.
Recorded twinning is a floor. Twin calvings are missed when one calf is born dead and not recorded, when the second calf is found later, when calving records hold only one calf, or when a twin pregnancy is lost early and never seen. A herd’s true twinning rate is likely higher than recorded.
Twinning also varies with season: it is highest for conceptions in August to October (Silva del Río et al. 2007). It rises with milk yield, because high-producing cows double-ovulate more often (Fricke & Wiltbank 1999).
Common mistakes
Section titled “Common mistakes”- Counting calves, not calvings. Twin calves ÷ all calves is close to double the twin calvings ÷ calvings rate.
- Treating recorded twins as the total. It is a floor, especially when one twin is born dead.
- Raising freemartins as replacements without checking. Test mixed-sex twin heifers before investing in rearing them.
- Ignoring twins when reading stillbirth and dystocia rates. A rise in twinning raises both; split singles from twins.