Skip to content
AgridianDairy KB

Calving interval

Definition

Section titled “Definition”

Calving interval (CI) is the number of days between two successive calvings of the same cow. It is also called calving-to-calving interval or intercalving interval.

A cow’s calving interval is her days open plus her gestation length. Gestation varies little (US Holsteins average 277.8 days for heifers and 279.4 days for cows, with a standard deviation of about 5–6 days; Norman et al. 2009), so almost all the variation in calving interval is variation in days open.

Why it’s a lagging indicator

Section titled “Why it’s a lagging indicator”

A calving interval only exists once the cow calves again. That is roughly 9 months after the conception that ends it, and 12–15 months after the start of the lactation it describes. A herd’s average calving interval this month mostly reflects breeding decisions made a year or more ago. It is useful for long-term trends and for comparing to historical records, not for managing current reproduction. For that, use pregnancy rate and cumulative % pregnant by DIM.

Survivorship bias

Section titled “Survivorship bias”

Only cows that calved again have a calving interval. Cows that were culled open, died, or were sold before recalving never get one, and those are often the cows that had the most trouble getting pregnant. So:

  • Herd average calving interval is biased low relative to the true experience of all cows that started a lactation.
  • A herd that culls hard for reproduction can show a good calving interval while its pregnancy rate is poor.
  • It says nothing about heifers (no previous calving) or first-lactation cows that haven’t recalved yet.

A projected calving interval for currently pregnant cows (days open + 280) is more current, but it is still limited to cows that got pregnant.

Formula

Section titled “Formula”

Plain text:

CI (days) = current calving date − previous calving date


CI ≈ days open + gestation length


Projected CI (pregnant cow) = days open + expected gestation (about 280 days for Holsteins)

LaTeX:

\mathrm{CI}_i = d_{\text{calving},\,k+1} - d_{\text{calving},\,k}
\qquad
\mathrm{CI} \approx \mathrm{DO} + \mathrm{GL}
\qquad
\mathrm{CI}_{\text{proj}} = \mathrm{DO} + 280

Herd average CI is the mean over cows that calved again in the reporting window. State whether it uses actual intervals (cows that recalved) or projected intervals (currently pregnant cows).

How to pull it in DC305

Section titled “How to pull it in DC305”

Actual calving intervals need the previous lactation’s fresh date. DC305 keeps lactation history, and the VAS and Select Sires analysis examples use a calving-interval item, CINT.

LIST ID LACT FDAT CINT FOR LACT>1                       recalved cows, with calving interval
SUM CINT FOR LACT>1                                     average calving interval, recalved cows
SUM CINT BY LCTGP FOR LACT>1                            by lactation group
LIST ID LACT FDAT DOPN DUE FOR LACT>0 DCC>0              pregnant cows, with days open and due date
SUM DOPN FOR LACT>0 DCC>0                               days open of pregnant cows, for projected CI
  • Projected CI for a pregnant cow = expected calving date (DUE) − current fresh date (FDAT).
  • Actual CI for a cow in her second or later lactation = current FDAT − previous lactation’s fresh date.
  • DUE is the standard due-date item. Select pregnant cows with DCC>0 (days carrying calf) rather than RC=5: dry cows carry RC 6.
  • Cows that leave the herd are no longer in the default (live-only) list. To show the bias, add \D (animals that left the herd) or \B (live and left), e.g. LIST\D ID LACT FDAT FOR LACT>0, and count how many never recalved.

Typical range

Section titled “Typical range”

Herd context: US Holsteins, freestall or dry-lot, VWP 50–60 days.

Level Average actual calving interval
Very good ≤ 13.0 months (about 395 days)
Typical 13.0–13.8 months (about 395–420 days)
Problem > 14 months (about 425+ days)
  • The older “12-month calving interval” goal (about 85 days open) is rarely reached in high-producing Holstein herds and is not always the economic optimum. Optimal interval depends on milk yield, persistency and prices (De Vries 2006).
  • Jersey and Holstein gestations are almost the same length: US Jerseys average 278.4 days (heifers) and 280.0 days (cows), about half a day longer than Holsteins (Norman et al. 2009). Breed makes almost no difference to calving interval at the same days open; Brown Swiss (about 287 days) and Guernseys (about 285 days) run a week or so longer.

Common mistakes

Section titled “Common mistakes”
  • Using calving interval to manage current reproduction. By the time it moves, the problem is a year old.
  • Ignoring culled cows. The cows that never recalved are missing from the average, so it understates the herd’s repro problems.
  • Mixing actual and projected intervals in one number without saying so.
  • Comparing herds with different culling policies. A herd that keeps late-bred cows looks worse on calving interval than one that culls them, even with the same pregnancy rate.
  • Expecting a 12-month interval without checking whether it pays for that herd.
  • Forgetting abortions. A cow that aborts and is rebred has a long interval that reflects pregnancy loss, not slow breeding. Look at days open and pregnancy loss alongside it.
Section titled “Related”