Days open

Definition

Days open is the number of days from calving to the conception that produces the next pregnancy. It is also called the calving-to-conception interval.

It is known exactly only for cows that are pregnant: the conception date is the date of the insemination that was later confirmed pregnant. For cows that are still open, days open is unknown and still growing. It can only be given as “at least” the current DIM, or as a projection.

So there are three different numbers people call “days open”:

Actual days open, pregnant cows: conception date − calving date.
Current days open, open cows: today’s DIM. This is a minimum, not a final value.
Projected days open, open cows: current DIM plus the expected days until conception at the herd’s pregnancy rate.

Survivorship bias

An average of days open over pregnant cows only leaves out every cow that has not conceived yet, and every cow that was culled without conceiving. Those are exactly the cows with the longest days open. The result:

The pregnant-cow average is too low, often by weeks.
It can improve while reproduction gets worse: if more cows fail to conceive and are culled, they never enter the average.
It lags: a cow that conceives at 250 DIM enters the average only when she is confirmed pregnant, long after the problem started.

Better measures: include open cows with projected values, use a survival (Kaplan–Meier) median days open that treats open and culled cows as censored, or use cumulative % pregnant by DIM.

Formula

Plain text:

Days open (pregnant cow) = conception date − calving date

Days open (open cow, current) = today − calving date = DIM

Projected days open (open cow) ≈ DIM + 21 × (1 − PR) / PR

LaTeX:

\mathrm{DO}_{\text{preg}} = d_{\text{conception}} - d_{\text{calving}}
\qquad
\mathrm{DO}_{\text{proj}} \approx \mathrm{DIM} + 21 \cdot \frac{1 - \mathrm{PR}}{\mathrm{PR}}

The projection treats each future 21-day cycle as an independent chance of pregnancy at the herd’s 21-day pregnancy rate (PR, as a fraction). The expected number of failed cycles before success is (1 − PR) / PR. At PR = 0.25 that is three cycles, or about 63 more days. It is a rough herd average, not a prediction for one cow, and it should not be applied to cows marked do-not-breed.

Days open and calving interval are linked through gestation length:

Calving interval ≈ days open + gestation length (about 280 days in Holsteins)

How to pull it in DC305

DOPN is DC305’s standard days-open item.

LIST ID LACT RC DIM DOPN CDAT FOR LACT>0 DCC>0       pregnant cows (milking and dry), actual days open
LIST ID LACT RC DIM DOPN FOR LACT>0 RC<5             not-pregnant cows (DNB, fresh, open, bred)
SUM DOPN FOR LACT>0 DCC>0                            average for pregnant cows
PCT DOPN<150 DCC>0 FOR LACT>0                        % of cows that conceived before 150 DIM
PCT DCC=0 DIM>300 FOR LACT>0                         % of cows open past 300 DIM

For pregnant cows, DOPN should equal conception date (CDAT) minus fresh date (FDAT). Spot-check a few cows.
Dry cows are pregnant too. RC 5 is pregnant and RC 6 is dry (see RC codes), so RC=5 alone leaves out the cows that conceived earliest. DCC>0 (days carrying calf) covers both.
LIST, SUM and PCT cover live animals only by default, so cows that left the herd drop out. Add \B to include sold and dead animals or \D for those alone, e.g. SUM\B DOPN FOR LACT>0 DCC>0; closed lactations sit in the archive (\L on EVENTS, BREDSUM, GRAPH).

Typical range

Herd context: US Holsteins, lactating cows, freestall or dry-lot, VWP 50–60 days, 21-day PR 20–30%.

Level	Average days open, pregnant cows
Very good	< 110 days
Typical	115–140 days
Problem	> 150 days

Median days open from a survival analysis is usually longer than the pregnant-cow average in the same herd, because it accounts for open and culled cows.
A herd’s VWP sets a floor: average days open can’t be much below VWP plus a few weeks.
The economic cost of an extra day open is real but depends on milk price, feed cost, persistency and the cow’s stage of lactation. It is small early in lactation and grows later (De Vries 2006). Don’t quote a fixed dollar figure.

Common mistakes

Averaging pregnant cows only and calling it the herd’s days open. See survivorship bias above.
Treating an open cow’s DIM as her days open. It is a lower bound.
Leaving out dry cows when averaging pregnant cows.
Using the wrong conception date. If a cow was bred twice and the pregnancy check didn’t establish which breeding worked, the conception date may be off by a cycle.
Not adjusting for pregnancy loss. A cow that conceived at 80 DIM and lost the pregnancy at 60 days of gestation is open again. Her days open run to the next conception, not the lost one.
Comparing herds with different VWPs or culling policies without saying so.