Pregnancy rate (21-day)

Definition

The 21-day pregnancy rate (PR) is the share of cows eligible to become pregnant during a 21-day period that actually become pregnant in it. 21 days is one estrous cycle, so every eligible cow gets about one chance per period.

A cow is eligible for a period if she is:

past the herd’s voluntary waiting period (VWP),
not pregnant,
not marked do-not-breed, and
in the herd for the period. DC305 counts cows who leave partway through as partially eligible. See the VERIFY note below.

PR combines two things: how many eligible cows get bred (heat detection rate, also called insemination or service rate) and how many of those breedings work (conception rate). Some literature calls it “pregnancy risk”.

Formula

Plain text:

PR (%) = cows pregnant in the period / cows eligible in the period × 100

Approximation:  PR ≈ HDR × CR

LaTeX:

\mathrm{PR}_{21} = \frac{N_{\text{preg in period}}}{N_{\text{eligible in period}}} \times 100
\qquad
\mathrm{PR}_{21} \approx \mathrm{HDR} \times \mathrm{CR}

The summary PR over a longer window (e.g. 12 months) is the pooled ratio (total pregnancies ÷ total eligible cow-periods). It is not the average of the period percentages.

How to pull it in DC305

BREDSUM \P FOR LACT>0     21-day table, cows
BREDSUM \P FOR LACT=0     21-day table, heifers (if heifers are bred in DC305)

[!VERIFY] \P as the 21-day pregnancy rate switch, and the column layout (Br Elig / Bred / Pct / Pg Elig / Preg / Pct / Loss), on the DC305 version in use.

Each row is a 21-day period with:

breeding-eligible cows, cows bred and the heat detection %,
pregnancy-eligible cows, cows pregnant and the pregnancy rate %, and
pregnancy losses.

The summary line pools all completed periods.

The last one to two periods are incomplete: their pregnancies haven’t been diagnosed yet, so DC305 shows them but leaves them out of the total. Never report the current period’s PR.
The VWP setting in DC305 drives eligibility. If the VWP is wrong, the denominator is wrong. Check it before trusting the table.

[!VERIFY] How DC305 counts cows who leave or are marked DNB partway through a period (partial eligibility vs. excluded).

Typical range

Herd context: US Holsteins, lactating cows, freestall or dry-lot, mostly timed AI with some detected heats, VWP 50–60 days.

Level	21-day PR
Below average	< 18%
Average (US, current)	20–24%
Good	25–30%
Top herds	> 30%

[!VERIFY] Benchmark bands. US averages have risen a lot since the early 2000s (about 14–16%) and need re-checking against current DRMS/DCRC or Ferguson & Skidmore data.

Heifers: usually 35–50%+, because heat detection and conception are both higher.

[!VERIFY] Heifer PR range.

Heat stress: summer PR in Southwest herds can fall to single digits or low teens without aggressive cooling and timed AI.

[!VERIFY] Summer PR range for Southwest herds.

Each 1-point gain in PR is worth a meaningful amount per cow per year, through fewer days open and fewer culls for reproduction. The exact dollar figure depends heavily on prices, so don’t quote a fixed number.

Common mistakes

Confusing PR with conception rate. CR is per service; PR is per eligible cow per 21 days. A 40% CR herd with 45% HDR has a PR of about 18%.
Reporting incomplete periods. The most recent periods always look low.
Averaging period percentages instead of pooling. Small periods get as much weight as large ones.
A wrong or changed VWP. Changing the VWP in DC305 changes historical eligibility and therefore historical PR.
Leaving DNB cows in. They inflate the denominator. Check that DNB is recorded consistently.
Mixing cows and heifers in one number.
Comparing herds with different VWPs. A herd with a 75-day VWP has fewer cows eligible early in lactation, when fertility is lowest. That flatters its PR compared with a herd using a 50-day VWP.