Milk letdown and milking routine

Definition

Milk letdown (milk ejection) is the release of milk from the udder’s alveoli into the cistern, triggered by oxytocin after the teats are stimulated. Only a small share of milk (the cisternal fraction, less than 20% of the milk yielded; Bruckmaier & Blum 1998) is available before letdown; the rest (alveolar milk) needs oxytocin, which takes about 1–2 minutes to act after stimulation starts. The delay is longer when the udder is less full: Bruckmaier & Hilger 2001 found milk ejection is delayed at short milking intervals and in late lactation, in proportion to udder filling.

The milking routine (prep) is the sequence of steps before attach: pre-dip, forestrip, wipe, and attach. Its timing decides whether the unit goes on a cow that is already letting milk down. Parlor milk meters measure how well that worked:

Prep lag time: time from first tactile stimulation of the teats to unit attachment.
2-minute milk: pounds of milk harvested in the first 2 minutes (120 s) after attach, often expressed as % of milk in the first 2 minutes. High values mean the cow was letting down at attach.
Bimodal flow: a flow curve that rises (cisternal milk), dips or stops, then rises again when alveolar milk arrives. It means the unit went on before letdown, so the teat end was milked on empty for a while.
Unit-on time (machine-on time): attach to detach. Poor letdown and bimodal flow lengthen it.
Average and peak flow rate (lb/min).

Poor prep wastes parlor time, increases teat-end damage and can raise SCC and clinical mastitis over time.

Formula

% milk in first 2 min = milk lb in first 120 s after attach / total milk lb at that milking × 100
prep lag time (s)     = attach time − first stimulation time
bimodal rate (%)      = milkings with bimodal flow / milkings measured × 100
average flow (lb/min) = milk lb / unit-on time (min)

\%_{2\,\text{min}} = \frac{M_{0\text{–}120\,\text{s}}}{M_{\text{total}}} \times 100

How to pull it in DC305

These measures come from parlor or milk-meter software, not from DC305. DC305 normally holds test-day milk and, in some herds, daily milk imported from the parlor, but not per-milking flow curves.

Pull 2-minute milk, bimodal flags, unit-on time and flow rate from the parlor system, by milking shift, parlor side or stall, and operator if available.
If a herd imports any of these into DC305 as user-defined items, the item names vary by herd; check the herd’s private dc305-vocab mapping.

Join to DC305 only to stratify by cow factors (e.g. LACT, DIM, pen), and match on milking date and cow ID.

Typical range

Herd context: US Holsteins, freestall or dry-lot, 3x milking in a rotary or parallel parlor with electronic meters.

Measure	Target
Prep lag time	60–120 s (90 s is a common target)
Tactile stimulation	≥ 10–15 s of forestripping/wiping per cow
% milk in first 2 min	≥ 40–50%
2-minute milk (lb)	≥ 15–20 lb in high pens
Bimodal milkings	< 10–20%
Unit-on time (3x, high pens)	about 4–6 min
Average flow rate	about 7–10 lb/min

Late-lactation and low-yield cows have less milk to harvest and lower 2-minute milk; compare like pens.

Common mistakes

Comparing 2-minute milk across pens with different yields. Use % of milk in first 2 minutes, or compare within similar pens.
Comparing across meter brands. Each brand defines its flow windows and bimodal detection differently.
Too long a prep lag. Waiting much past 2 minutes lets oxytocin fall before the unit is on; longer is not always better.
Reading one milking. Flow measures vary milking to milking; look at shift and week averages.
Ignoring shift and operator. Prep problems are usually tied to a crew, shift or parlor side, not the whole herd.
Blaming the routine for machine problems. Vacuum level, pulsation and detacher settings also change flow and unit-on time.