Heat stress and THI

Definition

Heat stress occurs when a cow gains more heat (from metabolism and the environment) than she can lose. High-producing cows generate a lot of metabolic heat, so they hit heat stress at lower temperatures than dry cows or heifers.

The temperature-humidity index (THI) combines air temperature and relative humidity into one number that tracks heat load. It does not account for solar radiation or wind; indexes such as the black globe humidity index add those.

Heat stress affects:

Dry matter intake: drops, often 10–25% or more in severe heat.
Milk yield and components: milk falls, with a lag of about 1–3 days after hot weather; fat and protein tests fall too.
Fertility: conception rate and heat detection fall in summer, and the effect persists 1–2 months after weather cools (developing follicles are damaged weeks before ovulation).
Health: more lameness, SARA, mastitis and higher SCC.
Dry cows: heat stress in the dry period lowers next-lactation milk and calf birth weight.

Cooling is the main response: shade, fans plus feedline soakers (sprinklers) in freestalls and dry lots, holding-pen and exit-lane cooling, and evaporative (misting or cool-cell) cooling in dry climates. Low-humidity regions can cool air evaporatively; humid regions rely more on wetting the cow and moving air.

Summer months

This KB defines summer as June through September (Northern Hemisphere) unless a page says otherwise. In hot regions such as the US Southwest, the heat-stress season often runs May through October. Define the window explicitly when comparing seasons.

Formula

THI, NRC (1971) form, with temperature in °C and relative humidity in %:

THI = (1.8 × T + 32) − (0.55 − 0.0055 × RH) × (1.8 × T − 26)

\mathrm{THI} = (1.8\,T + 32) - (0.55 - 0.0055\,\mathrm{RH})\,(1.8\,T - 26)

The same formula with temperature in °F:

THI = T°F − (0.55 − 0.0055 × RH) × (T°F − 58)

\mathrm{THI} = T_F - (0.55 - 0.0055\,\mathrm{RH})\,(T_F - 58)

Example: 95 °F (35 °C) at 30% RH gives THI = 95 − (0.55 − 0.165) × 37 = 95 − 14.2 ≈ 80.8.

Other THI formulas exist (e.g. dry-bulb/wet-bulb or dew-point forms), and they give slightly different values. State the version and whether THI is the daily maximum, daily mean or hourly value.

How to pull it in DC305

DC305 holds no weather data. Pull temperature and humidity from a weather station on the farm or the nearest public station (e.g. NOAA), compute THI hourly, then join to milk data by date:

Daily milk: parlor daily milk or bulk tank shipped milk by day (better resolution than test-day).
Test-day milk in DC305, e.g. SUM MILK BY PEN FOR LACT>0 on each test date, or PLOT for summer vs winter calvers. PLOT graphs test-day items against DIM; PLOT\R groups by test date instead, \Dddd limits the date range, and WITH overlays a second group (PLOT MILK FOR LACT>0 <summer calvers> WITH LACT>0 <winter calvers>).
Conception rate by month of breeding with BREDSUM\C FOR LACT>0 (by calendar month) to show seasonal fertility (see Conception rate).

Use a lag: compare milk to THI averaged over the previous 1–3 days, not the same day.

Typical range

Herd context: US Holsteins, freestall or dry-lot, high-producing (above about 75–80 lb/cow/day), US Southwest and Southeast.

THI	Interpretation
< 68	No heat stress for most cows
68–71	Onset of heat stress in high producers; milk loss begins
72–79	Mild to moderate heat stress (older threshold of 72 for onset)
80–89	Moderate to severe; large intake and milk losses
≥ 90	Severe; risk of death without cooling

The 68 onset comes from Zimbelman et al. 2009: in high-producing Holsteins (about 100 multiparous cows across 8 studies in an arid climate), milk fell by about 2.2 kg/day once average daily THI reached 68. Earlier work, used with lower-producing cows, put the onset at 72 (e.g. Ravagnolo et al. 2000, first-parity Georgia Holsteins). A peer-reviewed summary (Ouellet et al. 2021) gives the arid-climate milk-loss threshold as THI 68–72 depending on production level and notes that thresholds vary between studies and climates.

Milk loss: often quoted at about 0.4–0.6 lb/cow/day per THI unit above the threshold for high producers. Ravagnolo et al. 2000 found test-day milk fell about 0.2 kg (0.44 lb) per THI unit at THI ≥ 72 in first-parity Georgia Holsteins averaging 26.3 kg/day below that threshold; fat and protein yields fell 0.012 and 0.009 kg per unit.

Night recovery: cows recover when night-time THI drops below about 64–65 for several hours. Hot nights (common in humid regions) matter more than the daily peak.

Fertility: summer conception rate in uncooled or poorly cooled herds often falls by 10–20 points versus winter.

Common mistakes

Using the wrong THI formula or input. °F plugged into a °C formula, or dew point used as RH, gives nonsense values.
Using daily mean THI only. Mean THI hides the hours above threshold. Track daily max and hours above 68.
Comparing same-day milk to same-day THI. Milk responds with a 1–3 day lag.
Using a distant weather station. Airport readings can differ a lot from conditions in the barn or pen; THI in a dry lot with no shade is higher.
Blaming heat for every summer drop. DIM mix, ration changes and pen moves also shift summer milk. Compare at similar DIM.
Ignoring dry cows and heifers. Cooling only the lactating barn leaves later losses in next lactation and in calf performance.
Not defining the summer window. Say which months count as summer.