May THI Summary – What Did Dairy Cows Experience?

As we launch into summer, May’s shed-based THI data reveals a story of early heat spikes, cooler recoveries, and subtle stressors that may already be affecting herd performance. Using real-time in-shed data collected weekly across the UK through THI-live.com, here’s what May showed us:

Key Observations

• **Early May Heat** The start of May brought a surprise spike: in-shed temperatures hit 30.2°C in southern England and 28.7°C in Wales — levels more typical of mid-summer. Thankfully, cooler night-time conditions helped keep 24hr average THIs in check, reducing the risk of prolonged heat stress.
• **Cooler Averages – But Stressful Moments**: Despite daytime highs in the mid-20s°C, 24hr average THIs mostly stayed within the 58–63 range, which is generally comfortable for cows. However, THI 65+ days were recorded in Wales, the Midlands, and the South — particularly in weeks 1 and 4 — a level where fertility begins to be affected.
• **Humidity’s Hidden Impact**: In week 3, the role of humidity became clear. The South of England hit 28.4°C (THI 74, RH 35%), while mid-Wales recorded THI 77 at just 27.9°C, due to higher relative humidity (RH 58%). This underscores why THI — not temperature alone — is key in understanding heat stress.
• **Behavioural Changes May Have Already Started**: Though hours above THI 67 were mostly low, Wales recorded 27% of hours over this threshold in mid-May. It’s also likely we saw changes in respiration rates and water intake patterns during hotter spells.

What this means on farm

• Average conditions were mostly manageable, but early THI spikes and isolated high-humidity events could still impact conception rates — especially in high-producing herds or those in southern and western regions.
• Cows don’t just feel heat — they remember it. A few hot days early in the season can prime animals for stress later on. And as night-time temperatures rise, the risk will only grow.

Regional Heat Stress Trends (Early Summer Snapshot)

THI data shows clear regional differences in early-season stress exposure:

• South: Highest exposure so far — 31.5 hours above THI 67, peak of 28.9°C
• Wales: Close behind, with 45+ hours over threshold and highs of 28.6°C
• Midlands: Gradual increase, with 24+ hours above THI 67
• North & Scotland: Lower overall, but still peak THIs of 73–75 recorded

No region has seen a full day over THI 68 yet, but subclinical heat stress is building, especially in the South and Wales.

Looking Ahead: Prepare for Warmer Nights

Cool nights helped cows recover in May, but warmer night-time temperatures in June will push up 24hr averages, making it harder for cows to cool down. Early intervention now can safeguard performance and welfare during the peak season ahead.

Heat Stress Support from Galebreaker

• ✅ Live THI data via THI-live.com
• ✅ Annual reporting package — just £350, includes a THI sensor
• ✅ Monthly Heat Maps linking THI exposure to fertility, yield & health outcomes

More information about heat stress can be found on our webpage and to sign up for weekly THI updates just leave your details here.