Heat Stress Impacts on Lactating Cows Grazing Australian Summer Pastures on an Automatic Robotic Dairy

SIMPLE SUMMARY: Heat stress (HS) is a major challenge for sustainable livestock production, which compromises animal welfare and performance during the hot summer months, leading to multibillion-dollar losses to the global livestock industries.

In this study, we investigated the effect of summer heat events on lactating Holstein Friesian cows at the Robotic milking farm of the University of Melbourne in Victoria, Australia.

We followed the cows during the entire summer period (December 2018–February 2019) and measured the effect of high temperature and humidity on physiological variables such as respiratory rate, panting scores and body temperature.

All these parameters were significantly affected by high-temperature-humidity conditions with a significant drop in milk production.

Most cows stopped grazing, sought shade, panted and spent more time at the watering points.

These indicate that lactating dairy cows grazing summer pastures experience severe HS, compromising their welfare and leading to some changes in behaviour such as suspension of grazing and jumping into water troughs.

However, the quantum of production losses, though significant, can be reduced by the provision of shade and water for the cows to cool down, as was observed in this study where the production losses were lower than previously reported in heat stress studies.

ABSTRACT: The objective of this study was to measure the impacts of summer heat events on physiological parameters (body temperature, respiratory rate and panting scores), grazing behaviour and production parameters of lactating Holstein Friesian cows managed on an Automated Robotic Dairy during Australian summer.

The severity of heat stress was measured using Temperature-Humidity Index (THI) and impacts of different THIs—low (≤72), moderate (73–82) and high (≥83)—on physiological responses and production performance were measured.

There was a highly significant (p ≤ 0.01) effect of THI on respiratory rate (66.7, 84.7 and 109.1/min), panting scores (1.4, 1.9 and 2.3) and average body temperature of cows (38.4, 39.4 and 41.5 °C), which increased as THI increased from low to moderate to high over the summer.

Average milk production parameters were also significantly (p ≤ 0.01) affected by THI, such that daily milk production dropped by 14% from low to high THI, milk temperature and fat% increased by 3%, whilst protein% increased by 2%.

The lactation stage of cow had no significant effect on physiological parameters but affected (p ≤ 0.05) average daily milk yield and milk solids.

Highly significant (p ≤ 0.01) positive correlations were obtained between THI and milk temperature, fat% and protein% whilst the reverse was observed between THI and milk yield, feed intake and rumination time.

Under moderate and high THI, most cows sought shade, spent more time around watering points and showed signs of distress (excessive salivation and open mouth panting).

In view of the expected future increase in the frequency and severity of heat events, additional strategies including selection and breeding for thermotolerance and dietary interventions to improve resilience of cows need to be pursued.