CLIMATE CHANGE is leading to warmer Australian summers with an increased number of hot days and warmer nights.
This increases the risk of heat stress, leading to adverse effects on herd health and farm profitability.
Effects of heat stress
Reduced milk production (up to 40 per cent) is an obvious effect of heat stress in dairy cows but other losses include:
- Poorer herd fertility: Reduced intensity and length of oestrus, lower conception rates, increased risk of embryonic death, decreased in-calf rates/increased not-in-calf rates, decreased calf birth weight leading to poorer calf viability.
- Reduced milk quality components: Increased SCC from behavioural compensation and reduced protein.
- Loss of body condition score: Reduced feed intake and increased use of nutrients to maintain core body temperature.
- Poorer cow health and immune function: Hot and humid conditions can challenge the maintenance of rumen pH, increasing the risk of ruminal acidosis and ketosis. Strategies to cool down can increase the risk of opportunistic infections such as mastitis.
How do cows become heat-stressed?
Dairy cows need to maintain their core body temperature between 38.5°C and 39.3°C.
This core body temperature can change throughout the day, peaking in the early evening and reaching a low in the early morning.
In hot weather, dairy cows deal with heat load using a variety of strategies:
- Looking for areas with greater air movement or standing to increase exposure to air.
- Seeking water and shade.
- Changing their orientation to the sun.
- Panting or sweating.
- Stopping or reducing feed intake which decreases rumen heat production.
- Decreased feed intake (-10 to 20 per cent) when the air temperature is hotter than 26°C.
- Increased core body temperature.
- Altered blood hormone levels.
- Changed blood flow distribution — decreased blood flow to the gut, uterus and other internal organs; increased blood flow to the skin.
However, as the heat load increases, these physiological and behavioural strategies can no longer cope. It is important to recognise these subtle signs so that prompt action can be taken.
Prolonged heat stress can result in open mouth breathing, group seeking of shade and excessive drooling, by which point urgent attention is required.
Management of heat stress
Early management strategies are essential to reduce the short- and long-term negative effects of heat stress. An integrated approach works best and some strategies include:
- Milk cows later in the day: Cows milked two hours later in the afternoon can produce 1–2 litres more milk than cows milked mid-afternoon.
- Use of sprinklers: Sprinkling cows before the afternoon milking will cool cows down provided there is a breeze. It is important to provide a water droplet size which wets cows to their skin; providing a mist that sits on the coat surface will increase heat load by acting as an insulating layer around the cow.
- Nutrition: Feed a mixed ration of conserved feeds during the day (preferably in a shaded area). Allow access to good quality pasture at night to maximise pasture intake during the coolest part of the day.
- Provide shade: Natural shade trees, shade cloth and galvanised iron roofs have all been used. Research shows that cows usually prefer an iron roof. Any roofing system should be vented and be at a height of at least 3.7 m.
• Dr Gemma Chuck is an adviser with Apiam Animal Health.