As I am writing this article, it seems to me that heat stress may not be at the forefront of your minds, but it may be more prevalent than you think. At what temperature do you think cows begin to exhibit signs of heat stress?
Historically, ambient temperatures in the UK would not have been a limiting factor for summer milk production. However, there is an increasing body of evidence to suggest that even relatively low summer temperatures can negatively impact performance of our dairy herds.
Heat stress is known to have significant detrimental effects on milk production and quality, reproductive performance and immune function. Many of us will have had to live with the consequences brought about by our unusually hot UK summer last year, such as declining reproductive performance (including increased early embryonic losses), lower than anticipated milk yields and higher incidence of mastitis. So, are we all just getting hot under the collar for no reason or are their practical options that we can consider in order to help our cows better manage rising temperatures in the field and/or when housed?
Cow comfort zone
Dairy cows have a finite comfort zone within which they are happiest. At either end of this comfort zone are critical temperatures at which the cow becomes stressed and performance is reduced. Above the Higher Critical Temperature cows attempt to lose heat by sweating, but because they sweat at only 10% the rate of a human, they are much more susceptible to heat stress. The ideal ambient temperature for a dairy cow is between 41 and 77°F (5 – 25°C), however this range is affected by many variables, including liveweight, breed, feeding level and level of production. Therefore, at temperatures of 20°C or greater a cow is susceptible to heat stress.
Why do cows prefer to be cool?
Dairy cows are homeothermic animals and need to maintain a constant body temperature of around 38.6֯C (+/- 0.5֯C) for efficient metabolism. To maintain this temperature, the heat generated from normal rumen digestion and metabolism must be lost to the environment. When that environmental temperature is high, this heat is lost more slowly, causing the cow’s body temperature to rise, resulting in heat stress. To dissipate body heat, dairy cows react by reducing feed intake (typically by 10-15%, but as much as 30%) and rumination time, are selective in what they eat (forages increase rumen activity and therefore heat production so cows will seek out concentrates or feeds that produce less heat), increasing respiration rate (>80 breaths per minute), standing time and water intake (by up to 20% and can drink up to 20L per minute!), excessive salivation, drooling and panting. This leads to a suboptimal rumen environment and function: lower rumen pH, lower volatile fatty acid and microbial protein production and nutrient digestibility. Requirement for maintenance increases as the cow attempts to lose body heat.
Heat stress is a result of a complex relationship between temperature and humidity. The Temperature Humidity Index (THI) is used to determine occurrence and severity of heat stress. Do you know the ambient temperature and relative humidity in your sheds? Figure 1 shows the amount of time we spent above the heat stress threshold in the South West last summer.
Figure 1: South West Temperature Humidity Index for May to August last year
SOURCE: Cargill, CoolCow Weather Date Sheet, South West
Symptoms of heat stress
- Cows will become lethargic and will often stand with their heads lowered
- Cows will stand for much longer, favouring that over lying down
- Cows will often pant to increase heat loss
- Surprisingly, cows suffering heat stress will often move close together and stand in tightly packed groups
- Respiration rates will also increase (>80 breaths per minute) as cows attempt to increase their heat dispersion
Why are cows so sensitive to heat stress?
Metabolic heat production goes hand in hand with high milk production! High-yielding cows generate more heat than dry cows irrespective of ambient temperature. A cow yielding 18 litres/day will generate 28% more body heat than a dry cow. A cow yielding 31 litres/day will generate 48% more body heat. Broadly speaking, each cow produces the same heat output as a 1.4kW electric heater (NADIS). There is evidence to suggest that heat stress is most apparent when it comes in short bursts with no time for the cow to adapt to rising temperatures i.e. the UK!
Implications of heat stress on the dry and transition cow
Heat stress during the dry period impairs mammary gland development and alters metabolism in the dry and transition cow, in turn reducing subsequent milk yield. There is some evidence to suggest that a dry cow experiencing heat stress will give birth to a lower birth weight and lower weaning weight calf and may have a reduced milk production compared to their ‘cool’ counterparts (Tao et al., 2016). There is also a theory that heat stress might have negative repercussions on dry and transition cows for three generations: the cow herself, her growing calf and potentially the eggs contained within that growing calf – so please think about the environment for your dry and transition cows too!
Practical hints and tips to better manage heat stress:
- Provide cool, clean, fresh water always
- Ensure water flow rate is adequate for peak demand
- Ensure that you have the flexibility in your grazing platform to offer shady fields and/or housing to escape the direct sun
- Breed and animal selection – Jersey and light-coated colours
- Clipping the cows to help and increase evaporative heat loss
- Try to minimise the amount of time cows are gathering together in the collecting yard
- Avoid moving cows in the heat of the day
- Consider the use of additives such as a live yeast, rumen buffer, niacin, fat soluble vitamins and ensure that these additives are fed for up to four weeks following the period of heat stress as cows will need time to adapt
- Provide high quality, digestible feeds to reduce heat production from rumen fermentation
- Consider feeding out twice per day (if buffer feeding or housing cows) to keep feed fresh and consider time of feedout to avoid the hottest parts of the day
- Minimise the number of roof lights (especially those South facing) – they can increase heat within a building