In Brief:

  • Heat stress increases maintenance requirements, reduces feed intake and pushes cows
  • into negative energy balance.
  • Heat-stressed cows have increased risk of acidosis, poorer fertility and produce less milk.
  • Megalac increases ration energy density without increasing acid load in the rumen.
  • Megalac is not fermented in the rumen and has a high NEL value (27.3 MJ/kg DM) so
    minimal heat is generated by its metabolism – Megalac is a ‘cool’ ingredient.


Heat stress can have major detrimental effects on animal health and productivity and poses particular challenges to herds in ‘hot’ regions. As well as temperature, relativehumidity has a significant effect on heat stress, with the temperature-humidity index (THI) often used to indicate the degree of stress on dairy cattle see (Appendix 1). Optimal temperature for producing animals is determined by their ‘thermoneutral zone’, which for lactating dairy cows is between 5°C and 20°C. This represents the temperature zone within which no additional energy above maintenance is expended to heat or cool the body.

Can Megalac play a role in reducing the effects of heat stress ?

Megalac is commonly used as a strategic ingredient to help maintain herd performance under heat stress conditions. This bulletin summarises the major effects of heat stress on dairy herds and highlights how Megalac can play a beneficial role in countering these effects. Megalac is a ‘cool’ ingredient.

Effects of heat stress on productivity

The effects of heat stress on animal performance are well-documented, with some of the main effects as follows :

1. Reduced dry matter (DM) and energy intake

temperatures above 25°C, with feed intakes at 40°C typically reduced by 20 to 40% compared to cows in a thermoneutral environment. As a consequence, cows under heat stress are generally in a state of negative energy balance.

2. Reduced milk production

The reduction in DM intake, coupled with increased energy demands for maintenance
(see later), contribute to the negative effects on milk output recorded with cows under
heat stress; studies have reported a reduction in milk yield of 25 to 35% where heat
stress conditions have been evaluated.

3. Acidosis

Increased acidosis is a common factor associated with heat stress, reflecting the reduction in rumination time with lower feed intake and hence reduced production of bicarbonate-laden saliva to act as a rumen buffer (Figure 1). Drooling also results in loss
of the key saliva buffer while increased respiration rate causes increased bicarbonate production to maintain constant blood pH, further reducing the quantity available forrumen buffering.

Effects of heat stress on productivity

Figure 1 Summary of heat stress effects on acidosis

4. Reduced fertility

Heat-stressed cows can have significantly reduced reproductive performance, with studies reporting conception rates of less than 10% during hot months of the  year, compared to 40 to 50% in cool weather. The major consequence of heat stress on reproduction is a delay in re-breeding of cows after calving, resulting from, inter alia, a decrease in the number of cycling cows detected in oestrous and a decrease in the number of inseminated cows that establish and maintain a pregnancy. Problems associated with heat stress are exacerbated in high producing animals – these animals have greater metabolic activity and produce more body heat than low producers
and hence are more susceptible to heat stress.

Animal mechanisms to reduce heat stress

As environmental temperature increases, the animals’ physiology adapts to facilitate increased heat loss. These adaptive thermoregulatory response mechanisms have been extensively reviewed and include sweating, increased respiratory rate and active panting to help dissipate
heat through breath. However, such active processes require energy, and mild to severe heat stress is estimated to increase maintenance energy requirements by 7 to 25%, respectively. Heart rate also increases and greater vasodilation occurs with redirection of blood flow from
internal to peripheral tissues to facilitate heat loss to the environment.

Feeding to reduce heat production

Digestion and metabolism of nutrients in ingested food is associated with the production of heat. This heat, termed the heat increment, is generated from a number of sources including the work associated with the digestion and mastication of food, the heat produced during ruminal fermentation, and the work of nutrient metabolism (inefficiencies in the conversion of nutrients to ATP). Hence, metabolisable energy (ME) contained within feeds is converted to the net energy used for maintenance of the animal, and that contained within animal products (e.g. milk, tissue, foetus) with an efficiency of less than 1.0, with the inefficiencies corresponding to metabolic heat production. Fibrous feeds result in larger increases in heat production than concentrate feeds due to factors including the greater work of digestion and increased activity of the highly metabolically-active liver. Similarly, diets yielding high levels of acetate are utilised lessefficiently than those resulting in higher levels of propionate.

How can Megalac help reduce heat stress ?

In contrast to other nutrients, dietary fat undergoes minimal fermentation in the rumen and is used with much greater efficiency (i.e. lower heat production). Research work has determined the net energy value of lactation (NEL) of Megalac to be 27.3 MJ/kg DM based on in vivo studies with dairy cows. This indicates that ME in Megalac is converted to NEL with similar efficiency to that generally accepted for utilisation of mobilised body energy in lactating cows (0.82). This is substantially above the mean efficiency of use of common diets (0.64), such that heat production associated with digestion and metabolism of Megalac is considerably less than with other dietary ingredients. The high ME of Megalac (33.3 MJ/kg DM) also enables high energy density diets to be formulated to help ensure energy supply is increased even when total DM intake is low, thereby helping to improve the energy status of the animal. Furthermore, as a rumenprotected fat, Megalac does not add to the acid load in the rumen so is a ‘safe’ ingredient under heat stress conditions where acidosis is a particular risk.
These data suggest that reformulation of diets to incorporate Megalac is an important method of increasing energy intake under heat stress conditions, while also reducing the metabolic heat load imposed on the animal due to its properties as a ‘cool’ ingredient.

Appendix 1

  • Maintenance requirements of cows can increase by 25% in heat stress conditions.
  •  Heat stress can substantially reduce DM and energy intake, pushing cows into
    negative energy balance.
  •  Studies report reductions in milk yield up to 35% and conception rates of around 10%
    for cows under heat stress cf. those in thermoneutral conditions.
  •  Acidosis is a major risk factor for heat-stressed cows, primarily due to reduced
    rumination and lower production and availability of salivary buffer.
  • Megalac enables formulation of higher energy diets to supply increased energy
    without increasing risk of acidosis.
  • The proven high NEL of Megalac (27.3 MJ/kg DM) demonstrates the low heat
    increment associated with Megalac, helping reduce internal heat production.

Appendix 1

Temperature-Humidity index chart (University of Arizona)

Leave a Reply

Your email address will not be published. Required fields are marked *