What strategies can be implemented to maintain their thermal neutrality?
Thermal stress in dairy cattle significantly impacts during these hot periods. However, physiologically, animals have two ways to adapt to heat peaks.
To cope with heat waves, dairy cows can naturally regulate their temperature in two ways:
1 Increase heat dissipation through:
- Evaporation, increasing subcutaneous blood flow, respiration, salivation, etc.
- These activities increase energy needs for maintenance by 20% at 35°C.
- This means that part of the energy intended for production is diverted to thermal regulation.
2 Reduce heat production
- Reduction of all activities and changes in feeding patterns. (Heat production in cattle is mainly due to ruminal fermentations, so cows will reduce their dry matter intake by 10-30%)
- Dry matter intake will decrease, as will milk production.
Is a decrease (even if significant) in dry matter intake sufficient to explain reduced productivity during heat stress?
No, a more accurate understanding of the biological reasons why stress reduces productive performance will allow us to have a clearer idea of how to alleviate it.
As shown by studies from Rhoads et al., 2009, Wheelock et al., 2010, and Baumgard et al., 2012, during thermal stress in dairy cows, cows lose much more milk (about 45%) compared to cows not under thermal stress but underfed, with the same amount of dry matter (about 19%). Consequently, the reduced dry matter intake is only part, about 50%, of the causes of reduced milk production.
Consequently, the reduced dry matter intake is only part, about 50%, of the causes of reduced milk production.
The remaining 50% is attributed to metabolic changes that heat stress induces in the animal: Despite the negative energy balance and weight loss, fat tissue is not mobilized (unlike what can happen during transition and calving).
Indeed, severe thermal stress in dairy cattle induces extraordinary changes both in the distribution of nutrients (no longer destined for milk production but for defense against heat) and endocrine changes, with significant increases in epinephrine, norepinephrine, but especially cortisol.
Cortisol levels, during periods of thermal stress, will be 10 times higher than normal, with a significant series of problems, including health issues:
- Cortisol decreases protein synthesis and inhibits oxytocin release (resulting in difficulty during milking);
- Cortisol limits white blood cell function and replication, reducing protection against potential infections;
- High cortisol levels can also cause failure or delay in ovulation, reducing the animal’s fertility.
Thermal stress in dairy cattle: how is the NEFA deficiency explained?
As already mentioned, despite thermal stress in dairy cattle involving a significant reduction in dry matter intake and weight loss, it does not influence fat tissue mobilization.
The unusual deficiency of NEFA in heat-stressed dairy cows is partly explained by the high levels of circulating insulin, as insulin is a potent anti-lipolytic hormone.
As seen in Figure 1, under conditions of underfeeding but not heat stress, there is a normal production of propionic acid in the rumen, which reaches the liver and is converted into glucose, which in turn goes to the pancreas, stimulating moderate insulin production;
a lot of glucose is directed to the mammary gland for milk production.
Additionally, there is normal mobilization of fat tissue with the release of NEFA, destined both to return to the liver for glucose production and to increase fat content in milk and provide energy to muscle tissue.
In a situation of thermal stress in dairy cattle (Figure 2), however, ruminal activity decreases with limited production of propionic acid and consequently less glucose produced by the liver.
Part of this glucose goes to the pancreas and part goes to the muscle instead of the mammary gland (here is the significant production difference!);
Additionally, the high production of insulin inhibits fat mobilization, resulting in decreased fat in the mammary gland (and in milk), and reaching the muscles promotes mobilization of “gluconeogenic” amino acids, increasing circulating urea levels.
To summarize, thermal stress in dairy cattle results in:
1. High levels of insulin
2. Decreased levels of NEFA
3. Glucose directed to muscle
Consequently, animals need more energy, especially GLUCOSE.
Supplementing with hexose sugars like sucrose, dextrose, fructose, or glycerol (even though it is an alcohol) becomes a valuable aid during this delicate time of the year to address thermal stress in dairy cattle.
The sugars contained in forages are not always sufficient because they are pentoses, which are poorly digestible (no more than 50%).
To discover all the products for dairy cattle and to tackle thermal stress in dairy cattle, visit the dedicated section on the website.
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