Heat stress could cost the dairy producer up to 400 €/cow/year (Saint Pierre et al., 2003). Around 80% of these losses are associated with drop of productivity, and 20% with health issues: impaired reproduction and immunity, which translate into increased mortality and mastitis frequency etc. In 2011, Prof. Burgos Zimbelman and Collier, from University of Arizona revised the heat stress scale and revealed that the severity of heat stress was largely underestimated, especially for high producing cows, and consequently lowered the heat stress threshold. It appears today that heat stress is more common than previously thought and can impact dairy production even under temperate climates. We will see how to assess heat stress in practice before focusing on some nutritional approaches to limit its impact on performance.
How to diagnose heat stress?
It is generally admitted that cow’s comfort zone is between 5-20°C. But external temperature is not the only parameter: relative humidity is also important to define the cow’s comfort zone. Moreover, a cow’s thermo-neutral zone is dependent upon its physiological status and level of production: higher producing cows are more sensitive to thermal stress. It is referred to heat stress when environmental conditions exceed the cow’s thermal zone of comfort (Armstrong et al., 1993). There are two types of parameters to diagnose heat stress in dairy cows, either based on animal observations, or on environmental signs:
In practice, cows subject to moderate heat stress (around 25°C and 50% relative humidity) show visible signs such as:
- reduced milk production (around 10%)
- decreased feed intake
- shallow breathing
- profuse sweating
- lethargic behavior
- open mouth
- breathing with panting and tongue hanging out.
Under severe heat stress (for example at 34°C and high humidity), the milk production drop can reach 35% and feed intake is severely reduced.
The environmental indicator of heat stress risk is the Temperature Humidity Index (THI). It takes into account the combined effects of environmental temperature and relative humidity. Each THI value has been linked to a level of stress, associated to potential damages on cow’s production and health status. The THI scale has been revised in 2011, by Prof. Burgos Zimbelman and Collier from University of Arizona: they found out that the impact of heat stress was underestimated for today’s high producing dairy cow. Indeed, THI scale was established in the 1960s, when cows were less productive than today. Increased milk potential has contributed to increased cattle sensitivity to thermal stress and reduced the threshold THI at which milk losses occur. According to the new THI scale, a THI of 68 is low enough to cause adverse affects (Fig. 1). This corresponds for example to 22°C ambient temperature with 45% of humidity, a rather common condition in moderate climates, even in northern Europe for example.
In practice :
- A thermo-hygrometer can easily be used to quickly evaluate the level of risk of heat stress.
- Historical weather data, such as those recorded on the internet (e.g. www.wunderground.com) can also help evaluate heat stress risks and possible impact.
Figure 1: University of Arizona revised heat stress scale (2011): to each temperature/humidity couple corresponds a level of thermal stress for the dairy cow. Legend: yellow: Stress Threshold; Orange: Mild-Moderate Stress; Red: Moderate-Severe Stress ; Purple: Severe Stress (From Burgos Zimbelman and Collier, 2011).
The table illustrates the impact of the various heat stress levels on milk production with some practical examples.
Impact of heat stress on dairy productivity and health
Cows respond to heat stress by increased sudation, panting, drooling etc. to help cooling their body. These activities increase the animal maintenance energy, and part of milk production energy will be redirected to thermal regulation. Furthermore, heat production in dairy cow is essentially due to rumen fermentations. In order to reduce heat production, all activities are reduced and feeding pattern is altered. Feed intake and rumination decrease, cows will tend to eat less during the day, but more often and in small quantities. They will tend to consume more feed at night when it is cooler, slug feed, sort feed and tend to choose feeds that produce less heat during digestion, choosing grains and proteins over forages.
Moreover, heat stress generally increases the production of free radicals, leading to oxidative stress. In dairy cows, oxidative stress has a negative impact on immune and reproductive functions: increased mastitis frequency and higher somatic cells count in milk, decreased fertility, increased embryo mortality, post-partum retained placenta, and early calving, with consequences on the calves live weight, mortality and health.
The first short-term impact of heat stress is the reduction of milk yield. Reduced milk yield results from a combination of reduced feed intake, alterations in endocrine profiles, energy metabolism and other unidentified factors. It is usually acknowledged that milk production can decrease between 10 to 35% during warm summer months.
Consequences at longer term will be linked to the effects of heat stress on cows health, immune function (mastitis risks and increased somatic cell count in milk), reproduction, and even death. A 1999 study (Hansen and Aréchiga) showed that conception rate dropped under severe heat stress in dairy herds. It is currently estimated that conception rate decreases by around 50% under heat stress.
These effects are linked to the impact of heat stress:
- On oxidative stress,
- On the decreased buffer capacity of the rumen, due to reduced efficient fiber intake and drooling. Under heat stress, subacidosis risks are increased, with further consequences such as lameness etc.
In 2003, Saint Pierre et al. evaluated the overall financial consequences of heat stress for the US dairy industry. This study analyzed data for each of the 48 American states and showed huge discrepancies according to climate and heat stress level, with overall losses close to 700$/cow/year for the most affected states (Texas, Florida). Based on milk loss alone, they have calculated overall annual losses of $897 million, equivalent to almost $100 per dairy cow per year on average. To this must be added the costs linked to reproduction, early culling, health issues and mortality.
Nutritional management of heat stress
Appropriate nutritional programs are important to help limit the impact of heat stress. One of the goals should be to improve feed efficiency to compensate for the reduced feed intake, while protecting the rumen environment from acidosis risks.
Since cows will be consuming less as temperatures rise, increasing the energy density of the diet can be considered to compensate in part for the decreased DMI. High quality, highly digestible and palatable forage should be available to the animal if possible. More starch or added fat can be useful too.
It must also be kept in mind that rumen balance is stressed, cows are at higher risk of acidosis and extra care should be taken to manage the rumen. Feeding of a high quality fiber source in the diet that helps maintain a stable rumen, but still contributes energy rather than just gut fill, is therefore essential, especially for high-producing herds receiving high starch diets.
Moreover, the use of rumen modifiers which improve and protect the rumen environment, such as rumen specific live yeast is also advisable. In the following chapter a recent trial conducted under severe heat stress condition is detailed, which confirms the benefits of live yeast Saccharomyces cerevisiae I-1077 to limit milk production loss.
Another important aspect it to properly balance the mineral electrolytes in the milk cow diet since excessive sudation, (sweating), leads to losses of sodium and potassium,
Finally, since the increased respiration rate induced a higher production of reactive oxygen substances (ROS), it is recommended to increase antioxidant intake such as selenium, Vitamin E etc. In the case of selenium, in particular, it has been shown that dairy cow supplementation with organic selenium (supplied through selenium enriched yeast) can help reduce somatic cells cont in milk more efficiently than mineral selenite (Fig. 2; Malbé et al.,1995).
Figure 2: Selenium yeast, which is more effective than inorganic selenium for raising blood selenium level, has the ability to reduce SCC in milk (Malbé, 1995).
Live yeast S. cerevisiae I-1077 minimizes the impact of heat stress on rumen and feed efficiency of high producing dairy cows
Rumen specific live yeast Saccharomyces cerevisiae I-1077 has been selected and documented for its interaction with the rumen microflora and its positive effect on rumen environment and function (stabilization of rumen pH, increased fiber degradation leading to better feed efficiency…). It is recognized by dairy nutritionists as a natural, safe and effective rumen modifier, registered in many countries as dairy feed additive.
A meta-analysis that encompassed 14 trials in various production systems (1 600 dairy cows in total), shows that, under non-stressful conditions, S. cerevisiae I-1077 significantly improved Feed Efficiency by +3% on average (De Ondarza et al., 2009). Field trials indicate that this particular live yeast is very effective under conditions that exert stress on the rumen environment, such as feed transition or heat stress. From New York State to China (Shangaï), trials conducted under heat stress conditions have shown a positive effect on feed efficiency, resulting in increase of Income over feed cost by up to 9% during summer period.
Based on these results and the abundant literature about the positive effects of S. cerevisiae I-1077 on rumen function, the Department of Animal Sciences of the University of Florida (Gainesville, USA) conducted a trial in 2009 under well controlled conditions. The objective was to evaluate the impact of live S. cerevisiae I-1077 on both feed efficiency and rumen environment (pH) of high producing dairy cows under severe heat stress. Results of these trials were presented at the ADSA joint annual meeting in July 2010 (Marsola et al., 2010).
The trial was conducted between May and September 2009, under extreme heat stress conditions (THI=80 on average), on 60 dairy cows in total. The basal diet was: corn silage (41.1%), alfalfa hay (10.4%), brewer’s grains wet (5.2%) and grain mixture (43.0%).
The three-month study showed that dairy cow diet supplementation with S. cerevisiae I-1077 (Levucell®SC 20, Lallemand Animal Nutrition, France), at the stress-specific inclusion rate of 20 billion live yeast cell (CFU)/cow/day, lead to:
- Significant increase of feed efficiency by 7%, equivalent to an extra 120g of milk per kg DMI, bringing the producer a return on investment evaluated at 1:6 (Figure 3).
- Significant decrease of the number of cows at risk of acidosis: both rumen pH (Figure 4), and rumen lactate level, one of the causes of rumen acidosis, were reduced. 45% of cow had a pH lower than 5.8 in the control group versus 10.5 % of the cows treated with 1g/cow/day of LEVUCELL® SC 20.
Figure 3: Effect of supplementation of dairy cows diet with LEVUCELL® SC on feed efficiency under severe heat stress (Marsola et al., 2010).
Figure 4: Effect of supplementation of dairy cows diet with live yeast LEVUCELL® SC on rumen pH under severe heat stress (Marsola et al., 2010).
Two main modes of action
The live yeast strain Saccharomyces cerevisiae I- 1077 has been selected and extensively studied by leading international research institutes and universities for its modes of action in the rumen and benefits on rumen health and function.
Two main mechanisms can help explain its effects on dairy performance:
- Improved rumen pH: reduced acidosis risk
- Improved fiber digestion and nitrogen utilization: increased feed efficacy
It is easy to see how these effects can help counteract the detrimental action of heat stress, but also, of any stressful event for the rumen balance (feed transition, transportation etc.):
- Heat stress is a risk factor for rumen acidosis: S. cerevisiae I-1077 stabilizes rumen pH. This effect has been demonstrated under various conditions and with different diets (Bachet al; 2007, Thrune et al., 2009, Guedes et al., 2007).
- Feed utilization is decreased during heat stress: as shown by the Florida University trial, feed efficacy is improved during heat stress.
- This live yeast is proven to increase fiber degradation in the rumen, an effect shown for a variety of forages (Guedes et al, 2008; Chaucheyras-Durand et al, 2010). During heat stress, a better fiber degradation will lead to a better energy supply to the animal, minimizing the impact of decreased DMI and increased energy maintenance
Dairy production can be heavily impacted by heat stress and the recent reevaluation of heat stress threshold indicates that dairy cows could be affected earlier than previously suspected. the recent research on microbial natural solutions show that, when the best herd management practices are respected (housing and nutrition...), the additional help of these natural nutritional solutions could benefit the dairy producer by further helping to manage and limit the impact of heat stress on cows health and productivity: live yeast S. cerevisiae I-1077 helps to restore and maintain rumen function, improving rumen pH and feed efficiency during stressful conditions. Organic selenium restores and protects the animal antioxidant status with positive impact on milk hygiene, cow fertility and immunity.
The graph below summarizes major factors involved in heat stress.
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