Heat Stress increases maintenance energy requirements and lowers dry matter intake, making it difficult to meet energy needs. Therefore, feeding management and forage quality become critical during heat stress.
Heat Stress in Dairy Cows:
Implications & Nutritional Management (I) Dairy cows are very sensitive to heat stress, which can significantly impact economics for the producer, not only by lost productivity and milk quality (increased somatic cells count), but also by health-related problems. The producer is usually aware of essential and necessary herd management practices during this critical period; however, some nutritional solutions may not be as well known. In particular, probiotics, which, by improving rumen conditions and functions affected by heat stress, can help maintain the cow’s digestive health and overall productivity. During periods of heat stress, oxidative balance is also affected and it is very important to increase the antioxidant intake in order to preserve the cow’s reproductive health and immunity.
1. Defining heat stress and its implications:
The severity of heat stress is correlated to both the humidity level and the ambient temperature. The cow’s thermal comfort zone is approximately 12°C – 25°C. Within this temperature range, the animal comfort is optimal, with a body temperature between 38°C and 39°C (Lefebvre & Plamondon, 2003). Above 20°C the cow suffers from heat stress: its health status and general performance are affected.
Body temperature (rectal) >39.4°C Respiratory frequency >80 breaths per minute Dry Matter intake decreases – 10% = high stress; 25% = severe stress
2. The financial impact of heat stress:
Severe heat stress can result in substantial financial losses. An estimated 80% of these losses are associated with a loss of productivity, and 20% with health issues such as reproduction and immunity problems, which translate into increased mortality and mastitis frequency in particular.
3. A disrupted energy balance:
Cows have two ways of maintaining their thermal balance and regulating their body temperature under high heat (and humidity) conditions. They rely essentially on both: Favouring heat dispersion, in particular through evaporation, by increasing subcutaneous blood flow, panting, etc. These activities increase the maintenance energy needs of the animal by an estimated 20% at 35°F. In the case of a dairy cow, this means that part of her production energy will be redirected to thermal regulation. Limiting heat production, by reducing activity and changing her feeding pattern. Indeed, significant heat production in dairy cows results from rumen fermentation. The cow’s DMI can be reduced by 10- 30%. Also, rumination, which produces heat, decreases dramatically. Cows will tend to eat less overall during the day, but more often and in smaller quantities. They will tend to consume more feed at night when it is cooler, slug feed, sort feed and choose feeds that produce less heat during digestion, selecting concentrates over forages.
4. Acidosis risks:
In periods of heat stress, the risk of acidosis increases. Factors that can contribute to rumen acidosis problems may be: decreased DMI with a lower proportion of forage and higher levels of fermentable carbohydrates, decreased rumination, decreased saliva to the gut (a source of bicarbonate), with a reduction of its buffering power due to increased CO2 expelled (panting). Additionally, a decrease in rumen pH impairs fiber digestion efficiency: rumen fibrolytic bacteria are the most affected when rumen pH drops (below 6.0). All of these factors contribute to reduced feed efficiency, and consequently, milk yield, and often milk fat. Moreover, acidosis is proven to affect the cows’ overall health status, fertility and longevity. It is important to keep cows cool throughout the hot, humid summer months. We will have more on managing for heat stress in our next edition.
Nutritional Strategies to Alleviate Heat Stress
There are several nutritional strategies which can be used to mitigate heat stress in dairy animals. Increasing water availability to cows is one of most important strategy to alleviate heat stress. As feed intake is markedly decreases during heat stress, a common strategy is to increase the energy and nutrient densities (increased concentrates and supplemental fat) of the diet. In addition to the energy balance, reducing the fiber content of the diet is thought to improve the cow’s thermal balance and may reduce body temperature. However, increasing ration concentrates should be considered with care as heat-stressed cows are highly prone to rumen acidosis. The nutritional needs of the cow change during heat stress, and ration reformulation to account for decreased DMI, the need to increase nutrient density, changing nutrient requirements, avoiding nutrient excesses and maintenance of normal rumen function is necessary.
Water, the Forgotten Nutrient
Water is undoubtedly the most important nutrient for dairy cows subjected to heat stress. Milk contains about 87 percent water, and water is critical for dissipation of excess body heat. Moreover, water intake is highly correlated with milk yield and dry matter intake (Dado and Allen, 1994). The consumption of water increases sharply as the environmental temperature increases because of greater water losses from sweating and from water vaporization with more rapid respiratory rates (panting), both efforts aimed at increasing evaporative cooling for the cow. So, the normal water supply recommendations are inadequate in the summer. Water intake increases by up to 50% as the THI approaches 80. Supply unlimited clean, cool and fresh water under shade within easy walking distance for the cow. Water in tanks long distances from the feeding area, especially if tanks are not shaded or the area between the feeding area and the tank is not shaded may force the cow to choose shade over water, limiting performance. So, place extra water points close to where the cows spend most of the time.
If the cows are heat stressed, in spite of the use of management strategies designed to prevent it from occurring, then DMI will decline and the diet should be reformulated if the performance level of the cows is to be maintained. However, diet reformulation should only be used to manage the reduced DMI caused because the cows are heat stressed. Diet reformulation will not prevent heat stress from occurring. The best means to assess the extent of heat stress is to track the DMI of the cows directly. If this is not possible, then an alternative is to observe the respiration rate of the cows. Cows are heat stressed if their respiration rate rises above 75 breaths per minute. Fermentation of feedstuffs in the rumen creates heat. In a cool or cold environment this heat is beneficial by helping dairy cows prevent a decline in body temperature below the range of thermoneutrality. However, this heat of fermentation is not beneficial in a hot environment since it makes it more difficult for the cow to prevent its body temperature from rising above the range of thermoneutrality and becoming heat stressed. Reduced DMI associated with heat is almost certainly the body’s mechanism of reducing heat of fermentation simply by reducing the amount of feed to be fermented in the rumen. While heat stress will depress DMI, it has little impact on the amount of nutrients required to support a particular level of milk production. Thus, the nutrient density of the ration must be increased if milk production is to be maintained.
Increasing the amount of dietary fat has been a widely accepted strategy in order to reduce basal metabolic heat production. The heat increment of fat is less (up to 50%) than the forages, so it is seemingly a rational decision to supplement additional fat and reduce fiber content of the diet. Studies where fats have been fed to heat-stressed cows have shown inconsistent responses in improving milk production; some have improved milk production, and others have shown no response. Recently, Melo et al. (2016) reported that supplementation of palm oil significantly reduced rectal temperature and respiratory frequency, increased milk yield, reduced DM intake and increased feed efficiency in lactating cows. Similarly, Wang et al. (2010) observed that supplementation of saturated fatty acids improved milk yield and milk fat content and yield and reduced peak rectal temperatures in mid-lactation heat-stressed dairy cows. In contrast, Moallem et al. (2010) indicated that cows fed additional fat increased rectal temperatures and respiratory rates. Other researchers reported little or no differences in rectal temperatures (Knapp and Grummer 1991; Chan et al., 1997; Drackely et al., 2003).
Fiber Level in Ration
One common nutritional strategy involves reducing dietary fiber during an increased heat-load. However, adequate fiber in the diet is essential to maintain rumen health, and high-quality forage helps to maintain feed intake. NRC (2001) recommended that minimum dietary neutral detergent fiber (NDF) of 25% with the proportion of NDF from roughages equalling 75% of total NDF. However, its digestion and metabolism create more heat than compared to concentrates (Van Soest et al., 1991). Grant (1997) demonstrated that a roughage NDF value of 60% still provides sufficient fiber for production of fat-corrected milk. Cows fed diets containing NDF from soya hulls and cassava residue produced more 4% fat-corrected milk, lost less body weight, and had lower rectal temperatures (Kanjanapruthipong et al., 2015).
Protein Level in Ration
As feed intake is progressively depressed due to heat stress, it is necessary to increase the protein level of the ration (West, 1999). However, the proportion of the dietary protein that must be provided as rumen un-degradable protein (bypass protein) must be increased, since the net passage of microbial protein from the rumen declines with lower DM intake. The oversupply of rumen degradable protein will lead to its inefficient use in the rumen which in turn will require the animal to expend energy to convert this wasted protein (as nitrogen) to urea which will largely be excreted in the urine. The negative effect of increased dietary protein agrees with recent recommendations which suggest that addition of dietary crude protein is not helpful during heat stress (Arieli et al., 2006). How heat stress affects dietary protein requirements is not well defined and more research is needed in order to generate more appropriate recommendations.
Natural and synthetic antioxidants in the feed as well as optimal levels of minerals, principally selenium, help to maintain efficient levels of endogenous antioxidants in tissues. Selenium protects tissues against oxidative stress, as it is a component of the glutathione peroxidase (GPX) enzyme, which destroys free radicals in the cytoplasm. Calamari et al. (2011) reported an improvement in the preventive antioxidant systems in terms of the prevention system of free radical formation and chain breaking antioxidants in cows fed selenium-yeast, with a lower lipid peroxidation during hotter periods, when animals are subjected to more oxidative stress. Similarly, Thatcher (2006) reported an increase in immune-competence at parturition, an improvement in uterine health and second service pregnancy rate during the summer months in cows fed selenium-yeast prior to calving.
Chromium is a micronutrient that facilitates insulin action on glucose, lipid, and protein metabolism (Mertz, 1993). Spears et al. (2012) reported that heifers supplemented with increasing amounts of chromium had increased insulin sensitivity, suggesting that chromium plays an essential role in glucose metabolism in ruminants. Because glucose use predominates during heat stress, chromium supplementation may improve thermal tolerance or production in heat-stressed animals. In addition, supplementing heat-stressed early lactation dairy cows with chromium reduced the degree of weight loss, improved milk production, reduced plasma NEFA concentrations, and improved rebreeding rates (Soltan, 2010; Mirzaei et al., 2011). Dietary inorganic chromium supplementation in summer-exposed buffalo calves improved heat tolerance, immune status and potency of insulin hormone (Kumar et al., 2015). However, further research using varying concentrations and lengths of chromium supplementation should be done to determine the ability of chromium to alleviate the deleterious effects of heat stress in dairy animals.
Niacin (vitamin B3) is known to increase peripheral vasodilation to increase sweat gland activity in dairy cattle (Gille et al., 2008). Recently, Zimbelman et al. (2010) demonstrated that cows supplemented with encapsulated niacin at a dose of 12 g/cow/d during acute thermal stress had a lower core body temperature and increased sweating rates, which are adaptive mechanisms to allow for dissipation of more body heat to the surface area through peripheral or vasomotor function and/or increased sweating. This prevents some of the decrease in DM intake due to heat stress thereby improving milk production (Di Constanzo et al.,1997). Di Constanzo et al. (1997) reported that cows undergoing mild to severe heat stress fed encapsulated niacin at 12 and 24 g/cow/d had reduced skin temperatures and increased milk production during summer heat. Feeding encapsulated niacin had different responses on milk production and milk components depending on the degree of heat stress and/or, possibly, the stage of lactation of the cows (Zimbelman et al., 2013). However, Yuan et al. (2011; 2012) found that supplementation with 12 g/cow per day of the rumen protected niacin modified lipid metabolism but did not affect milk yield over the first 3 weeks of lactation or oxidative stress of transition dairy cows. Rungruang et al. (2014) also observed that supplementation of encapsulated niacin did not improve thermo-tolerance of winter-acclimated lactating dairy cows exposed to moderate thermal stress.
Dietary Cation Anion Difference (DCAD)
Dietary cation anion difference (DCAD) is the difference between certain dietary minerals nominated as cations (Na, K) and anions (Cl, S) on the basis of charges they carry and is usually measured as milliequivalents of (Na+K) – (Cl+S) per kilogram of DM (Sarwar et al., 2007). Wildman et al. (2007) stated that keeping the DCAD at a healthy lactating level (200 to 300 mEq/kg DM) remains a good strategy during the warm summer months. Animal productivity is influenced more by the difference between these cations and anions than their individual effects when fed as a sole independent mineral source. In addition, Shahzad et al. (2007; 2008) reported that a diet having DCAD 330 mEq/kg DM has promoted feed consumption, water intake and resulted in greater milk yield and milk fat in early lactating buffaloes.
Unlike humans, bovines utilize potassium (K+) as their primary osmotic regulator of water secretion from sweat glands. As a consequence, K+ requirements are increased (1.4 to 1.6% of DM) during the summer, and this should be adjusted for in the diet. Kumar et al. (2010) reported that supplementation of electrolyte relieved oxidative stress and improved cell mediated immunity in heat stressed buffaloes. In addition, dietary levels of sodium (Na+) and magnesium (Mg+) should be increased as they compete with K+ for intestinal absorption (West, 2002).
Rumen Fermentation Modifiers
Monensin (an ionophore) is a well-described rumen fermentation modifier that increases the production of propionate, which is predominate gluconeogenic precursor in ruminants and thus improve the glucose status of heat-stressed cows (Baumgard et al.,2011). The use of live yeast cultures (Saccharomyces cerevisiae), has been extensively used to enhance nutrient utilization in ruminant animals (Francia et al., 2008). Many studies with dairy cows (Schingoethe et al., 2004; Bruno et al., 2009; Shwartz et al., 2009) reported that yeast culture supplementation increased feed intake during heat stress. In addition, supplementation of yeast culture (Bruno et al., 2009; Singh et al., 2011) and other fungal cultures (Huber et al., 1985; Gomez-Alarcon et al., 1991; Higginbotham et al., 1993) decreased rectum temperature and respiration frequency during heat stress. Bruno et al. (2009) and Huber (1998) reported that the supplementation of increased milk production in dairy cows during heat stress.
Heat stressed animals are more likely to have lower reproductive and productive performance. Feeding high quality forages and balanced rations will decrease some of the effects of heat stress and will boost performance of the animals. Some nutritional management tips to manage heat stress are:
- Provide high quality feeds like total mixed rations
- Increase the frequency of feedings
- Feed during cooler times of the day
- Keep feed fresh as much as possible
- Provide high-quality forage
- Provide adequate fibre
- Use of by–pass proteins can enhance the milk yield and protein content.
- Intake of sufficient cool water is probably the most important strategy for animals to undertake during heat stress.
Steps dairy producers can take to manage heat stress include:
- Use high-quality forages, and increase the energy density of the diet to reduce gut fill.
- Consider the addition of high-fat feeds, bypass fats or lower fiber feedstuffs.
- Alter feeding times to allow more feed intake at night, when it is cool.
- Utilize total mixed rations to reduce feed sorting by cattle.
- Ensure an adequate source of cool, fresh water. As temperatures rise into the mid-90s, dairy cows will increase their water intake by as much as 50 percent.
- Consider adding a nutritional supplement to help support normal immune function.