Heat stress in dairy cows can be mitigated

Modern breeds of farm animals are highly productive, but they have a high basic metabolic rate making them more sensitive to the extreme heat. For example, milk production of dairy cows was doubled in the last 50 years and this requires more intensive ways for cooling in hot weather. Nowadays, high-productive milking cows are affected by THI values less than 72.

In warm blooded animals normal physiological processes require certain limits of ambient temperature. The temperature range in which farm animals feel comfortable and are able to give maximal productivity is called the thermoneutral zone. The mechanisms of body thermoregulation in young animals are not fully developed, meaning that their thermoneutral zone is narrower compared to adults.

Elevation of ambient temperature beyond the thermoneutral zone complicates maintaining normal body temperature and leads to changes in metabolism and heat stress. As a consequence, productivity, fertility and health status of farm animals are negatively affected.

The harmful effects of high ambient temperatures are enhanced by high humidity and insufficient air flow. The so-called temperature and humidity index (THI) is used in dairy cows to estimate the effective temperature based on ambient temperature and relative humidity values. When the THI exceeds 72, cows suffer heat stress.

Modern breeds of farm animals are highly productive, but they have a high basic metabolic rate making them more sensitive to the extreme heat. For example, milk production of dairy cows was doubled in the last 50 years and this requires more intensive ways for cooling in hot weather. Nowadays, high-productive milking cows are affected by THI values less than 72.

To maintain normal body temperature in heat stress conditions cows try to increase heat output by panting, sweating and dilation of blood vessels in the skin. At the same time, for reducing heat increment, cows decrease feed intake which is directly related to milk production. Higher respiration rates are related with higher losses of CO₂, leading to respiratory alkalosis. Thus, acid-base balance of the body is impaired.

Heat stress (HS) affects cows in all physiological phases. In dry cows it results in reduced blood flow to uterus, reduced placenta weight and birth of smaller calves with lower vitality. Even when the ambient temperature is back to normal range, milk yield in early lactation can be reduced by 10-12% as a result of impaired endocrine function of placenta and body during HS. High yielding cows, especially in peak lactation, maintain a very high metabolic rate and are more prone to heat stress than low yielding cows. Milk production can be reduced by 10-25% not only due to decreased appetite and feed intake, but because of lower levels of lactogenic hormones. In heat stressed cows dry matter intake is decreased by 8-12%, and milk yield reduced by 20-30%. In addition, endocrine status can be negatively affected together with reduced intensity and duration of oestrus plus impaired fertility and elevated early embryo mortality. Milk yield and quality can be affected as well, with reduced butter fat content and increased somatic and bacterial cell counts, most likely due to impaired immunity.

Excessive ambient heat can cause significant economic losses. According to a study of St-Pierre et al. (2003) the U.S. dairy industry lost between $897 – $1,500 million/year in revenue due to heat stress.

The following key management and nutrition actions can help to mitigate the negative effects of heat stress:
• Protect from direct sun radiation
• Increase ventilation rate in barns
• Apply cool water spray
• Reduce animal density
• Allow access to cool drinking water
• Use high quality dietary ingredients to improve nutrient content
• Ensure gradual and on-time adaptation to corrected diet
• Diet to be fed mainly during the cooler part of the day
• Increase feeding frequency with smaller portions.

Specific nutritional strategies to reduce the physiologic impact of heat stress, to support milk yield and health status of cows:
• Time and frequency of feeding. These two factors can influence feed intake. For appetite stimulation 60-70% of the ration should be fed overnight, when the ambient temperature is lower. Fresh feed portions should be provided after milking. Increased feeding frequency reduces the overall heat produced by the animal after ingestion. Offering smaller portions also reduced the risk of feed spoilage and keeps if fresher.

• Forage/concentrate ratio. High production dairy diets normally have a roughage (forage) content of 45-60% of total DMI, providing a significant part of dietary fibre. More heat is produced as a result of the fermentation of more fibrous diets and more acetate is produced, compared to diets with higher levels of grain were starch is fermented to propionic acid. Hence, it seems logical to reduce ruminal production of acetic acid at the expense of propionic acid by reduced proportion of roughages in the diet. However, such transition should be done carefully by maintaining sufficient levels of physical effective fibre and Neutral Detergent Fibre (NDF) and preventing concentrates exceeding 60% of DMI. There is an increased risk of sub-acute ruminal acidosis (SARA) if forages drop too low resulting in impaired rumen function and further depressions in DMI and milk butterfat. Forage and fibrous feeds should be of the highest quality and harvested at the correct maturity stage. Highly digestible roughages stimulate DMI while reducing heat increment and therefore aiding thermoregulation. Brewers grains, cotton seeds and beet pulp are recommended ingredients in situations during heat stress. Their nutrient content favours a more balanced fermentation pattern in the rumen.

• Fat level. Increased fat addition in the diets of dairy cows during heat stress is recommended due to the fact that less heat is released during their metabolism compared to carbohydrates and proteins. This contributes to a reduction in heat production and it is beneficial for maintaining normal body temperatures. Supplementing with fats increases energy density of the diet, partially compensating for reduced DM and energy intakes. Total fat content should not exceed 6-7% dietary DM content however. In practical diets fat can be added in the form of oil seeds (sunflower or cotton) or ruminally protected fats. Oil seeds should be provided whole to prevent fast release of free oil which in turn can suppress rumen fermentation.

• Nitrogen balance. In conditions of extreme heat cows might experience negative nitrogen balance due to reduced feed intake and higher excretion of nitrogen. This results in reduced milk production. The target should be improving nitrogen balance but not simply by elevated dietary crude protein (CP) levels. There needs to be a good balance of rumen degradable and undegradable protein. Excess degradable protein can release too much rumen ammonia that cannot be fully captured by rumen microbes and the liver can struggle to deal with this. A study during hot temperatures showed that cows fed diets with 16% CP with a low degradable protein content had 1.5 kg higher milk yield compared to animals fed a diet with 18% CP which was highly degradable (Higginbotham et al., 1989). Correct nitrogen balance in the rumen can be achieved by introducing ingredients with lower protein degradability: brewers grains, distilleries dried grains and soluble (DDGS). When feeding some co-products in fresh moist form, spoilage can be an issue in hotter climates and needs to be taken into account. Spoiled/mouldy feed should be carefully separated and never fed to animals.

• Electrolyte balance. The Electrolyte Balance (EB) of the ration is considered as another factor that can help to mitigate the harmful effect of heat stress in milking cows. The final dietary EB depends on the content of ions Na⁺, K⁺, Mg²⁺, C^1-, S^2- & P^2- and can be manipulated by the restriction of one type of ions and supplementation of of desirable electrolyte minerals. It is recommended to maintain the following concentrations of minerals as % DM: К – 1.3-1.5; Na ≥ 0.5; Mg ≥ 0.35 & chlorine ≤ 0.35. Higher levels of K and Na should compensate for greater losses due to sweating compared to chlorine. Excessive chlorine levels may depress DMI and milk yield, respectively. The elevated levels of K can suppress Mg absorption in the rumen, requiring a higher Mg intake. In conditions of heat stress increasing EB from 120 to 460 mEq/kg DM has a positive effect on appetite and DMI of milking cows (West et al, 1992). It is recommended to maintain EB close to 300 mEq/kg DM. Sodium bicarbonate helps in keeping desired dietary Na levels and EB without excessive chlorine content.

• Mineral buffer additives. Improving the buffer capacity of the diet can contribute to maintaining normal pH in the rumen, milk production and butter fat during extreme ambient temperature.

• Live yeast (Saccharomyces cerevisiae) supplementation. Heat-stressed cows suffer decreased rumination (Collier et al., 2006) and reduced amplitude and frequency of rumen contractions (Bernabucci, 2012). Decreased rumination results in reduced saliva flow and buffering capacity is also reduced due to increased CO₂ loss via panting. Furthermore, a decreased rumen pH impairs fibre digestion efficiency, with rumen fibrolytic bacteria being most affected when rumen pH drops below 6.0.

The intraruminal temperature may affect rumen metabolism (Gengler et al., 1970). These authors found that an increase in intraruminal temperature is related to a decrease in volatile fatty acid (VFA) production and a shift in their profile with a significant decrease in the acetate to propionate ratio.

To counteract these negative effects in ruminal efficiency and metabolism and to maintain health status, fertility and performance, live yeast has successfully been used in dairy cow diets during periods of heat stress.

As live yeast removes oxygen in the rumen, it improves the conditions for growth of those bacteria that convert lactic acid to propionic acid which is a major energy source for the ruminant. In addition, live yeast effectively competes with starch degrading bacteria for sugars and reduces their growth and subsequent lactic acid production. Thus, live yeast can help to prevent accumulation of lactic acid in the rumen, helping to regulate the rumen pH and limit the risk of both clinical and subclinical acidosis in animals experiencing HS. In the summer of 2019 during the heatwave affecting Europe, in those herds which were treated with a double dose of live yeast (100×109 CFU/head/day) no significant drop in milk yield was seen and strong daily fluctuations were not experienced.

In conditions of HS, cows show increased respiration rates and higher losses of CO₂ through the lungs and higher bicarbonate loss in the urine. As a result, less bicarbonate is available to help buffer acids in the rumen and pH is reduced, leading to SARA.

Systematic measurement of ruminal pH for 24-h period indicated that daily live yeast supplementation contributed to a higher pH versus non-supplemented animals (Krizova et al., 2011) (Figure 1).

In SARA challenged cows Alzahal et al. (2014) (Figure 2) found a positive influence of live yeast on rumen pH and in particular the time spent below pH 5.6. Levels of fibrolytic fungi and the main fibre degrading bacteria were increased, indicating better fibre digestion and also the number of lactate producing microorganisms was reduced.

CONCLUSIONS/IMPLICATIONS
There is no single approach to alleviate heat stress for dairy cows. It is necessary to introduce effective management and nutritional strategies to balance the homeostasis of the animal to help support feed efficiency, productivity and reproduction.

Supplementation with live yeast (Saccharomyces cerevisiae) can be a successful part of those strategies to help alleviate the negatives effects of heat stress.

For further information or references please contact [email protected]

About Diego Parra
Diego Parra, Technical Manager EMEA at AB Vista, is responsible for some of the Mediterranean countries in Europe such as Spain, Portugal, Italy, Turkey and Switzerland. Diego was born and raised in Spain where he studied veterinary science at Complutense University in Madrid. Besides, he holds a master of science and a master of business administration. Also, he has already started a PhD in agricultural engineering with a focus on animal nutrition. He has 6 years of practical experience in the feed and poultry field, mainly, managing feed mills and farms, before joining AB Vista 3 years ago.

About Dr. Dimcho Djouvinov
Dr. Dimcho Djouvinov, PhD, Technical Manager CEE, AB Vista, provides technical service and nutritional solutions to customers in Bulgaria, Czech Republic, Romania and Ukraine. Dr. Djouvinov has more than 20 years’ experience in animal nutrition at university level and feed industry and brings scientific knowledge into practice.