CALORIC STRESS

Characteristics and strategies for defending in cattle (meat or milk)

Cattle (meat or milk) begin to experience some type of heat stress when several adverse parameters are present simultaneously: ambient temperature above 26-28°C, relative humidity greater than 50%, and wind speed less than 5 km/h. The temperature and humidity index (TIH) synthesizes 2 of the 3 parameters mentioned above that affect animal welfare (Thom, 1959).

The formula for calculating the temperature-humidity index (THI) is:

ITH = 0.8°Ta + ((relative humidity/100) * (Ta – 14.3)) + 46.4

where Ta = air temperature (ºC)

Monitoring heat stress in cattle (meat or milk)

This index is currently used to determine the degree of heat stress that cattle may be exposed to, and its correlation in milk production, meat production, and reproductive efficiency. In general, experts have determined that with an IHT index below 72, “dairy cattle” do not experience stress (green colored area), if the IHT is between 72.1 and 78 the stress is moderate (yellow color), IHT values between 78.1 and 88 (blue color) are characterized by severe stress and above 88.1 there is a danger of death (red color) (Table 1).

Table 1 shows a comfort zone (green). For beef cattle, this area varies between 7 to 26°C and for dairy cows between 5 to 21°C (NRC, 2015). The comfort zone is characterized by animals achieving maximum productivity (meat or milk) and reproductive performance, as long as the diet is balanced (energy-protein) and adequately supplied in quantity and quality.

The comfort zone for cattle is reached, in addition to the above-mentioned temperatures, when the relative humidity ranges between 10-50% and the wind speed varies between 5-8 km/h, both below and above these values the animal does not reach the best productive and reproductive status.

How do cattle lose heat?

• By radiation: This is the elimination of heat in the form of infrared radiation from the skin to the environment. For this to occur, there must be a temperature gradient, higher in the skin and lower in the animal’s environment (10-15 mm thick on the skin).
• By convection: By the movement of air.

• By conduction: Heat passes from a warmer system to a colder one, by contact. This is how heat diffuses from the inside of the cow to the skin.

• By evaporation: When perspiration water and moisture from the external mucous membranes evaporate. The passage of water from the liquid state to vapor uses heat energy from the environment, cooling a few millimeters at the level of the animal’s skin.

Precisely, shade (natural or artificial), fans, and spraying with fine water droplets (drizzle type), used by many dairies located in very hot areas, seek to increase heat losses from the animal to the environment.

Although cattle have sweat glands to eliminate heat through perspiration (evaporative cooling), they do it in a limited (profuse) way, very different from horses.

What are the indicators of heat stress?

The stress indicators, specific to cattle (meat or milk), they all interact with each other in an environment with high temperatures and humidity.

1. Respiratory rate: Normal respiratory rate varies between 26 to 50 breaths (gasps)/minutes. However, under heat stress, it can range from 65 to 120 breaths (gasps)/minute.
2. Rectal temperature: Normal temperature varies between 36.5 to 37.5°C and in stressful environments, it may exceed 39°C.
3. Dry matter (DM) consumption: With high temperature and humidity, DM consumption is reduced by more than 20%, and can exceed 50% in extreme situations (+35°C temperature and +70% humidity).
4. Effects on production: In stressful environments, the loss of production (meat or milk) exceeds 10%, being able to reach in extreme cases falls of more than 50% (tropical and subtropical regions). The effect of high temperature and relative humidity in these regions is added to the lower quality of fresh forages (higher levels of fiber with lower digestibility and low protein content).
5. Effects on reproductive indicators: As a result of heat stress, it has been observed:
   • Reduction in the duration of estrus during the hours of higher temperature. Table 2 shows the information obtained by Gallardo and Valtorta (2000), where 71% of the estrus lasted 7 h or less.
   • Higher frequency of estrus during the night.
   • Less detection of estrus. In the state of Florida (USA), 82% of “undetected” estrus was observed in summer compared to winter. The occurrence of a high percentage of nocturnal estrus and the absence of another amount of them because of high temperatures and humidity produce “failures” in the detection of estrus, affecting reproductive and productive efficiency.
   • Lower conception rate. In the Santa Fe dairy basin (Argentina), a decrease of more than 15% in the conception rate in summer compared to spring was found.

In a study conducted in Israel[2] with high producing dairy cows (12 to 15,000 kg milk/cow/lactation), in summer months with temperatures above 40°C, the following reproductive parameters were affected:

• Increased calving interval (IEP), between 10 to 15%, consequently affecting overall production.
• An 8% increase in calving problems.
• Conception rate was reduced by more than 30%.
• Lower fat and protein content in milk and higher somatic cell count, producing a significant decrease in milk quality.
• Reduced milk production per cow by 15 to 20% (minimum) and lower production in peak lactation compared to winter months.
• When heat stress occurs during gestation, it affects the development of the fetus and placental mass, limiting the development of the mammary gland and, indirectly, affects subsequent lactation.

 STRATEGIES TO DEFEND AGAINST HEAT STRESS

Strategies should be implemented jointly and harmoniously. Among them, the following stand out:

1. Cold diets vs. hot diets

In pastoral systems (meat or milk), summer pastures or green summer pastures are used in the summer season, which, in general, grow and develop very fast, losing quality (they flower and seed). These forages have a high content of fiber (neutral detergent fiber -FDN-) and lignin (acid detergent fiber -FDA-) which reduces their digestibility. This fibrous material causes, when fermented in the rumen, high proportions of acetic acid, and with it, a greater amount of heat is generated, which raises body temperature (Table 3). Under heat stress conditions this heat is more difficult for cattle to dissipate. These are the so-called “hot” diets.

Table 3 shows the different products of fermentation within the rumen and the energy yield of each of them, i.e., the amount of ATP (adenosine triphosphate) generated for each molecule of glucose degraded (glycolysis).

Meanwhile, “cold” diets are those that have fiber with low lignin contents (lower levels of NDF and FDF) that are easily fermented. In addition, these cold diets should contain an adequate proportion of energy concentrates (starchy grains) and proteins of lower rumen degradability (“bypass”). In this way, both starches and bypass protein would arrive “intact” to the duodenum to be digested there.

To counteract the decrease in dry matter (DM) intake experienced by cattle due to heat effect, the diet should have a higher energy concentration to cover the animal’s requirements, which would be affected by this lower intake. The use of by-pass fats is a good alternative because they do not ferment in the rumen and increase the energy density of the diet, but care should be taken because some fats can depress DM consumption, which would aggravate the problem. The use should be exclusive with high-producing dairy cows and using inert fats, such as fats protected with calcium salts.

An adequate cold diet should have a balanced energy and protein fraction, with a lower content of low digestible fiber and a higher proportion of concentrates, thus promoting less acetic fermentation in the rumen and making better use of the generated energy.

However, some care should be taken not to exceed the use of energy concentrates (starchy grains) because there may be a higher incidence of “acidosis”. This can occur because the animal, having a higher respiratory rate, has a greater loss of carbon dioxide (alkalosis) that is compensated with a greater elimination of “bicarbonate” (kidneys) through the urine that, together with a lower concentration of “salivary sodium bicarbonate”, because of drooling, there is a lower “buffer” effect of ruminal pH, increasing the dangers of acidosis when high concentrations of starchy grains are used.

Meanwhile, “hot” diets are characterized by a high proportion of fiber (cellulose, hemicellulose, and lignin) of low digestibility. These complex molecules are fermented in the rumen, generating high proportions of acetic acid (higher amount of heat) (Tables 3 and 4).

2. Shade (natural or artificial), sprinklers, and fans.

To dampen the high temperatures, both in pastoral livestock systems as well as in barnyard systems, artificial or natural shade should be used, especially between 11 am and 6 pm, so that the animals can rest adequately. In addition, some type of shade should be provided where feeders and waterers are located.

Meanwhile, in dairies located in areas where high temperatures (>32°C) prevail during several months of the summer, it is also advisable to place large fans under open sheds or half shade where the animals rest and eat (Photo 1). The objective is to operate them during the hottest hours, even at night. Meanwhile, in the holding pens, before milking, the best result is achieved when a combination of spraying (30 seconds) followed by ventilation (4 to 5 minutes) is repeated every 30-45 min, while the cows are in these places (Photo 2).

Eng. Ghiano (EEA INTA Rafaela) also advises the use of shade in the places where the feeders are located (outside the milking parlor) and over the drinking troughs. In most cases, this investment is recovered in 2 to 3 years with the increase in milk or meat production.

For every 0.5 l of water applied to the animal, 255 kcal of body heat can be dissipated. For this to occur, the droplet size must vary between 3 to 5 mm so that the water can pass through the hair and reach the hide. On the other hand, if the water droplets are smaller (mist), a waterproofing of that area may occur, not allowing the internal heat of the animal to radiate to the environment.

Characteristics of sprinklers[3]

• Placement height: 3.5 m from the floor.
• Distance between sprinklers: 4 m
• Adjustable wetting angle from 0 to 360º.
• Working pressure: 2.1 bar.
• Flow rate per peak: 12.7 to 16.0 liters/minute (8.5 to 10.6 liters per cycle)
• Drop size: 3-5 mm
• Wetting diameter: 4.5 m to 1.2 m (cow height).

Along the same lines, Dr. Flamenbaum commented that in Israel the main method used is based on increasing evaporation from the body surface. For this purpose, the following are applied in combination and simultaneously: shade (large open sheds on the sides), showers (sprinklers), and forced ventilation in the pre-milking parlor and the resting pens (open places) (Photos 3 and 4).

3. Access to grazing or feed in the troughs during the night

In different research works, among them, the one carried out in the spring of 1996 by the author of this article (Magister thesis at EEA INTA Balcarce), animals enclosed in pens and consuming a diet based on corn silage and concentrates, consumed 30% of the total DM during night hours (Graph 1). Meanwhile, in hot regions (above 32ºC), the amount of feed that animals can eat during the night hours can be as high as 60% of the total DM consumed during the day (Purechena 1999).

In this study, a tachograph placed on the neck board of the animals was used, which was marked on a circular diagram with carbon paper each time the animals bent down to eat. As a result of this study, 3 conclusions were confirmed:

1. The animals eat for 24 h in different proportions, as long as there is feed (quantity and quality) and there are no “heat stress” conditions. This is observed in both feedlot and pasture fattening.
2. Two consumption peaks are observed, the first and higher proportion occurs between 1.5 to 2 h after sunrise (pastoral) or delivery of the new feed (feedlot fattening), and the second and lower proportion occurs between 1.5 to 2 h after sunrise (both pastoral and feedlot).
3. Animals eat a high proportion of DM during night hours. In the months of cold or cool temperatures (autumn-winter and spring), it varies between 20 to 30% of the total and in the summer months, they can consume between 40 to 60% of the total, depending on the temperature of the environment.

All this shows that, in the summer months, it is key that the animals have access to “fresh” food, in quality and quantity throughout the night, either by grazing fresh forage (pastures or summer green pastures) or by having fresh food in the feed bunkers supplied at the end of the afternoon (feedlot fattening). Some technicians speak that 60% of the total daily feed should be supplied in the late afternoon. However, this issue should be subject to each production system (meat or milk), to the animal category, and the characteristics of each diet.

The important thing is that the animals have access to “fresh” food, with adequate quality and quantity during the night hours, to cover this higher table and to be able to sustain high meat or milk production.

4. Water Consumption

Table 5 shows the water consumption (liters/day) of different categories of dairy cattle as a function of ambient temperature. This reinforces the importance of having “fresh” water of sufficient quality and quantity to cope with the higher demand that occurs during the high-temperature season.

5. Animal movements

Finally, and as preventive management, herding, work in pens or large animal movements should be avoided during the hours with the highest incidence of heat stress. They should be done early in the morning or late in the afternoon.

REFERENCES.

• Bovinos: cómo cuidarlos del estrés por calor http://intainforma.inta.gov.ar/?p=41871
• Cruañez, M.J, 2017. Clima y Producción lechera. Universidad Nacional de Entre Ríos.
• http://www.fca.uner.edu.ar/files/academica/deptos/catedras/leche/Tema%203%20Clima%20y%20produccion%20lechera.pdf
• Fernández Mayer,A.E., Santini,F.J.,Rearte,D.H.,Mezzadra,C.,Garcia,C.S. y Manchado,J.C. 1998. Engorde a corral: comportamiento productivo de novillos alimentados con silaje de maíz como dieta base, harina de girasol y grano de maíz. Tesis de M.Sci. Fac.Cs Agr. UNMdP-EEA INTA Balcarce.
• Gallardo, M y Valtorta,S. 2000. Suplemento lechero del INTA. EEA Rafaela
• NRC (National Reasearch Council), 2015. Nutriente requirements of beef cattle revised edition.  Nat. Ac. Press. Washington   D.C.pp 60
• Oberto, M.F, Reitú, M.A y Pirra,M.A 2006. Estrés calórico: ¿Qué podemos hacer?
• Producción de energía en los microorganismos (UCV) 2015. Universidad de Venezuela.
• http://www.ucv.ve/fileadmin/user_upload/facultad_farmacia/catedraMicro/08_Tema_4_Metabolismo.pdf
• Purechena, C, O, 1999. Suplementación de bovinos para carne sobre pasturas tropicales. Aspectos nutricionales, productivos y económicos Conferencia. XXXVI Congreso Anual de la Sociedad Brasileira de Zootecnia,   Porto Alegre, Brasil.
• Taverna, M; Ghiano,J; Gastaldi, L y Tec. Emilio Walter, 2016. Estrés calórico. Enfriamiento de vacas mediante la combinación de mojado y ventilación forzada. INTA EEA Rafaela.
• https://inta.gob.ar/sites/default/files/script-tmp-inta_estres_calorico_sistema_ ventilacion_y_aspersion_.pdf
• Thom, E. 1959. Oficina de Climatología del US Weather Bureau)
• https://wikivisually.com/lang-es/wiki/%C3%8Dndice_de_temperatura-humedad
• [1] Technician of the EEA INTA Bordenave (CERBAS) (Buenos Aires). Master Sc, and Doctor in Veterinary Sciences, (Univ. Agraria La Habana, CUBA). Specialized in bovine nutrition. E-mail: afmayer56@yahoo.com.ar; fernandez.anibal@inta.gob.ar
• [2] Dr. Flamenbaum, personal communication
• [3] Taverna et al, 2016