How to reduce heat stress in dairy cattle
Abatement of heat stress in dairy cattle
In dairy cattle, heat stress occurs when the animal generates and absorbs more heat than it can get rid of through the normal channels of respiration, sweating and evaporation. In dairy production, the focus is on dairy cows. Cows mainly generate heat during digestion and biosynthesis of milk and absorb solar heat when exposed to the sun.
In general, livestock experience heat stress when the ambient temperature exceeds the upper critical temperature of the thermoneutral zone, which occurs in lactating dairy cattle when the temperature-humidity index (THI) is above 68. The comfort zone or thermoneutral zone for dairy cows arranges from -15°C to +25°C; the lower temperature is referred to as the Lower Critical Temperature (LCT) and the higher temperature is called the Upper Critical Temperature (UCT). At temperatures below the LCT, what is observed is that the cow increases its dry matter intake to keep warm by generating heat at the expense of milk production. On the other hand, at temperatures above the UCT, cows thrive to maintain thermal balance by increasing heat dispersion and limiting heat production; these strategies compromise several physiological and production characteristics of the animal. The challenge of heat stress is exacerbated by today’s ever-increasing global temperatures.
The negative impact of heat stress
The effects of heat stress on dairy cows are reduced feed intake, reduced fertility, lower milk production, and increased risk of diseases such as mastitis. The physiological status is compromised through observed increases in respiration rate, body temperature, and sweating. Heat stress negatively affects cow welfare as cows tend to spend more time standing; dairy cows will reduce lying time to increase body surface area for heat dissipation.
Reduction in milk yield
When exposed to heat stress, dairy cattle increase heat dispersion through evaporation by mainly increasing the subcutaneous blood flow, panting, drooling, and time spent standing; these unusual activities increase the cow’s maintenance energy needs, re-directing the cow’s production energy partition towards thermal regulation instead of milk production. As most of the heat production in dairy cows is due to rumen fermentation, the cow will reduce her dry matter intake or feed intake; lower feed intake equals lower milk yield. It is also important to know that high-yielding cows generate more heat than dry cows or heifers, irrespective of ambient temperatures.
Respiration rates and body temperatures
To assess how well cows cope with the weather conditions and existing cooling practices, one can observe and record respiration rates or measure body temperatures at different times of the day. Normal respiratory rates for adult dairy cattle range between 40 and 60 breaths per minute (bpm). Immediate action should be taken if more than 10% of cows have a respiratory rate exceeding 100 bpm.
Physical appearance and susceptibility to diseases
When cows experience heat stress, the usual signs involve breathing with mouths open and panting with their necks extended, appearing lethargic and showing distressed behavior. The incidence of clinical mastitis and lameness are strongly associated with heat stress in dairy cows. As previously described, long-standing times compromise animal welfare; animal welfare is a major concern for today’s consumers of animal products.
Heat abatement strategies focus on management solutions and nutritional interventions with the goal of balancing heat gain and loss over a 24-hour period.
Provision of shade
To help lower the cow’s body temperature and respiration rate, protecting the cow from direct solar radiation is a must. As reported by several heat abatement studies, providing proper shade helps reduce the solar heat load and remains one of the first recommendations to help dairy cattle cope with heat stress during hot conditions. It is important to avoid overcrowding when providing shade. To minimize the effect of direct sunlight exposure, buildings, and feeding areas outside are typically placed in the east–west direction.
Focusing on ventilation
To help get rid of heat, air velocity flowing past a cow is increased. To increase air velocity, different ventilation systems such as tunnel ventilation and cross-ventilation with baffles and high-speed mixing fans are recommended. Barn ventilation cools cows by providing air exchange between the inside and outside. In hot weather, give as much air exchange as you can; usually between 60 and 90 air exchanges per hour. In all ventilation systems, it is important to ensure the fans are well maintained and the inlets are open to get the desired air velocity.
Cooling with water
When it comes to cooling with water, the use of sprinklers and misters pops up as the most common ways to help cows get rid of body heat. Some points to remember:
- It is important to allow sprinklers to cycle on and off to allow time for water evaporation.
- Excessive sprinkling increases the risk of mastitis as it causes water to run down and wet the udder, creating suitable conditions for infections.
- For misters to be more effective, ventilation should not blow the mist out of the barn before the air cools.
For a cow to regulate body temperature, hydration during both times of heat stress and cold stress is of utmost importance. In situations where cows have access to outdoor space or grazing, it is important that the water supply is close to a proper shade and feed source as cows will try avoiding walking long distances to drink. It is recommended to add temporary water access points for cows as they leave the milking parlor. The following points are worth considering:
- Increasing drinking space for each animal during hot conditions.
- Water troughs should be deep enough to allow cows to submerge their muzzle when drinking.
- Ensure water flows to the trough and water capacity is adequate to account for large quantities needed at once.
High-quality forages and concentrates
Heat stress results in lower feed intake, meaning, for the animal to meet its nutritional requirements, the concentration of nutrients should be increased. Low-quality forages generate more heat by fermentation in the rumen; high-quality forages with high digestibility values are recommended to reduce heat production. To increase the energy density of the dairy cattle diet, using high-quality forages, and feeding more concentrates are recommended strategies even when the animal has a lower feed intake, but with close monitoring of the inclusion levels to avoid digestive disorders such as acidosis.
There is a need to increase the amount of protein as heat stress results in a negative nitrogen balance. Essential amino acids such as methionine and lysine are recommended during heat stress conditions; methionine plays an important role in dairy cows by improving milk production and antioxidant capacity, whereas lysine is key to milk protein synthesis.
Vitamin and mineral supplements
In hot conditions, potassium is lost in sweat, bicarbonate through panting and sodium through urine, exposing cows to a higher risk of respiratory alkalosis. Therefore, supplementation of potassium and sodium during heat stress is recommended. Of the vitamins, niacin helps alleviate heat stress by increasing evaporative heat loss from the body and by reducing the effects of heat at the cell level; supplementation of niacin at 6 g/cow/day reduces skin temperature and increases milk yield. The supply of vitamins A, C, E, and selenium is highly recommended during hot days as they are protective against oxidative stress. In addition, supplementation with plant extracts such as Bupleurum extract (0.5 g/kg dry matter basis) benefits heat-stressed cows in several ways.
Frequency and time of feeding
Several studies show that increasing feeding frequency in dairy cattle reduces heat production as this promotes a uniform absorption rate of nutrients, spreading the total heat increment. A well-documented strategy is to feed cows at night till the early morning (8 pm to 6 am); this allows the heat of digestion to peak early before the day gets hot and allows the cow to dissipate some of the heat before the hotter parts of the day.
Detecting the signs of heat stress in dairy cattle is the first aspect of any heat abatement strategy. Proper monitoring and recording of the conditions in the barn are next to detecting the signs of heat stress. Both management and nutritional strategies are crucial strategies that can be implored separately or combined to reduce heat stress’s negative effects.
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