High temperatures in a humid environment have a significant negative impact upon the dairy cow. Cows suffer from heat stress at temperatures as low as 22°C, as it is the combination of heat and humidity that influences the cow’s ability to thermo-regulate body temperature. Figure 1 shows cows will suffer heat stress when the temperature-humidity index (THI) value is above 68. A THI value of >75 cows will suffer from severe heat stress causing a significant decrease in performance due to not being able to maintain her normal body temperature (38.5 – 38.8°C).
Figure 1: THI values and risk level of heat stress
Signs of heat stress include panting more than 80 breaths per minute, reduced laying times and activity, with cows seeking shade. Dry Matter Intake (DMI) decreases, 68-78 DMI will drop 9.6% (Bouraoui et al) and decline in milk production.
Maintenance requirements for heat stressed cows increases by around 30%, so combined with often a lower DMI for cows under heat stress, they can go into negative energy balance easily. For every THI value over 69 milk yield declines by 0.41kg/cow/day (spiers et al 2004). Protein and milk fat content is negatively affected when the THI goes above 72.
In the dry period heat stress causes reduced mammary cell proliferation, therefore, decreases milk yield in the subsequent lactation (Teo et al). Calves born to a heat stressed mothers are often lighter at birth and have a poorer ability to absorb IgG’s due to gut closure being accelerated (Ahmed et al, 2015). Long term implications of heat stress can be profound, affecting the whole lifetime health and production of offspring.
Heat stress also impacts fertility as higher temperatures will affect the cow’s ability to display natural bulling behaviour. This is due to a reduction in the duration and intensity of oestrous leading to a silent heat. Pereira et al found that conception rates at 60 days declines when rectal temperature is greater than 39.1°C at the time of AI. Oocyte quality also declines due to poorer growth and development.
Lameness risk also increases with heat stress because cows will stand more to expel heat, or to stay under shade. Increased standing time is well established as being a cause of foot lesions such as sole ulcers and bruising.
There are many strategies to consider for controlling Heat Stress. Water is an important resource to a cow suffering heat stress so ensure supply is adequate. Make sure shade is available to the cows at all times, giving protection from solar radiation. It is better to house cows in the day and graze at night to ensure shade and good water supply, letting them graze at night, where they will maximise intakes. Fans increases cooling by convection, but this is only effective when air temperature is lower than body temperature. The best place to cool cows is often in the collecting yard, where aggressive air movement along with sprinklers can cool all cows regularly.
Heat stress is associated with a lower DMI, a reduced metabolic rate and poorer performance. It is the cow’s natural strategy to help her maintain her normal body temperature but with proper control strategies the effects can be minimised.
Bouraoui R, Lahmar M, Majdoub A, Djemali M, Belyea R. The relationship of temperature-humidity index with milk production of dairy cows in a Mediterranean climate. Anim. Res. 2002;51(6):479–491
Spiers D.E, Spain J.N, Sampson J.D, Rhoads R.P. Use of physiological parameters to predict milk yield and feed intake in heat-stressed dairy cows. J. Therm. Biol. 2004;29(7-8):759–764
Ahmed, B.M.S., A.P.A. Monteiro, U. Younas, T.O. Asar, J-D. Liu, J. Hayen, S. Tao and G.E. Dahl. 2015. Maternal heat stress affects calf passive immunity: Effects on intestinal cell apoptosis. J. Dairy Sci. 98(Suppl. 2):713. Abstract W268
Tao S, Dahl G.E. Heat stress effects during late gestation on dry cows and their calves. J. Dairy Sci. 2013;96(7):4079–4093
Pereira, M.H.C., Rodrigues, A.D.P., Martins, T., Oliveira, W.V.C., Silveira, P.S.A., Wiltbank, M.C., and Vasconcelos, J.L.M. Timed artificial insemination programs during the summer in lactating dairy cows: Comparison of the 5-d Cosynch protocol with an estrogen/progesterone-based protocol. (24054286)J. Dairy Sci. 2013; 96: 6904–6914
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