Discipline: stress; Keywords: Economic evaluation, feed intake, heat tolerance indicators, water intake.
Dairy breeds originated in the United Kingdom and Western Europe. Both regions have a wet and cool to cold climate. As a result, the environmental comfort zone for dairy cows varies between -5 and 21 °C. Anatomical features that help cows withstand cold conditions include a thick skin, a dense hair coat, subcutaneous fat layers, large muscles and a digestive system that is based on fermentation processes in the rumen which creates internal heat. These features, however, make dairy cows sensitive to a warm to hot environment as in South Africa which in comparison is warm and drier with more actual sunshine hours because of less cloud cover. Direct and indirect solar radiation is the cause of heat stress. Cows will avoid direct sunlight seeking shade while also using water to cool down and preferring wet places to stand or lie down. Heat is also lost through panting, which increases the respiration rate. As cows have limited means to reduce the effect of heat stress, environmental manipulation is often required to help cows maintain production levels under hot conditions. In the study by the authors cited below, results on the effect of a shade structure on the production, physiological parameters and behaviour of Holstein-Friesian cows have been obtained.
In the study production performance, stress levels and behaviour of Holstein-Friesian cows with and without access to a shade structure during summer was compared. The shade structure was 3.65m high, orientated lengthwise north to south and which provided at least 4 m² of roof space per cow. Summer days were characterized by high day-time temperatures, about 28 °C, and cool nights with minimum temperatures of about 14 °C. Ambient temperatures were higher than 21 °C for about 11 hours per day. Black globe temperatures integrating the net effects of solar radiation from the sun, ground surface and other objects close-by underneath the shade structure and in the sun differed, being 30 and 40 °C, respectively.
The daily free-water intake of cows without shade was higher than that of cows with access to shade, being 114 vs. 97 litres per cow per day, respectively. Overall, cows with access to shade produced 5.5% more milk than cows without shade. Although linear regression analyses showed that the average daily milk yield of cows was not affected by increasing maximum temperatures, the difference in the cumulative milk yield for shade and no-shade cows increased for each experimental period indicating an increasing negative effect as summer progressed, probably indicating the long term effect of heat stress. Cows with access to shade had higher feed intakes during the day. However, both cow groups maintained their daily feed intake as they had adapted their feeding pattern towards the cool times of the day. About 55% of the total daily feed intake was at night when ambient temperatures were below 24 °C. Most cows would also have completed most of their daytime feed intake before 09:00 in the morning. Only a small number of cows would eat again during the hot time of the day from 11:00 to 13:00. Providing shade resulted in reduced stress levels as observed in lower cortisol levels. Thyroxine levels were reduced by increasing maximum temperatures with no-shade cows showing a greater reduction. The rectal temperatures and respiration rates of cows using shade were lower than that of cows without shade. Rectal temperatures increased during the day for both shade and no shade cows although less so for shade cows. Cows without shade also adapted their daily behaviour to cope with heat stress. They converged at the water trough with some cows standing with their front feet inside the trough while other cows would use another cow’s body to shield their heads from the sun.
In the study the positive milk yield response to shade was relatively small (5.5%) in comparison to studies conducted in other parts of the world. The reason probably was because of cool conditions at night, i.e. ambient temperatures below 21 °C. This cooler period allowed cows to recover from the day-time heat stress. However, even at this modest increase in milk yield, the construction cost of putting up a shade structure was recovered within three summers.
C.J.C. Muller & J.A. Botha, 2018. Heat stress in dairy cows and the effect of a shade structure. Applied Animal Husbandry & Rural Development 11, 1 – 4.