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a stress condition and compensate imbalances created (Guo et al., 1999; Sahin et al., 2009; Renaudeau et al., 2011; Sahin et al., 2009). Even considering the use of protective facilities and auxiliary control devices, producers experience high economic losses when high humidity is present or heat waves are hitting their production complexes (West, 2003). The best approach is to use all the elements available – considering assortment of facilities, installing proper equipment and control devices. All these are being designed by qualified professional agricultural engineers. Also, the nutritional approach should be considered, as well as trying to take some advantage of acclimation. Sharp common sense and customised management should never be left aside for technology to give its best performance. Dairy cows During heat stress, cows display reduced activity, reduced dry matter intake (DMI), increased respiratory rate, sweating and seeking shade. The primary heat dissipation mechanisms become less effective as temperature rises and then the cow relies more on the evaporative cooling in the form of sweating and panting (West, 2003). In high relative humidity (RH) conditions this means that cooling is restricted to limited times in the day, then the deficiency in heat loss leads to reduction in dry matter intake (DMI) and consequently lower milk production. The critical values for minimum, mean and maximum thermal humidity index (THI) obtained in a study were 64, 72 and 76 respectively (Igono et al., 1992). It is difficult to separate what influences reduced milk production more – either low feed intake or direct effect of heat on hormones. Thyroid hormone levels are reduced by hot weather and therefore metabolic activity is reduced. Respiratory alkalosis derived from prolonged panting later becomes metabolic acidosis by the compensatory loss of bicarbonates. This affects the acid-base balance of the cow, especially when high grain feeds are fed. In one study done in Florida, USA, milk yield declined by 0,2kg per unit increase in THI when THI exceeded 72 (Ravagnolo et.al., 2000). The immune system can also be compromised during heat stress (Renaudeau et al., 2011, Roth et al., 1982). The following approaches are classified by the target objectives pursued when cows are exposed to heat stress conditions: Reducing exposure The use of shade is an important resource to mitigate heat stress. Shade is a requirement in any environmental management programme for dairy. In an experiment cows under shade yielded 10% more milk, had lower rectal temperatures and slower respiratory rates (Roman-Ponce et al., 1977). It is recommended to provide shade in all areas, at least 4m 2 /cow oriented to cover most of the sun path during the hottest season. It can be provided either by solid roofs or by tensed sheds. Feeding and watering areas should always be covered by shade. All open areas should provide a shade refuge – holding pens, maternity pens, cows in lactation, managing area and if possible also pathways. Less body heat production Reformulate diets to account for reduced DMI, adjust increased requirements to cope with heat, reduce heat increment and avoid any excess in nutrient supply. Avoid excess nitrogen as energy might be distracted to produce NH 3 . Feed low fibre diets since acetate is not efficiently used in hot weather conditions (West, 2003). Low forage and higher concentrate diets increase total DMI, so measures to manage a low DMI should be taken as it is the key factor to effectively reduce heat increment and provide the right amount of nutrients. To compensate this, the dietary neutral detergent fibre (NDF) content needs to be adjusted to keep DMI at the desired level. Reduced DMI requires an increase in the dietary mineral concentrations. Cationanions should be increased (Na, K; West, 2003), as well as trace minerals (Zn, Mn, Cr, Cu; Renaudeau et al., 2011) to reinforce the antioxidant status and also to prevent problems derived from tissue weakness like pododermatitis. In facilities with low mechanised cooling capacities, try to feed cows at a time when temperature is comfortable, early and later in the day. Provide a well-balanced diet with accurate energy supply calculated according to the production stage and weather (DMI). Increasing airflow Mechanised ventilation has shown to be a useful resource to improve comfort, especially when combined with the use of sprinklers or foggers. Fans running at 6,5 to 9,7km per hour, depending on the season and the facilities, would provide sufficient cooling. Fans should be placed at the most efficient height (2,40 to 2,50m; Brouk et al., 2004). Fan companies provide the service of calculating the optimal capacity for the requirement of the area. Remember to provide frequent maintenance to fans. All must be running fine before the hot season. A thermostat should be calibrated and cleaned frequently. Evaporative heat loss The use of sprinklers on the cows has shown to provide comfort as they increase their evaporative heat loss. Low pressure coarse droplets sprinklers (1,8 to 2,8l/min, 1,25l/cow) are preferred, as the less the air is moving the more times the cow needs to be soaked. Once the cow is wet, time should be allowed for the water to evaporate. Humid weather requires more frequent soaking. An 11,6% improvement in milk yield was obtained when cows were sprayed for 1,5 minutes every 15 minutes (Strickland et al., 1988). Give proper maintenance to nozzles and flush pipes. Care must be taken to avoid manure accumulation creating muddy areas. Due to metabolic acidosis that develops during heat stress, vasoactive changes usually weaken claws (hooves). Lameness can result from the softening and later wear of the claw (Tomlinson et al., 2004; Tomlinson et al., 2008). Compensating the negative effects in the animal Provide full water access (linear water access: 1-1,20m/cow), preferably in shaded areas, in free stall barns and parlour exit lanes. Keep water tanks clean and check water flow rates at peak hot times. Supplementation of vitamins A and C, and trace minerals lost and depleted dur- Processing <strong>AFMA</strong> MATRIX ● APRIL <strong>2013</strong> 19