Title:Hőstressz tejelő tehenekben II. Az alkalmazkodást segítő takarmányozási megoldások - Irodalmi összefoglaló
SUMMARYThe present review describes the possible nutritional strategies to alleviate the heat stress related decrease in the milk yield of dairy cows. Heat stress affects the energy metabolism both directly and indirectly. Loss of appetite and reduced feed intake lowers the energy sources available for production, while the increase in basal insulin concentration – as an indirect effect – is determi-nant in glucose and fatty acid utilization. Nutritional strategies that can lead to improved energy status and moderate increase in production are reviewed in detail. Altering the schedule of feed distribution, supplementing dairy rations with fat or feeding NDF (neutral detergent fibre) components with improved digestibility are effectively increasing energy intake. Yeast supplementation that is widely used in periods of negative energy balance can enhance ruminal fermentation, and it is also effective in heat stress. Niacin supports adaptation to heat stress through several different metabolic pathways, however, studies have come to contradictory conclusions. The enhanced insulin action characteristic to heat stress can be promoted by dietary chromium supplementation and an adjusted ratio of rumen degradable and undegradable protein in the ration. Supply of major cationic and anionic macro elements supports the acid base status and compensates the heat-related electrolyte-deficiency. Other, less investigated supplements, such as betaine, flavonoids and selenium act through mechanisms that are not fully known. The primary goal of feeding solutions is the improvement of the general health status and adaptive capacity of animals which often results in increased production, though understandably to a limited extent. The role of nutritional interventions can thus only be secondary to cooling efforts in preventing heat stress. Increasing knowledge on heat stress physiology can promote the development of nutritional strategies.