ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2023 43 The financial impact of heat waves on the industry will certainly become more of an issue in the future as high-yielding animals have more natural metabolic activity and will be more affected by high temperatures as they produce more body heat (Jones and Stallings, 1999). BEHAVIORAL AND PHYSIOLOGICAL CHANGES DURING HEAT EVENTS The behavioral coping strategies of dairy cows facing heat challenges include increased standing time, shade seeking and decreased activity and movement (Schutz et al., 2009). Furthermore, cattle may increase water consumption, and change or reduce feed consumption. If rough and concentrated feeds are provided as alternatives, animals will prefer concentrated feeds, as they induce less metabolic heat to digest (Sireli et al., 2017). However, the overall effect of heat stress on growth performance cannot be explained solely based on reduced feed intake (Slimen et al., 2015). A study by Rhoads et al. (2009), provides data that indicate heat-induced reductions in nutrient intake only account for approximately 35% of the decrease in milk synthesis. As for physiological changes, heightened environmental temperatures are shown to alter the basic mechanisms of the rumen, with increased risks of metabolic disorders and health concerns (Soriani et al., 2013). For example, the rumen’s microbial population changes and its pH fluctuates (Hall, 2009). Shifts in the cow’s acid-base balance in the blood may reduce production of bicarbonate and secretion of this buffer into the saliva. Plus, water consumption may be inadequate, which reduces acid dilution in the rumen. Declines in rumen pH can predispose cattle to sub-acute rumen acidosis. When TNZ conditions are exceeded, the cow’s body attempts to adapt to the new environmental situation. The animal can radiate body heat through increased respiration rate, panting and sweating, otherwise known as the principles of convection, conduction, radiation and evaporation. During extreme heat events, however, the cow’s adaptive mechanisms may fail to remove the excess heat generated.
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