1. The respiratory rate of dairy cattle experiencing heat stress can increase from 30 bpm to more than 50 bpm (Zimbelman et al., 2010).
  2. Cows pant during heat stress, resulting in a significant amount of saliva which is lost through open mouth panting (West, 2003). Reduction in available saliva flowing into the rumen to act as a buffer in the rumen increases the risk of rumen acidosis.
  3. In order to be an effective blood pH buffering system, the body needs to maintain a 20:1 HCO3 (bicarbonate) to CO2 Due to hyperventilation, blood CO2 decreases. As a result, the kidney secretes HCO3 to maintain this ratio. This reduces the amount of HCO3 that can be used (via saliva) to buffer and maintain a healthy rumen pH (Baumgard et al., 2006).
  4. Heat stress can reduce uptake of P from the portal-drained viscera in lactating cows by 50% compared with that of cows with the same DMI in a thermoneutral environment (Sanchez et al., 1994).
  5. High concentrations of sulphate and chloride in drinking water can jeopardize productivity of cows during hot weather (Sanchez et al., 1994).
  6. Sweating rate can increase to ± 120 g/m2/h in dairy cows experiencing heat stress (Zimbelman et al., 2010).
  7. Heat stress can decrease DMI and therefore, digestibility of DM can increase due to slower ingesta flow. This can result in an increase ruminal acid production per unit of fermentable feed, and therefore a possible reduction in ruminal pH (Lippke, 1975; Sanchez et al., 1994).
  8. Reduction in DMI can only account for ±40-50% of the decrease in production when cows are heat-stressed. Therefore, ± 50-60% can be explained by other heat-stressed induced changes (Baumgard et al., 2006).
  9. Heat-stressed induced negative energy balance does not result in elevated plasma NEFA (Baumgard et al., 2006).
  10. Glucose disposal (rate of cellular glucose entry) is greater in heat-stressed pair-fed cows compared to thermal-neutral pair-fed cows (Wheelock et al., 2006).
  11. The mammary gland requires glucose to synthesize milk lactose. Lactose production is the primary osmoregulator and thus determinant of milk yield. However, to generate less metabolic heat, the body appears to utilize glucose at an increasing rate. Therefore, the mammary gland may not receive adequate amounts of glucose and thus mammary lactose production and subsequent milk yield is reduced (Baumgard et al., 2006).
  12. Heat stress has been reported to induce increased blood cortisol concentrations, which have been shown to inhibit the production of certain cytokines. Cytokines are messengers used by the immune system to improve immune response (Bagath et al., 2019).

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