ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2022 35 decrease feed intake which is directly related to milk production. Higher respiration rates are related with higher losses of CO2, leading to respiratory alkalosis. Thus, acid-base balance of the body is impaired. Heat stress (HS) affects cows in all physiological phases. In dry cows it results in reduced blood flow to uterus, reduced placenta weight and birth of smaller calves with lower vitality. Even when the ambient temperature is back to normal range, milk yield in early lactation can be reduced by 10-12% as a result of impaired endocrine function of placenta and body during HS. High yielding cows, especially in peak lactation, maintain a very high metabolic rate and are more prone to heat stress than low yielding cows. Milk production can be reduced by 10-25% not only due to decreased appetite and feed intake, but because of lower levels of lactogenic hormones. In heat stressed cows dry matter intake is decreased by 8-12%, and milk yield reduced by 2030%. In addition, endocrine status can be negatively affected together with reduced intensity and duration of oestrus plus impaired fertility and elevated early embryo mortality. Milk yield and quality can be affected as well, with reduced butter fat content and increased somatic and bacterial cell counts, most likely due to impaired immunity. Excessive ambient heat can cause significant economic losses. According to a study of St-Pierre et al. (2003) the U.S. dairy industry lost between $897 - $1,500 million/year in revenue due to heat stress. The following key management and nutrition actions can help to mitigate the negative effects of heat stress: • Protect from direct sun radiation • Increase ventilation rate in barns • Apply cool water spray • Reduce animal density • Allow access to cool drinking water • Use high quality dietary ingredients to improve nutrient content • Ensure gradual and on-time adaptation to corrected diet • Diet to be fed mainly during the cooler part of the day • Increase feeding frequency with smaller portions. No heat stress Moderate heat stress Severe heat stress Dead cows Source: https://www.nadis.org.uk/ C 22 24 26 28 30 32 34 36 38 40 20 66 68 70 72 74 76 78 80 82 84 30 66 69 71 73 75 77 80 82 84 86 70 69 72 75 78 81 84 87 90 93 96 40 67 70 72 74 77 79 82 84 86 89 80 70 73 77 80 83 86 89 93 96 99 50 68 70 73 76 78 81 84 86 89 91 90 71 74 78 81 84 88 91 95 98 101 60 69 71 74 77 80 83 85 88 91 94 100 72 75 79 82 86 90 93 97 100 104 Temperature Humidity Index (THI) Relative Humidity %
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