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RESEARCH ▶▶▶


Late-gestation heifers and heat stress


Increasing global environmental temperatures have detrimental effects on the dairy industry. While the effects of heat stress on multiparous lactating and non-lactating dairy cows have been studied extensively, the effects on gestating heifers have received less attention.


BY MATTHEW WEDZERAI H


eat stress reduces cow milk yield and results in sig- nificant economic loss for the dairy industry. Heat stress is experienced by livestock when ambient temperature exceeds the upper critical temperature


of the thermoneutral zone, which occurs in lactating dairy cat- tle when the temperature-humidity index (THI) is above 68. A study shows that the economic loss from heat stress in both lactating and non-lactating dairy cows combined is estimated at more than $ 1.2 billion in the US. Exposure to stressors during late gestation (a critical phase of development) could be detrimental to the energy partitioning towards foetal and mammary gland development in gestating heifers. The exponential growth of the foetus, coupled with in- creased energy demands and simultaneous mammary gland growth add to the overall heat load, decreasing the ability of the heifer to effectively dissipate heat during the late gestation


period. Studies show that a greater tolerance for increased en- vironmental thermoneutral zone and less metabolic heat pro- duction prolongs the onset of physiological responses to heat stress in dry cows, pregnant heifers and calves but does not prevent them from experiencing heat stress. Researchers from the University of Florida, US, have charac- terised the effect of late-gestation heat abatement in heifers.


The study A total of 31 gestating heifers were used to characterise the effect of late-gestation heat abatement on thermoregulatory responses and subsequent milk production of nulliparous Holstein heifers. The heifers were allocated either to a heat stress group (shade of free stall barn) or a cooling group (shade of free stall barn, water soakers and fans) during the last 60 days of gestation. Prior to the study, all heifers were managed in a single group raised on pasture with access to artificial shade.


Heat effects on mammary gland Research shows that about 48–94% of mammary growth occurs during pregnancy, with most of the growth occurring during the last two months of gestation. The developing foetus is also growing at a rapid rate during the last trimester of gestation. Several studies have shown that heat stress during a cow’s dry period decreases mammary gland growth, resulting in reduced milk yield in the subsequent lactation. Therefore, ex- posure to stressors during this critical phase of development could be detrimental to the energy partitioning towards foe- tal and mammary growth in nulliparous heifers. Adding to that, some studies indicate that heifers born to cows that experienced late-gestation heat stress have lower milk yield during their first lactation, implying that the mater- nal environment may alter mammary gland development of the offspring; it is not clear if this is only due to a non-genetic modification of prenatal mammary gland development by maternal heat stress.


Thermoregulatory responses Under heat stress, physiological processes (i.e. respiration rates, sweating rates and core body temperature) are altered to maintain homeostasis. Sweating and movement of blood


32 ▶ DAIRY GLOBAL | Volume 8, No. 2, 2021


PHOTO: PIXABAY


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