Summer is coming for everyone
Arianna Zanella, DVM
When talking about heat stress in dairy cow breeding the first thought goes to the consequences in the group of animals in production. Deteriorating production and reproductive performance is the first concern when facing the summer season. But contrary to popular belief, heat stress is a problem that affects ALL animals on a dairy farm. The consequences of high summer temperatures are often also important on calves and heifers, even though they produce less metabolic heat and have a higher dissipation efficiency. For the youngest group of animals, the consequences of heat stress can result from exposure both during intrauterine life and after birth.
Thermal stress in the prenatal period
There is now growing evidence that the uterine environment of the mother can transmit an indirect effect of heat stress by triggering adaptation mechanisms in the foetus as well. Uncooled cows during the last period of pregnancy give birth to calves with reduced weight (Amaral et al, 2011; Tao et al, 2012). Foetal growth is compromised because heat stress tends to shorten the duration of pregnancy (Amaral et al., 2011) and since the foetus grows rapidly in the last two months, a decrease in the number of days in utero compromises the final weight of the unborn. In addition, high temperatures impair placental development and vascularisation (Regnault et al, 2003) causing hypoxia and fetal malnutrition. Calves born to cows exposed to heat stress will have a lower weight and shorter stature at weaning and struggle to reach the same weight at 12 months as daughters of cooled dry cows. This is probably related to the fact that intrauterine heat stress causes a permanent change in metabolism with peripheral energy storage and less use of energy for growth (Tao and Dahl, 2013). Maternal heat stress also worsens the immune capacity of the offspring, resulting in greater susceptibility to disease and lower survival rates from birth to puberty. This is caused by the fact that newborns have significantly less circulating immunoglobulin than calves of cooler mothers (Tao et al, 2012). The mechanism is not yet fully understood, possibly related to the reduced quality of colostrum produced, but it is also thought to be related to the intestinal absorption capacity of newborns. Prenatal stress also alters blood parameters and postnatal cellular immune capacity (Tao, Monteiro et al., 2012). Furthermore, the data available so far tell us that heat stress in dry conditions has consequences that seem to be negatively reflected also on the future productive and reproductive capacity of the offspring (Dahal et al., 2013). Several studies report that heifers born to heat-stressed mothers require more surgeries at fecundation to conceive, resulting in a higher calving age. Production in the first weeks of first lactation is also compromised: Monteiro et al. (2013) report the average production of two groups of primiparae born to cooled (CL) or heat stressed (HT) cows. A higher production of about 5 kg/day in the first 35 weeks postpartum was found for daughters of cooled cows.
Heat stress in the postnatal period
It is now certain that even from birth onwards, exposure to excessively high environmental temperatures can worsen the growth performance of newborns. In contrast to dairy cows, it is not yet clear what thresholds of environmental indicators indicate the need to intervene to alleviate heat stress in this part of the herd. We do, however, have the possibility of basing our evaluations on certain parameters derived from direct observation of the animals. Increased respiratory and heart rates, open-mouth breathing, shade seeking and grouping, increased time spent standing and/or outside the cage, increased rectal temperature and water consumption are some of the indicators of heat discomfort. Adaptation to prolonged heat stress conditions has a very important impact on calf welfare and farm profitability. All studies to date on the consequences of thermal stress on the growth performance of newborns agree on a lower daily weight gain during seasons with higher environmental temperatures (Donovan et al., 1998; Broucek et al., 2009; López et al., 2018). The lower growth rate is due both to lower food intake, higher energy expenditure for thermoregulation and the consequences of any thermal stress suffered during intrauterine life. However, it has been proven that the impact of postnatal heat stress is also negative for calves born under thermoneutral conditions (Dado-Senn et al., 2020). For heifers, the most important effects of exposure to prolonged heat stress occur mainly in the reproductive sphere: high summer temperatures affect follicular development causing the production of poor-quality oocytes.
Techniques for reducing the thermal load
As already mentioned, until now, the issue of heat stress with regard to heifers has often been put aside. However, research has highlighted the fact that maintaining physiological conditions during summer periods with high temperatures, high humidity and strong sunlight is a major energy drain on the animals and causes economic losses for the herd. As the limits of the thermoneutral zone for younger animals are not yet perfectly clear, unlike for dairy cows, knowledge of the factors influencing thermoregulation in calves is important for making environmental and management changes that allow calves and heifers to use their energy for health and growth. The main management strategies to support the animals in the warmer season concern the management of feeding:
- for lactating calves, it is best to distribute the main meals at times when the ambient temperature is not too high, if possible supplemented with hydration solutions and fresh water at all times;
- for heifers, the energy density of the ration should be adapted to the reduction in intake. Supplementation with vitamins, yeasts and minerals with antioxidant effect can also alleviate the negative effects of heat stress.
In most cases calves from birth to weaning are housed in single cages and often outdoors. In summer, the microclimate in these cages becomes unbearable if the cages are not properly insulated and shaded. When possible, the cages should be oriented northwards to further reduce solar radiation. A further way of reducing the thermal load is to increase air circulation. In the case of single cages by opening the air inlets on the sides and top, in the case of animals housed indoors by installing horizontal fans. At an ambient temperature of 29°C a 2011 study (Hill et al.) reports a 23% increase in daily weight gain and 21% increase in feed efficiency in calves with the use of fans. In the case of heifers, however, the implementation of ventilation with wetting results in a reduction of rectal temperature, respiratory rate and an increase in weight gain of 26% compared to animals under heat stress (Marai et al., 1995).
Based on the literature data available to date, it can be concluded that although calves and heifers are supposed to be more tolerant to heat stress, they actually suffer the negative effects of high summer temperatures, compromising their growth and reproductive performance. It is therefore absolutely necessary to address the issue of heat stress also for these components of the dairy herd. Even though these animals are not in production, their impact on the profitability of the herd is proving to be very important.