The effects of heat stress in dairy cows can have long-term impacts. In the milking herd a reduction in feed intake and milk yield can be clear to see but what about the impacts during the dry period? There are both short-term and long-lasting impacts of heat stress, not only in dry cows but on their unborn calves as well.
Effects on the dry cow
During periods of heat stress, feeding behaviour is altered, leading to a reduction in dry matter intake as cows strive to reduce body heat production. This means less energy and protein is consumed which could affect colostrum yield and quality, depending on the stage of the dry period and how long the heat stress period lasts for. In addition, mineral intake may also be reduced, making cows more susceptible to milk fever and related conditions.
Immunity can also be affected, leading to a greater risk of infections such as mastitis and retained cleansings, as many cells of the immune system are involved in the process of placental detachment.
Lastly, heat stress can reduce lying times by as much as three hours a day, as cows stand more to try and dissipate heat. Prolonged standing times can increase the pressure on the feet and the incidence of claw horn lesions (sole bruising and ulcers) around two months after heat stress was experienced.
Effects on the unborn calf
The effects of heat stress can also impact the calf. A study by the University of Florida monitored dry cows from dry off to calving. The dry cows were in two groups, one which had access to shade, fans and sprinklers and the other group (the heat stress group) has no shade, fan or sprinklers. The calves born to cows from the heat stress group were nearly 8kg lighter at birth, due to shortened gestation periods, making these calves less likely to achieve target weights for age and calve down at the target of 24 months.
Calves born to heat stressed mothers also tend to have compromised immunity, with poorer efficiency of immunoglobulin absorption. Effects on fertility are also evident, with calves cooled in utero requiring less services to get pregnant and therefore calve down at an earlier age compared to calves that were heat stressed in utero. Even longer-term effects of calves born to heat stressed mothers are evident in the first lactation, with these calves producing on average 5kg less milk per day over the first 245 days of lactation, even though they were of similar body weight and condition score at calving compared to their cooled counterparts (Dahl et al 2016).
Heat stress mitigation strategies
Therefore, the management of dry cows is critical to ensure good cow health and performance over the transition period, as well as the health and development of the calf through its rearing period and lifetime performance in the milking herd. Steps that can be taken to mitigate the effects of heat stress during the dry period include the use of fans, provision of shade for those at grass and good availability of water to accommodate higher intakes. Always have a minimum of two watering points per group to minimise the effect of bullying.
From a nutrition perspective, a reduction in dry matter intake during heat stress conditions can be counteracted by providing a higher energy diet. Ensure the diet is also well mineralised with those trace elements and vitamins essential to immunity – particularly zinc, selenium, and vitamin E. Inclusion of chelated trace elements can help improve availability where antagonists are present in the forage.
Push up feed regularly to help keep it fresh and monitor for heating. If heating is an issue, feed twice a day instead of once a day. Offer fresh feed during the cooler parts of the day i.e. early in morning and late in the afternoon to reduce the heat load, as heat is generated from forage digestion.
Lastly, consider including yeast in the dry cow diet. Yeast can help improve digestion and feed conversion efficiency, leading to higher dry matter intakes. Biosprint is a proven strain of live yeast, available here. Explore how Biosprint can help support your herd’s transition period and overall performance.
Reference:
Dahl, G.G., Tao, S. and Monteiro, A.P.A. 2016. Effects of late-gestation heat stress on immunity and performance of calves. Journal of Dairy Science, 99:3193-3198. https://doi.org/10.3168/jds.2015-9990
