The negative effect of heat stress on milk production in dairy cows has been researched for many years, but there is now growing interest in how heat stress during conception and gestation affects the subsequent health, growth and productivity of calves. The extent and duration of heat stress varies by geographical location, but is particularly common in Florida, Georgia and Texas, and certainly can be seen during the summer months in Virginia. This review will consider recent studies from these states that investigated the future performance of calves born to heat-stressed dams. Heat stress occurs when a cow’s heat load exceeds her capacity to dissipate heat. Most heat is lost via the skin; therefore, in times of heat stress, blood will be redirected to the skin to aid heat loss, and away from internal organs. For lactating cows, less blood flows to
the mammary gland, and for pregnant cows,less blood flows to the uterus and placenta to support fetal development. The degree of heat stress exposure for dairy cattle is considered
as a temperature humidity index (THI). At a THI of 68, and relative humidity of 50%, cows may experience heat stress and an associated drop in milk yield at temperatures as low as 72°F.
Year-round calving systems mean that heat stress is inevitable for many cows during some stage of gestation. In a study using 10 years of records (n= 75,000), when cows conceived in summer, their daughter’s milk production was lower compared with offspring that were conceived during winter (Brown et al., 2015). The difference in milk yield between the daughters of heat-stressed and thermoneutral cows ranged from 82 to 399 kg=per lactation. Consequently, planning to breed cows outside of the hot season or utilizing cooling systems might lessen the impact of reduced milk production from the daughters of these cows.
Heat stress of cows during late gestation was also demonstrated to have negative effects on daughter performance in a series of studies from the University of Florida. The final two months of gestation are critical as the fetus gains approximately 60% of its total birth weight (Bauman and Currie, 1980). Fetal growth was compromised in cows that were heat stressed in the 45 days prior to calving, demonstrated by the lighter birth weights of their calves, relative to those from dams that were cooled using fans and sprinklers (Monteiro et al., 2014; 2016). The difference in birth weight could be explained by shorter gestation times (by 4-5 days), lower maternal feed intake, impaired placental function, and reduced blood flow to the placenta. In addition, the transfer of immunity via absorption of antibodies in colostrum was impaired in calves born to heat-stressed dams, regardless of whether they received colostrum sourced from heat-stressed or cooled dams (Monteiro et al., 2014). Data from 5 consecutive summers also indicated that maternal heat stress negatively affected milk production and survival in the herd in the first lactation (Monteiro et al., 2016). Heifers from heat-stressed dams produced, on average, 5.1 kg milk/d less than those from cooled dams (31.9 ± 1.7 kg milk/d), equating to a total difference of approximately 1,250 kg milk/cow during the first 245 DIM.
These studies provide good reasons to consider the next generation of animals, well before they are born. Avoiding exposure to heat stress during the entirety of gestation is difficult, because gestation length spans three quarters of the year, and especially as climates become warmer. Therefore, management strategies to reduce the impact of heat stress, such as cooling cows with shade and fans, provide the most practical way to mitigate the lower productivity from the offspring of these cows, setting them up for a successful first lactation and beyond.
Virginia Cooperative Extension materials are available for public use, reprint, or citation without further permission, provided the use includes credit to the author and to Virginia Cooperative Extension, Virginia Tech, and Virginia State University.
Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.
May 2, 2017