The negative effects of heat stress on reproduction are well documented, but we sometimes forget that heat stress reduces fertility in heifers as well as lactating cows. Heifers generally seem to tolerate higher environmental temperatures before showing signs of heat stress, but once heat stress begins, the results are the same as in cows: reduced conception rates and increased early embryonic death. Researchers in Iran studied the impact of short-term cooling strategies on pregnancy rate in 90 Holstein heifers. Their results were published in the June 2009 issue of Veterinary Research Communications.
During the trial, average daily temperature was 97 degrees F with 49 percent relative humidity (Temperature Humidity Index of 86.3). Estrus was synchronized with two prostaglandin injections given 11 days apart. At the second injection, all heifers were moved into shaded, enclosed housing and separated into three groups: control (no cooling), sprinkler cooling and sprinkler plus fan cooling. Cooling treatments were applied for two hours before breeding by artificial insemination and for two hours after breeding.
All heifers had similar, elevated body temperatures (103.8 degrees F) when measured 2.5 hours before breeding. Body temperature at breeding was lowest in the heifers cooled with sprinklers and fans (102.7 degrees F), intermediate in sprinkler-cooled heifers (103.1 degrees F) and highest in control heifers (103.8 degrees F). By 3.5 hours after breeding, body temperature of heifers in the sprinkler and fan group had fallen to 101.6 degrees F and was significantly lower than that of control and sprinkler-cooled heifers (102.7 degrees F). All heifers remained in the shaded structure for 45 days, but no further cooling treatments were applied. Pregnancy rates (determined by palpation at 45 days after breeding) were 57 percent, 40 percent and 23 percent for sprinkler and fan, sprinkler only and control heifers respectively, and the difference between sprinkler and fan and control groups was statistically significant.
This study suggests that short-term cooling applied around the time of insemination could be useful in improving the success of heifer reproduction during heat stress.