We are midway through the summer and on pace for above average heat for the season. The effects of summer heat and humidity are well documented in lactating dairy cows, as we see decreases in dry matter intake and subsequent production losses in the bulk tank. However, heat stress in dry cows is subtler and can linger long into lactation.
    In a recent roundtable discussion, Dr. Geoff Dahl, University of Florida, shared results from three studies the university conducted to evaluate the effects of heat stress on dry cows and the benefits heat abatement strategies can provide through this period. He explained that cows were moved to a freestall barn at dry off and exposed to either heat stress or cooling with fans and sprinklers. Both treatment groups received the same dry cow ration from dry off until calving. After calving, the animals from both groups were moved to one common sand-bedded freestall with fans and sprinklers. It is important to note that the cows were only heat stressed during the dry period and the milking cow ration was the same for both groups.
    Results showed cows that were cooled during the dry period yielded significantly more milk over the first 20 to 30 weeks of lactation. The average across all three Florida studies was 11 pounds more fat-corrected milk per cow per day in early lactation. Heat stress during the dry period also significantly reduced DMI prior to calving compared to cooled cows. This trend for higher feed intake continued post-calving and progressively increased to the end of the trials.
    During the dry period, udder cells from the previous lactation are replaced with new milk secretory cells to support milk production in the next lactation. Mammary tissue biopsies were conducted in the third University of Florida study to determine the impact of heat stress on mammary cell turnover. Results indicated heat stress decreased mammary epithelial cell proliferation, or creation of new cells, which may be a contributing factor to lower milk yield.
    Heat stress during the dry period has a negative effect on animal health and immune function during the transition period. Neutrophils, specialized immune cells responsible for fighting infections, do not function well in heat-stressed cows. The University of Florida studies showed an increase in body temperature and respiration rates on heat-stressed dry cows.
    On average, cows exposed to heat stress had a dry period that was seven days shorter than those cooled. In addition, calves born from the heat-stressed cows had significantly lower birth weights, ranging from 12 to 42 percent lower birth weight. University of Florida researchers have also continued to follow the calves born from these two dry cow treatment groups and found significant improvements in milk production for the calves born to the cooled cows.
    Heat abatement in dry cows may require significant investment in facilities or equipment, but the research shows it is worth the investment. Not only does it support greater DMI, improved mammary cell growth, better immune function and subsequent milk production, but it also lends itself to healthier calves, which will pay dividends two years down the road.
    Barry Visser is a nutritionist for Vita Plus.