Not all the negative effects of heat stress take place in lactating cows, according to University of Florida researcher Geoffrey Dahl.
Dahl recently shared the results of his heat-stress research with the audience at the Dairy Calf and Heifer Association Annual Conference. He noted that dry cows exposed to heat stress with no evaporative cooling suffered setbacks in mammary cell development, which translated into reduced milk production in their subsequent lactations. They also showed increased disease incidence and were more reproductively challenged compared to their herd mates whose dry periods were in cooler months.
Equally concerning was the impact of heat stress that Dahl and his colleagues observed on the female fetuses that were being carried by heat-stressed dry cows. Through an exhaustive evaluation of data, they noted that, compared to herd mates born to cooled dams, fetuses exposed to late-gestation heat stress:
- Were born 4 to 6 days earlier, and at a lower birth weight.
- Remained at a lower weight through weaning, and also were shorter at the withers at weaning.
- Experienced less efficient passive transfer of immunity from colostrum, due to accelerated post-birth gut closer (and not due to a difference in colostrum quality).
- Continued to have lower body weight through the first year of life.
- Had significantly lower survival rates in the herd – especially before puberty – and thus had a lower overall entry rate into the lactating herd.
- Produced 10 pounds of milk per day less -- despite no apparent genetic, nutritional or management differences -- in their first lactations.
- Never reached the same level of production in their second and third lactations;
- Passed on their lower milk production performance to their own offspring.
Dahl concluded that, while heat-stress abatement is important for dairy animals of all ages, the most far-reaching benefits may be in providing heat-stress relief to dry cows.