Understandably, calving during hot weather can negatively affect dams. But recent research indicates that the effects are far reaching, even to the grown calves of those heat-stressed dams.
Geoffrey Dahl presented “Baby It’s Hot in Here: Late Gestation Heat Stress Effects on Cow and Calf” as a recent webinar hosted by Animal Agricultural Modeling & Training Systems.
Dahl is a professor in the Department of Animal Sciences at the Institute of Food and Agricultural Science at the University of Florida. He shared research conducted in Gainesville, FL, on the effects of heat on cow and calf. Although a controlled study, the researchers attempted to mimic the typical commercial production barn.
The animals in the study enjoyed the cow comforts of sand-bedded free-stalls, fans over the stalls, soakers over the feedline, fans on at 70º F and soakers on for 1.5 minutes every five minutes at 72º.
But without active cooling, the researchers can induce heat stress. As expected, the heat stress increased average rectal temperature among the group that wasn’t actively cooled. The cooled cows’ temperature was lower.
“For our studies, the animals are dried off. They’ll come back to a pen where they may or may not have cooling. For a six- to seven-week dry period, they’re in a situation where they never have a normal body temperature for that time. They’re chronically heat stressed in our studies,” Dahl explained.
On many farms, dry cows don’t receive the special treatment lactating cows do, which is why Dahl and his team mimicked those environmental differences with the two cow groups: dry hot cows and dry cooled cows.
Allowing cows to become heat stressed “causes a reduction in milk production in their next lactation,” Dahl said. “There’s not a difference in how they’re managed after they’re calved. The only difference is what happens to them during the next dry period.”
Throughout the entire next lactation, the cows that were heat-stressed while dry produced less milk. Dahl compared his findings with 14 studies from around the world and they all offered the same conclusions.
Dahl’s team believes mammary capacity was affected. “Cows typically accumulate or activate cells early in lactation and ramp up to the peak in lactation,” he said. “That drop is driven by lack of epithelial cells in late lactation. We have to look at both of these.”
Cooling dry cows before their next lactation offers benefits for the farm.
“Those animals that were cooled have a greater capacity for milk production than those that were heat stressed,” Dahl said.
The researchers also looked at how long cows must be cooled during the dry period. In another experiment, they had four groups: cooled dry cows; dry cows heat stressed three weeks and then cooled three weeks; dry cows cooled three weeks and then heat stressed; and dry cows heat stressed the entire six-week dry period. It went as we expected, Dahl said.
The cows experiencing any heat stress at any point evidenced higher rectal temperatures and lower milk production.
“What we didn’t expect is gestation length,” Dahl said. “They can have calves three to four days early. It affects dry period length. Those who were cooled have gestation three to four days longer than the hot cows. The switch groups responded the same way whether cooled first or next.”
It suggests that heat stress at any time of the dry period, whether early or late, affects the cows negatively.
Heat stress on placental function may affect first calf heifers.
“In this case, we took heifers from pasture and took them into a free-stall barn and exposed them to cooling or not,” Dahl said. “They had shade and access to stalls, but they didn’t have active cooling. It’s reflected in their respiration rate relative those to contemporaries that had access to active cooling.”
By reviewing data from a large dairy, Dahl and his team agreed that cows dry in cool months exhibit better performance, including higher milk production – a few more liters daily – and low occurrence of mastitis, digestive and respiratory problems.
“Reproduction was poorer among animals that were dry during the hotter part of the year,” Dahl. “It suggests there’s an effect on reproductive performance on animals heat stressed late in gestation.”
Dahl’s research even shows evidence of long-term impacts on the calf of a dam that experienced heat stress while carrying it. And they tend to experience reduced birthweight.
“That placental dysfunction will potentially reduce nutrient delivery and limit growth in the heat stressed dams compared with cooled dams,” Dahl said.
The average weight of a calf of a heat stressed dam was 36.7 kg; those cooled were 42.4 kg. The littler calves remained smaller at weaning as well.
“There is a lower immunoglobulin circulation in calves born to heat stressed dams,” Dahl said.
The calves were fed the same colostrum, at the same time, at same immunoglobulin level. Dahl attributed the lower level in heat stressed calves to lower uptake. Since that trait persists through their first month of life, it “alters their ability for immunoglobulin transfer,” he said.
The researchers tried feeding calves the same, pooled colostrum to look for the effects of heat stress on calves; however, it made no difference. The heat stressed calves still started life behind and stayed behind with lower weight and height than their cooled peers.
Feeding colostrum from cooled cows also did not boost the growth of the heat stressed calves.
The researchers followed records of calves to puberty and the heat stressed group still lagged behind the cooled calves in bodyweight. In utero heat stress also decreases the chances of calf survival, with 85.4% of heifers completing first lactation in the cooled group and only 65.9% in the heat stressed group.
“It confirms that early life has an effect on a calf’s survival overall,” Dahl said.
The effects appear to last for the heat stressed heifer calf for life. The age at first artificial insemination is later for calves born to heat stressed dams. They also have more services per pregnancy, older age at pregnancy and older age at calving.
“If anything, there were only positive indications for calves born to cooled dams versus those born to heat stressed dams,” Dahl said.
Furthermore, heifer calves born to heat stressed dams have lower milk production. The dam’s dry period heat stress also affects lifetime lactation for their heifer calves.
“Daughters of dams that were cooled give two liters more milk in their first and second lactations and it gets more extreme beyond that point,” Dahl said.
These effects also impact the cow’s life in the herd – about a year less than their cooled herd mates.
“More concerning is that they appear to pass this phenotype on to their offspring,” Dahl said.