When producers think about keeping cows cool, keeping lactating cows cool, especially fresh cows, is usually top of mind. After all, those cows are the most susceptible to heat stress as they recover from calving, overcome a negative energy balance and reach peak milk.
But let’s not forget the importance of keeping cows in late lactation and the dry period cool as well. Even though these cows are usually left to coast through the end of their lactation and the first six weeks of the dry period before the transition period starts, research is showing that any heat stress these cows face not only impacts the next lactation, but has startling effects on the calves they carry.
Heat and humidity is a common, year-round occurrence in Florida which makes the climate ideal for conducting research on the impact of heat stress on dry cows. Heat stress studies conducted by Geoffrey Dahl at the University of Florida showed how the impact of heat stress on dry cows can impact the next lactation and beyond.
“We put cows in a heat-stress environment as soon as they were dried off and they didn’t get cooled again until they actually calved,” Dahl says. Over the six week dry period cows average a full degree increase in average body temperature over this period. “So this is a chronic elevation of body temperature and there’s never a time for a six week period where cows achieve a normal body temperature.”
When that elevation of body temperature happens over the full dry period, Dahl says, cows in the next lactation produce a lot less milk compared to cows that are cooled. “You get a pretty profound drop in milk yield in that next lactation when animals are heat stressed for the entire dry period,” he says.
What it comes down to is the number of mammary cells developed during the dry period in cows that are heat stressed versus cows that are cooled. Udders were examined from cows at about three weeks into the dry period, and Dahl says cows that were cooled had developed significantly more milk-producing cells than cows that were under heat stress.
“That’s suggesting that there’s going to be greater capacity for milk yield in that next lactation in the animals that are cooled versus those animals that are heat stressed,” he says. “It’s not that cows that are stressed when they are dry have a more rapid loss of cells, it’s more related to cell proliferation when they are dry.”
While milk yield is one casualty of heat stressed dry cows, immune function is another. When Dahl looked at data on a large commercial dairy, mastitis incidence, respiratory disease and retained fetal membranes were all higher for cows that had dry periods during hotter months versus cows that were dry during the winter.
“Cows that were dry during the cooler months of the year had better reproductive performance despite the fact they were at a higher level of production,” says Dahl.
Several research studies have also shown that cows under heat stress during the dry period also calve sooner as well. This explains some of the reason for production differences with cows that are cooled and carry calves full term, and also explains some of the impact heat stress has on calves.
“When that cow was experiencing heat stress obviously that calf in utero is experiencing that same heat stress over the last six weeks of development,” says Dahl. Research shows that cows heat stressed in the dry period produce lighter calves, and that negative growth rate continues as the calf gets older.
Calves may have challenged immune systems as well, starting with their ability to absorb immunoglobulins (IG) from colostrum. In a study conducted by Dahl, Even though two groups of animals received the same amounts of colostrum, calves born to heat stressed dams had lower blood IG concentrations than calves born to cooled dams.
“At first we looked at if there were certain gross differences in colostrum quality between these two groups,” says Dahl. “In fact when we look at IG concentration, levels in dams that are heat stressed is typically at or above what we had in cooled animals, they just make less of it.”
Further examination shows that it’s not necessarily the colostrum that comes from heat stressed dams that’s the issue. Two groups of calves from heat stressed and cooled cows were fed colostrum from the same source, and the heat stressed calves still had lower absorption rates than calves born to cooled dams.
“That’s pretty strong evidence that there’s a direct effect on that calf, and it’s limiting their ability to take up IG,” says Dahl.
Calves also have a shift in their metabolic pathway as well, and tend to shift resources to adding fat rather than lean growth which, Dahl says, could account for some of the issues that occur later in life. Calves born to heat stressed dams are more likely to leave the herd prior to puberty. Calves from heat stressed cows that made it to first lactation produced less milk than calves from cooled cows.
“In fact, it looks like what we’re seeing is an epigenetic effect that is limiting those calves’ ability to reach their genetic potential,” says Dahl. “When we look at second and third lactation production, those calves continue to lag behind those calves if they experienced heat stress.”
There is also a transgenerational effect when the calves pass on this effect to their own offspring, which Dahl says is one of the indicators of epigenetic influence.
Taking a look at the opportunities for heat stress in different areas, Dahl says the top 23 dairy states average about 90 days of potential heat stress situations each year. Given the impact heat stress has on the cow, the calf and future offspring, Dahl says it should make economic sense to cool dry cows.
“If you have a facility that just needs to be retrofitted with fans and soakers to make sure that we get those cows cooled it’s really a no brainer,” he says. “It makes sense to get those cows cooled off.”