Managing heat stress in calves and heifers
by Stephen Wagner
Dr. Jennifer Trout is a calf and heifer specialist with Cargill Nutrition and works with large calf ranches, heifer yards and dairies in the American West and Southwest. She earned her DVM from the University of Florida; she’s practiced dairy medicine in California, Texas and Maryland on dairies ranging from as small as 50 cows to as large as 12,000.
“With increasing temperatures around the world, people are looking at ways to be concerned about the welfare of their animals with heat stress as one of the big issues,” she said. In fact, there is an initiative in Holland to plant 100,000 trees to afford shade for livestock, so the heat issues aren’t just in the U.S. High temperatures combined with humidity and radiation from the sun can cause heat stress in young stock.
“This was on Instagram last week: 70º in Lancaster County at the end of April,” Trout reported. “So already these calves would be experiencing some heat stress.” Where a calf is comfortable, and where her body really likes to hang out, is a thermo-neutral zone – usually 50º to 77º. Lower than that, they begin to experience cold stress. “We know that calves who have experienced heat stress calve at an older age for their first lactation than those who don’t,” she said. “We also know that it costs more money to raise those calves … Their milk yield is decreased.”
Calves will drink more water when it’s hot. They don’t move around as much because they don’t want to. If they start to become dehydrated for whatever reason, they will have less urine output, “and the manure pats that we check in the calf hutch or in the calf barn will be firmer than we normally see,” Trout said. Another measurement is the temperature humidity index, which looks at the environmental temperature in combination with relative humidity. It estimates degree of discomfort. For a calf, the threshold is higher here than it is for cattle because calves have a larger surface area, relative to their body weight, for heat dispersal. That helps them tolerate warmer temperatures, and they generate less heat because they don’t ruminate at full capacity, like a lactating milk cow or a yearling.
When does heat stress really start for calves? The University of Florida reported that dry period heat stress affects the cow in her subsequent lactation and future birth weights. Birth weights drop and are reduced about 11 pounds and they don’t last as long in the herd. Longevity is reduced by about 30%. This can be generational.
“Interestingly enough, the heat stress that this cow experiences during the dry period affects her daughters,” said Trout. This calf, as she matures and comes into the milking stream, experiences reduced production over three lactations. Then, to compound this intergenerational effect even more, the daughter’s calf, which is now the original dame’s granddaughter, has lactations on her first two productions reduced as well. The takeaway is that the first step on mitigating heat stress on calves and heifers is to cool our dry cows. The total losses from dry cows: $1.4 billion a year.
Trout said, “We know that bacteria doubles every 20 minutes in warm weather and that is especially true in warm colostrum. You can start with a product that has minimal bacteria in it, and it replicates every 20 minutes, and you can end up with a bottle that is chock full of bacteria.” If you’re housing calves in hutches, make sure all the vents are open; elevate the backs of them to help with airflow and circulation. It also reduces the CO2 levels and bacteria levels in the hutch as well. Make sure that bedding isn’t obstructing airflow. Space your hutches four feet apart with 10 feet between rows to allow for circulation and airflow. Put shade on them, if at all possible, to help reduce temperatures inside. If you have a calf barn you might want to orient it perpendicular to prevailing winds. “Make sure your side curtains are open,” Trout suggested, “and that fans are going, tubes are going. You want to make sure if it gets humid in there that you can get airflow so there’s no humidity.”
With heifers, it’s the same thing. “Shade out in the pastures, shade on wean-kept pens, on breeding pens, anything to help reduce the load,” she said. “Heifers or calves need 24/7 access to water. We know that water consumption increases 20% to 35% in these animals during the summer.”
With calves, we realize that starter intake is reduced in summer. Maintenance requirements can also increase 20% to 30%. “Consider feeding electrolytes to these calves during the day,” Trout recommended. “It would be an added source of hydration for them.”
Trout concluded with water. “Ideally, it’s delivered at 30 minutes past milk replacer. Being delivered the calf is still up, the calf is still curious, the calf will start drinking and the calf will go to grain and start eating, which is great for rumen development, encouraging that grain intake. You want it to be always available.”
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