ST. Paul, Minn. — Dairy cows are marvelous calcium athletes.
“We think of cows being marvelous energy athletes, but they are also calcium athletes and they will struggle with calcium balance post calving just like they struggle with negative energy balance post calving,” said Gary Oetzel, professor in the School of Veterinary Medicine at the University of Wisconsin.
“Calcium homeostasis is fragile around calving time. It’s a difficult time metabolically for the cow because milk is really high in calcium,” said Oetzel during a webinar organized by Hoard’s Dairyman. “That’s great for people because one pint of milk provides 60% of my daily requirement for calcium.”
Cows pump all this calcium into their milk to support the skeletal growth of their calf, the veterinarian said, but they put themselves at great risk.
“Colostrum is even higher in calcium than milk, about twice as much,” Oetzel said. “A dairy cow can easily put 15 grams of calcium into the first colostrum and she repeats that 24 hours later.”
The risk of clinical milk fever in dairy cows is low, the veterinarian said, but the outcomes are really bad.
“On the good side the incidence is declining from 6.2% in 2002 down to 3.7% in a recent survey,” he said. “I have many herds that do much better below 0.5%, so we’ve done some good things to reduce the risk for clinical milk fever.”
Subclinical hypocalcemia risk in dairy cows is high and it is bad for the average cow, Oetzel said.
“This study shows persistent subclinical hypocalcemia affected future reproductive performance more than cows that were hypocalcemia on day one or two,” he said.
“Most older cows develop subclinical hypocalcemia and it’s most harmful if it persists out past 48 hours post calving,” Oetzel said. “When we see it, it may be secondary to another condition.”
“We need to think about prevention,” he said.
In a study of a Jersey herd, 11 cows received nothing and they got along quite well by 48 hours after calving.
“The cows that received oral calcium showed the classic response,” Oetzel said. “It gave gentle support, but it did not create a rebound hypocalcemia. That’s what happens when you give an IV calcium.”
The third group of cows received one dose of IV calcium.
“By 24 hours they were significantly lower than if they got nothing at all and it stayed that way until 48 hours,” Oetzel said. “Do not give IV calcium to a standing cow, only give it if she needs it.”
Oral calcium provides support during the critical time period, Oetzel said, but it doesn’t fix the problem.
“Lame and higher producing cows responded best to oral calcium in this study,” he said. “Lame cows had fewer health events if given two calcium boluses around calving.”
Cows have a complex response to oral calcium, Oetzel said.
“We think the cows are shunting it off to milk,” he said. “They are doing a little better in early lactation, but not fixing some of the other issues that go with having subclinical hypocalcemia.”
Dairymen may choose dietary means of prevention.
“Calcium binders create a functional calcium deficiency,” Oetzel said. “This is easier to manage than anionic salts because you don’t have to monitor urinary pH and you don’t have to adjust the dose you’re feeding.”
However, Oetzel said, there are disadvantages.
“The cost is rather high at $1.25 per cow per day which tends to push producers to a shorter pre-fresh period, so they feed the expensive stuff for a shorter period of time,” he said.
“If you have cows that don’t spend enough time in the pre-fresh pen then you’ll have a bunch of energy-related problems so don’t shorten the per-fresh period.”
Another option is acidogenic diets.
“We know they work, but the tendency is to expect too much from them,” Oetzel said. “It results in about a 50% reduction in clinical milk fevers but we can’t shut it down to zero.”
And researchers don’t know very well how these diets affect the subclinical form.
“Management is important to make acidogenic diets work,” Oetzel said.
“You have to check the urinary pH,” he said.
“The goal is for a mean pH around 6.5 and I’m concerned about cows that are over-acidified with a pH below 5.5,” he said. “The cows are under-acidified if they are over 7.”
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