By John Lovett
University of Arkansas System Division of Agriculture
Arkansas Agricultural Experiment Station
FAYETTEVILLE, Ark. — One percentage point of feed conversion loss in large-scale poultry production can cause millions in lost revenue, so even small improvements — like fine-tuning a single nutrient in feed — can make a big difference.
Calcium plays a key role in poultry feed conversion. In addition to bone density, enzyme activation, muscle contraction, and other critical functions, calcium affects the efficiency of turning food into weight gain. But in broilers, it's not just the amount of calcium that matters. What really counts is bioavailability: how much of that nutrient the bird can actually use.
Accurately measuring calcium bioavailability has been tricky for poultry scientists.
"The biggest challenge we have is analytically picking up the calcium content of our samples accurately," said Ben Parsons, an assistant professor of poultry nutrition with the Arkansas Agricultural Experiment Station, the research arm of the University of Arkansas System Division of Agriculture, and the Dale Bumpers College of Agricultural, Food and Life Sciences. "Our main calcium sources — dicalcium phosphate and limestone — are rocks. You may get a big piece or a little one in the sample, and it causes variability."
A new study by Parsons and his colleagues in the Center of Excellence for Poultry Science compared the results of two calcium availability tests — a classic approach and a newer, speedier test — and found that both tests offer reliable results that can help poultry producers optimize calcium digestibility.
Arkansas has consistently ranked as the third-largest producer of broiler chickens in the nation, producing more than 7.4 billion pounds of broiler meat in 2023, according to the latest Arkansas Agricultural Profile . Bringing in $6.5 billion, broiler production represents about 45 percent of all agricultural cash farm receipts in Arkansas.
In the quest to formulate optimum-performance poultry feed, poultry nutritionists have been looking at not just how much calcium is in the feed but how much is digested and absorbed by the bird.
Currently, the feed is formulated to meet a total calcium requirement in the diet, which does not account for differences in calcium availability among sources. Even among different sources of the same ingredient, calcium availability can vary due to factors such as solubility and particle size.
Value in the millions
Limestone is the most common calcium source in poultry diets, Parsons said. It's inexpensive and widely available, but its digestible calcium content can range from as low as 20 percent to nearly 80 percent. To be safe, feed producers often add more than needed — but too much calcium can reduce the availability of other nutrients, such as phosphorus, and may even worsen disease challenges, Parsons explained.
"Some recent work is showing that excess calcium can exacerbate disease and pathogen challenges," he said. "We don't understand how right now, but we know that there's value in trying to get more precise in how we're meeting the animal's calcium requirement."
The value could be worth millions. As Parsons explained, feed conversion ratio is a numerical value that can have a big effect even when it's a small number due to the high volume of birds being grown by the largest poultry producers.
"If you have a 1-point performance loss, that equates on a yearly basis to around $20 million to $24 million," Parsons said of large-scale poultry farms. "Small things that impair performance could end up costing a lot of money because that little reduction in performance gets amplified."
Two ways to measure calcium availability
Parsons and Rebekah Drysdale, Ph.D. poultry science student in Bumpers College at the University of Arkansas, conducted studies showing that a relatively new and speedy method of measuring calcium digestibility in the small intestines reflects results of an older and more time-consuming method of measuring bioavailability using ash or mineral content of a chicken's leg bone.
Drysdale developed the method to measure calcium bioavailability using bone ash in modern broiler chickens as part of her master's thesis, Parsons noted.
"The bone ash method is a classic approach that has been around for decades, mainly for trace minerals and phosphorus but very recently we've developed a regression approach for bone ash to measure calcium," Parsons said. "Our goal was to compare it to the new method, and if we could show they are similar, then we could move forward in using these tools to evaluate a lot of different calcium samples."
The newer method is called the "apparent ileal digestibility test" and involves collecting the partially digested feed known as digesta in the ileum, which is the end of the small intestines. The difference in calcium that was in the feed and what was left after digestion in the dried digesta can then be calculated.
While the apparent ileal digestibility test can be done in 24 to 72 hours with as few as one diet, the bone ash test process takes two weeks before the nutrients are absorbed in the bone and twice as many diets are needed compared with the newer digestibility test.
Results of the study indicate that the newer, more rapid digestibility test can be used to assess calcium availability in feedstuffs. Also, relative calcium bioavailability values based on bone ash content can also be used to predict or estimate calcium digestibility values.
Parsons said while the digestibility test is useful in providing direct measurements of calcium availability, tests for calcium bioavailability based on bone ash can be helpful to confirm results from digestibility tests while also eliminating analytical errors. The calcium bioavailability based on bone ash will also account for absorption, transportation and usage within the body, which offers additional insights beyond absorption or digestibility, Parsons added.
Finding the 'sweet spot'
Parsons said a long-term goal for poultry nutritionists is to move from total calcium requirements in poultry feed to digestible calcium levels.
"There's a big challenge in that because you've really got to have a good robust data set or database of calcium availability of different sources," Parsons said.
With varying levels of calcium digestibility in different sources of limestone, Parsons said both methods of testing can help poultry producers screen calcium sources that are causing problems and find sources with a "sweet spot" of digestibility based on solubility rates.
"You want a limestone that's in the middle of this solubility distribution," Parsons said. "If you get something that solubilizes rapidly in the GI tract, that's actually problematic and leads to reduced availability of other nutrients like phosphorus. If you get it solubilizing too slowly, the bird can't use it."
Drysdale and Parsons published the results of their experiments in the journal Poultry Science under the title "Comparison of relative calcium bioavailability based on bone ash and apparent ileal Ca digestibility in broiler chickens."
Seth Hufford, a program technician in the poultry science department, has also been a collaborator on the calcium studies.
To learn more about the Division of Agriculture research, visit the Arkansas Agricultural Experiment Station website
