By Fae Holin
Dave Combs has been traveling the U.S. the last two years, explaining the merits of total-tract NDF digestibility (TTNDFD). He says the standardized in vitro test can accurately tell how well a forage will feed.
In October, the University of Wisconsin dairy scientist added low-lignin alfalfa as part of his presentation. He and Rock River Laboratory, Watertown, WI, have been working with Alforex Seeds, which had just made public its low-lignin alfalfa, called Hi-Gest.
“What Alforex is doing with selecting a low-lignin line – TTNDFD dovetails very nicely with it – because it defines what that low-lignin level is actually doing to fiber digestibility within the plant,” Combs says.
TTNDFD, he adds, looks at fiber digestibility much the same way as producers and nutritionists look at corn starch digestion.
Farmers and consultants can get the most starch from corn by making sure their kernel processors are set up right, Combs says. “They know it means 3-5 lbs of milk production to get that starch digestibility optimized.
“The variance in fiber digestibility is twice what it is for (starch in) corn grain, so we’re missing opportunities unless we can really track that very well. We can lose 5-7 lbs of milk production if we’re not optimizing fiber digestibility,” he says.
Fiber digestibility “ranges all over the board,” Combs adds. In alfalfas, it varies from 25% to 70%; in grass hay and silages, from 15% to 80%, according to thousands of samples from Rock River Lab, analyzed over a three-year period.
“That’s why this tool, I think, will become a standard in the industry,” he says. TTNDFD incorporates three important factors that affect fiber digestion: the amount of potentially digestible fiber in a forage, how fast the fiber digests and the rate of passage from the cow.
“Those three numbers get integrated together to give us total-tract digestibility – one number,” Combs says.
“On average, 42% of the fiber that a cow consumes is actually digested. And every two- to three-unit change in fiber digestibility is enough energy to support about a pound of milk. So if we get 48% TTNDFD, and that’s going to be our goal, we expect 1-2 lbs more milk production potential from that ration. We can do that oftentimes by selecting high fiber-digestibility forages.”
To check the accuracy of the TTNDFD test, diets with similar levels of neutral detergent fiber (NDF) were compared.
The forage in the diets was 100% corn silage, or two-thirds corn silage and one-third alfalfa, or one-third corn silage and two-thirds alfalfa or 100% alfalfa. All four diets contained roughly 50% forage and 50% concentrate.
“We ran the diets through our in vitro system and predicted what the dietary total-tract NDF digestibility should be. We got 38% for the diet with 100% of the forage as corn silage. As we added more of this high-digestible alfalfa, the fiber digestibility we predicted would go up, from 38% to 45%.”
His predictions based on the lab test, Combs says, were then “matched to what the cows say” in feeding trials. The feeding study results matched what the in vitro TTNDFD test predicted. “They’re measured totally independently, and we see how closely they match up. The TTNDFD predictions have been matching up very, very well with what we’ve actually measured in the cows.”
A number of Hi-Gest low-lignin alfalfa check-plot samples were put through the TTNDFD test and compared to conventional alfalfas. The Hi-Gest variety averaged seven units higher in digestibility, he says.
“Total-tract digestibility is going to face some pretty strong headwinds” against other ways of measuring forage quality, including “the gold standard” – in vitro 30-hour NDF digestibility, says Mike Hutjens, emeritus University of Illinois Extension dairy nutritionist.
“But the 30-hour was an index, and this is a hard value. TTNDFD is based on really great science.
“Beyond that, it’s going to give a little bit of an edge to alfalfa and grasses” over corn silage, he adds.
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