By Neil Tietz
A new forage test that actually measures how much forage a cow can digest will help producers and nutritionists maximize dry matter intake and milk production.
So says Cornell University animal scientist Mike Van Amburgh. The test uses in vitro fermentation to measure undigested NDF (uNDF), sometimes called indigestible NDF (iNDF), in alfalfa, corn silage and other forages.
Forage-testing labs have traditionally used a relationship between lignin and NDF to estimate fiber digestibility, but that’s not always an accurate measure of what happens in cows’ rumens, says Van Amburgh. Two corn silage samples with identical amounts of NDF and lignin, for example, can vary in digestibility by 30 or 40 percentage points, he says.
The test he and his colleagues developed delivers much more accurate readings. They’ve used it to partition NDF into three pools – fast-digesting, slow-digesting and uNDF. The pools are inversely related, so forages low in uNDF tend to have large fast-digesting pools.
“The forage with the low uNDF can disappear very, very quickly, and when it does, that’s when you pick up feed intake and milk,” he says.
“The uNDF content of the TMR looks like the factor the cow actually fills to,” he adds. “The lower we can make the uNDF, the more digestible NDF she’s going to eat and the more total dry matter she’s going to eat.”
He says the test can help producers optimize the use of available forages. If the uNDF in a producer’s forages is low, he may be able to feed a ration that’s 60-70% forage. But if it’s high, he might need to cut back on forages and add a non-forage fiber source.
The test is available from Cumberland Valley Analytical Services, Hagerstown, MD; Dairyland Labs, Arcadia, WI; Dairy One, Ithaca, NY; and Rock River Lab, Watertown, WI.
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