If you want to use NDFD to help rank silage-corn hybrid performance, don’t compare your lab’s NDFD results with another’s, warns Dave Combs, University of Wisconsin dairy scientist.
Dairy nutritionists and producers would like to use NDFD (neutral detergent fiber digestibility) to help formulate their herds’ rations, too. There’s a general consensus among scientists, Combs says, that increased fiber digestibility (the digestible part of total fiber, or NDF) produces more milk.
But the labs that perform NDF digestibility tests haven’t agreed on a common methodology for measuring fiber digestibility. So results from one lab to another are inconsistent and of little value if compared to each other, he says.
“If you’re trying to evaluate your corn silage and asking, ‘Is it higher or lower than normal in terms of fiber digestibility?’ you can only do that by comparing your result to the average corn silage (NDFD) that went through that lab.”
Forage-analysis laboratories offer databases in which they have tested hundreds to thousands of silages, so each can provide an average NDFD value. "You can look at your results compared to that lab average and answer the questions, ‘Did mine fall close? Are they significantly better? Or are they significantly worse?’"
NDFD may differ only three to four units from the lab’s NDFD average. Or it may vary too much, Combs says. “Your consultant or you should be able to say, ‘If I’m having a problem on my farm and I think it’s related to corn silage, if that number is greatly different from the average, that may be a clue. Unfortunately, that’s about as sophisticated as you can get with NDFD results right now.”
To drive his point home, Combs did what he tells producers to avoid. He compared average NDFD values among four labs that analyze corn silage in the Midwest. One lab reported an average 42% NDF while the other three averaged 43% NDF. But each of those labs, testing 24-, 30- and 48-hour NDFD values, showed numbers that were “all over the map,” Combs says. (Click to view chart.)
“That doesn’t mean that NDFD is a useless number. It just means that labs have different methodologies and you can’t compare lab to lab.”