Jim Linn and Dave Mertens believe NDF digestibility (NDFD) testing can be a helpful tool for monitoring forage quality in dairy rations. But their data doesn't show that fiber digestibility has a significant effect on milk production.

The two dairy scientists — Linn at the University of Minnesota and Mertens with the U.S. Dairy Forage Research Center in Madison, WI — got inconclusive results in feeding trials comparing high- and low-NDFD alfalfa hay.

They had hoped cows fed highly digestible hay would produce more milk. That would have validated the merits of both NDFD analysis and relative forage quality (RFQ), the new forage testing index that includes a measure of fiber digestibility.

“I hate to hedge, but it just didn't come out clean enough to be able to say anything convincingly one way or the other,” says Mertens. “That puts us in a bit of a quandary as to exactly what to say.”

The 12-week trials were designed to feed early lactation cows four diets containing hay high in both NDF and NDFD, high in NDF and low in NDFD, low in NDF and high in NDFD or low in both NDF and NDFD. Hay supplied 15% of ration dry matter in Minnesota; 30% in Wisconsin. At both locations, the TMRs also contained corn silage, corn, a concentrate mix and other ingredients.

In Minnesota, neither the level of NDF nor its digestibility affected milk production. The difference between high- and low-NDFD hay was 4-5 percentage points, determined by both 48-hour in vitro and NIRS testing. With that narrow range and hay accounting for only 15% of ration dry matter, Linn didn't expect to see a production difference.

However, he did expect a difference in a short-term trial in which high- or low-NDFD hay was fed at 96% of the diet for midlactation cows.

“If NDFD is going to be a factor, that's the kind of diet where it really should exhibit itself,” he says.

Again, though, milk production was statistically the same among both groups. However, Linn says that, with the high-NDF, low NDFD hay, there appeared to be a reduction in feed intake, and cows needed to support milk production through body weight loss.

Mertens saw a small response in one part of his trial. When he fed high-fiber (38% NDF) hay to high- and low-NDFD cow groups, those fed the more digestible hay produced 3 lbs more milk per day. But the difference wasn't statistically significant given the low cow numbers (17 per treatment).

That hay had almost a 10 percentage point difference in NDFD — 53.1% vs. 43.4%. Linn and Mertens had hoped for a 10-point difference between high- and low-NDFD hay throughout the trials. But the range was narrower than that, especially in the low-NDF hay.

They aren't sure why they didn't get more response, but wish the hays' digestibility difference had been greater. Finding large quantities of hay with the right analysis was difficult.

“One of the problems is, we don't know all the factors that affect digestibility,” says Linn.

Growing conditions are one factor. Hay is thought to be most digestible when grown in cool climates. So the researchers bought hay from a fairly wide geographic area — Idaho, Minnesota, Illinois and Kansas — but probably ended up with all first-cutting hay.

Getting reliable NDFD analyses was another challenge. In vitro testing utilizes rumen fluid, which can vary, even when extracted from the same cow. NIRS testing, if done correctly, can be more consistent, say Linn and Mertens.

They sent hay samples to several forage testing labs for NDFD testing. They found that the labs use a variety of testing methods and get differing results.

“Eventually, this is something we need to confront,” says Mertens. “We need to make sure NDFD values are consistent within and among labs for this tool to be useful.”

He says NDFD testing is still in its infancy, and will get better.

“There are improvements to be made,” says Mertens. “We have a technique that has a good conceptual basis for getting us information about forage quality, and we need to work to continue to improve it.”

Meanwhile, the two researchers suggest that dairy producers and their nutritionists monitor the NDFD of forages.

“I would use NIRS and I would use the same lab so that it's a consistent process,” says Linn.

They agree that NDFD is not a magic bullet that will explain all forage quality differences.

“It's one more bit of information that will be beneficial,” says Mertens.

But don't get excited about small differences. NDFD test results are variable estimates, not absolute numbers. Based on one test, it may take more than five units difference in NDFD to indicate that forages are different.

Finally, don't expect a big milk production change if your hay or silage gets significantly more or less digestible. Forages are only part of the ration, and any difference in forage is diluted by the other feeds in the diet.

“I'm not willing to throw the baby out with the bath water, but I have to say that it's maybe not as striking as some people may believe,” says Mertens.