A dairyman's desire is to feed his best forages to his highest-producing cows. Yet that “best” usually isn't based on fiber digestibility — and should be, says Mike Allen, Michigan State University dairy nutritionist.

“Dairy managers might feed the highest-protein forage, like the alfalfa that's the leafiest. But that might not be the alfalfa with the highest fiber digestibility,” he explains.

Allen, who has been testing forages using in vitro digestibility since 1987, has found that the more fiber a high-producing cow can digest, the more milk she's likely to produce.

“We found that we got a much greater response to a more digestible fiber source for higher-producing cows than for lower-producing cows,” Allen says. “Cows producing around 70 lbs of milk had no response to a more digestible fiber source. Cows over 110 lbs of milk increased milk production more than 15 lbs/day.”

To identify fiber digestibility, dairy producers can run either an NIR test or an in-vitro wet-chemistry test, says Mike Hutjens, University of Illinois extension dairy nutritionist. Although the $15-18 NIR test is quicker than in vitro — and sometimes half its cost — some nutritionists aren't convinced of its accuracy.

Hutjens says that in vitro testing is the “bottom line.” A fermentation system, such as a glass container with no air, ferments a forage sample for 30 or 48 hours, allowing the bacteria to show just how much of the fiber is digestible in the rumen.

A 30-hour in vitro test measures the forage neutral detergent fiber (NDF) digested in rumen fluid. A 48-hour in vitro test is a better estimate of the maximum amount of digestible NDF.

Some labs choose to use the 30-hour test because it represents the normal time forages remain in the rumen — and shows what high-producing cows will digest, Hutjens says.

The percent of NDF cows digest, also called NDF digestibility or NDFD, varies widely — even among forages that are otherwise similar. But in vitro testing for fiber digestibility is most useful if forages can be segregated, Allen says.

“If you have your forages segregated,” he says, “you'll want to test them for fiber digestibility after ensiling for 30 days or so. The farms that can make the best use of in vitro digestibility testing would have their forage lots more segregated and test after ensiling or, if it's hay, after it's been harvested. Then they make decisions on which forage goes to which animals.”

In vitro testing can also be used in troubleshooting, Allen says. He recommends freezing a silage sample from the previous year's crop and then, if milk production drops, a dairyman can compare the fiber digestibility from one year's crop to the next.

“My recommendation,” Hutjens agrees, “is that I would like to test my major forages every year to get a baseline. Then, if something goes wrong, whether in forage harvesting or storage, I would have my forage tested and compare it to the baseline.”

Just be sure to run the same type of test — 30- or 48-hour — for each sample so you're comparing results correctly, he adds.

Expo Exhibit Will Explore Digestibility

The latest lab method used to determine fiber digestibility of forages will be showcased at this year's World Dairy Expo, running Sept. 30-Oct. 4 in Madison, WI.

“Our display will demonstrate the meaning, measurement and application of a 48-hour in vitro digestion of neutral detergent fiber,” says Neal Martin. He's director of the U.S. Dairy Forage Research Center, which is sponsoring the exhibit.

Lab equipment will show how samples of various forages with fast and slow digestion rates are fermented in rumen fluid. That demonstration will be accompanied by an interactive computer display that shows varying digestion rates.

Staff from the center will be on hand to answer questions, adds Martin.

The display will be adjacent to the World's Forage Analysis Superbowl exhibit area in the Arena Building at Alliant Energy Center.