Use This to Track Fiber Digestibility

Feed intakes, passage rates depend on ration fiber. ( Wyatt Bechtel )

Your eyes can quickly glaze over as you scan down the alphabet soup of test results on any forage analysis. But there’s a new test you and your nutritionist should focus on to gain optimal cow performance, particularly for forage targeted toward early lactation and high groups, advises David Combs, a dairy nutritionist with the University of Wisconsin.

That test: total tract neutral detergent fiber digestibility, or TTNDFD. The TTNDFD test was developed to predict fiber digestibility in high-producing dairy cattle. It can be used across forage types and byproduct feeds, and it can be used in ration balancing and evaluation, Combs says.

Watch Digestibility

Fiber digestibility is important because most performance swings in early-lactation cows is usually do to the energy concentration of the diet, Combs says. In addition to monitoring starch levels in diets, farmers and their nutritionists also need to keep an eye on the digestibility of forage neutral detergent fiber (NDF) levels. Remember, 30% of energy from corn silage comes from its fiber component.

“If fiber digestibility is compromised, it can create problems and decrease milk production,” he explains. “A two- to three-unit drop in fiber in ration TTNDFD corresponds to a 1-lb. change in milk yield.”

“We think two thirds or more of the variation in fiber digestibility is likely due to growing and environmental conditions, such as moisture, temperature and sun intensity. And we can’t control that,” Combs says.

Harvest and Digestibility

But there are things farmers can do to affect fiber digestibility. These include the timing and stage of growth at harvest as well as the timing of irrigation. In addition, low-lignin alfalfa varieties can have 10% to 14% higher digestibility levels than conventional alfalfa varieties. Stubble height can also affect fiber digestibility in both alfalfa and corn silage. Brown midrib corn hybrids also typically have higher fiber digestibility.

Alfalfa harvest management also can have an effect. Most of the digestible fiber in alfalfa is found in leaves, so rain damage, increased respiration losses due to slow dry down or increases in leaf loss will usually lower fiber digestibility.

The good news with the TTNDFD test, performed with near-infrared technology, is that results don’t change. Starch content of corn silage typically increases the longer the silage is in storage. But the TTNDFD test is unaffected by fermentation, so farmers and nutritionists can have a good idea of a forage’s fiber digestibility when it is ensiled, Combs says.    

Ash Content Increasing

“We’re seeing a significant increase in ash content in forage samples, and much of that can be attributed to harvest management,” says David Combs, a dairy nutritionist with the University of Wisconsin.

“It’s tough to get milk out of a rock, which ash is. Increased ash content is also displacing other feed in the ration.”

Much of the increased ash in forages is coming from harvest management. For example, using a disk-type forage cutter can suck up dry soil into the windrow. Or, improper settings on hay rakes can also result in more soil in the forage.

Corn fields kept weed-free can also result in more soil splashing up on stalks during rain events. If the corn stalk is then harvested low to the ground, that soil can end up in the silage pile. Soil can also be picked up by skid steers removing haylage or silage from silage bags.


Note: This story appears in the February 2018 magazine issue of Dairy Herd Management.



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