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Forage digestibility varies widely

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MARSHFIELD, Wis. – “This ain’t your daddy’s fiber or your daddy’s cow,” was Dave Combs’ message at a recent nutrition conference.

Combs, a University of Wisconsin-Madison dairy scientist, spoke at a recent Professional Dairy Producers of Wisconsin-sponsored Feed and Nutrition Conference.

Total Tract Neutral Detergent Fiber Digestibility isn’t daddy’s forage-quality test either. Combs developed the test to encompass the amount of digestible fiber in forage – and how fast it might be digested by a cow. The Rock River Laboratory in Watertown, Wisconsin, has been the only lab licensed to offer the UW-patented test. Combs said the UW’s own forage-testing lab is moving in the direction of also providing the test.

Neutral Detergent Fiber is a slow-to-digest component of forage that can restrict feed intake. It’s only part of the fiber story, Combs said. Digestibility varies between forages, varieties, plant maturities, growing conditions and harvest management. Moisture, hotter temperatures and sun intensity contribute in positive ways to digestibility, which impacts feed intake and milk production. The higher the Total Tract Neutral Detergent Fiber Digestibility value, the better the fiber can be utilized. Combs said the total-tract test is used across forage types and byproduct feeds and in ration balancing and evaluation.

Combs said typical fiber-related values are as follows.

1) Alfalfa and corn silage

  • Neutral Detergent Fiber, less than 40 percent;
  • average Total Tract Neutral Detergent Fiber Digestibility, 42 percent;
  • high Total Tract Neutral Detergent Fiber Digestibility, greater than 48 percent;
  • low Total Tract Neutral Detergent Fiber Digestibility, less than 36 percent

2) Grasses

  • Neutral Detergent Fiber, less than 45 percent;
  • average Total Tract Neutral Detergent Fiber Digestibility, 42 percent;
  • high total-tract value, greater than 48 percent;
  • low total-tract value, less than 36 percent.

3) Dairy-quality alfalfa and corn silages will be less than 40 percent Neutral Detergent Fiber with a total-tract value of at least 42 percent.

A two- to three-unit change in ration Total Tract Neutral Detergent Fiber Digestibility corresponds to a 1-pound change in milk yield.

A lot has changed in the present decade in terms of forage genetics and technology impacting digestibility, Combs said. Shredlage has increased starch digestibility in corn silage, while Brown Midrib Corn has increased fiber digestibility by reducing lignin. Feeding Brown Midrib Corn can result in 2 to 3 pounds more milk compared to standard corn silage.

Reduced-lignin alfalfa varieties – HiGest and HarvXtra – have less lignin and greater Total Tract Neutral Detergent Fiber Digestibility.

“There’s been a resurgence in planting improved grasses for high-produced dairy cows, with higher fiber digestibility than alfalfa or corn silage,” Combs said. “And there’s more interest in grasses in plant-breeding (circles).”

In one UW study, digestibility of fiber in a total mixed ration for high-producing cows was increased by replacing some of the corn silage and alfalfa with high total-tract-value fescue silages. Fiber digestibility of the corn-silage-and-alfalfa diet was 26 percent. The digestibility value was increased to more than 40 percent by replacing about a third of the other two forages with either tall fescue or meadow-fescue silage. Adding that much grass increased total Neutral Detergent Fiber in the diet, but the fiber was more digestible. The result was better feed intake and milk butterfat, showing that Total Tract Neutral Detergent Fiber Digestibility can be a valuable tool for optimizing forage utilization.

There’s definitely a place for tall fescue, meadow fescue and potentially late-maturing orchard grass on dairies. Combs suggested adding 30 percent to 40 percent grass in an alfalfa stand and lengthening the rotation beyond the typical three years for straight alfalfa. A grower might see five or six years from a fescue-alfalfa mix. The grower should maintain the same alfalfa-cutting schedule, ensiling recommendations and inoculants.

Straight fescue stands also work well. Grass benefits from manure applications, Combs said.

Forage quality of most new later-maturing cool-season grasses can complement alfalfa, Jim Paulson said. Paulson is a former University of Minnesota-Extension dairy educator who’s now doing private dairy and forage consulting out of Houston County, Minnesota.

“Too much focus is often placed on crude protein content of forage and not its digestibility,” Paulson said. “At the same stage of maturity, grasses exceed alfalfa in the amount of digestible (Neutral Detergent Fiber).

“For lactating-cow forage, the mixture of alfalfa with meadow fescue and tall fescue results in higher yields and higher-quality forage compared to alfalfa alone. And it appears to tolerate the cutting intensity of a four-cut system. Meadow fescue and improved orchard-grass varieties also perform well.

“Another critical aspect of managing grasses in alfalfa stands is to watch the cutting height. Grasses need to re-grow from the stubble above ground left in the field. Alfalfa re-grows from the crown, which is below ground. If a mixed stand of grass and alfalfa is cut at a 2-inch height, the grasses will not grow back as fast as the alfalfa and will be more prone to not surviving. This is particularly a concern with disc mowers. A disc mower needs to be adjusted to a 3- to 4-inch cutting height. This cutting height will also reduce soil contamination in forage.”

Contact or 507-251-4694 for more information.

Jane Fyksen writes about crops, dairy, livestock and other agricultural topics; she is the crops editor for Agri-View based in Wisconsin. Email to contact her.

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