by Amanda Smith, Associate Editor
The rumen is akin to an orchestra. It requires a variety of elements, working in concert with one another, to achieve its end goal. When the rumen is working harmoniously, dairy cattle are able to transform the high-forage diets we provide into nature's most nearly perfect food: milk.
"To optimize milk component production from forages, it is essential to understand rumen fiber digestion and passage," noted Rick Grant in a Miner Institute Farm Report. Fiber digestion characteristics influence feeding and rumination, rate of particle breakdown, rumen fill, dry matter intake, and overall rumen and productive efficiency.
Grasses, legumes and corn silage all behave differently in the rumen. Legumes such as alfalfa have more fragile NDF (neutral detergent fiber) than grasses, and their forage particle size is reduced more rapidly with rumination.
Grasses, meanwhile, tend to increase the amount of large fiber particles in the rumen compared with legumes. In addition to creating greater rumen fill, higher forage diets with slower fermenting fiber require longer to process by the cow. This can pose an often-overlooked time budget constraint, especially with overstocked feedbunks.
Diets containing highly fermentable fiber that is too fragile can lower chewing time, rumen pH and fat output. This lower rumen and productive efficiency can be corrected by adding forages that elicit greater chewing per unit of fiber.
High-producing cows will be more quickly limited by rumen fill with average quality grasses versus legumes. NDF digestion curves for legume and grass forages show that legumes have a 15 to 20 percent faster initial rate of NDF digestion.
But the extent of NDF digestion is 30 to 40 percent greater for grasses, reflecting 30 to 40 percent less lignin. The normal range in 30-hour NDF digestibility for grass silage is about 55 to 70 percent. To capitalize upon grasses, they need to be managed for harvest at the upper end of this quality range.
(c) Hoard's Dairyman Intel 2015
April 6, 2015
