Choppers and box trucks aside, there are many moving parts to a successful corn silage harvest. Monitoring plant moisture to hit target dry matter and starch levels is key to maximizing feeding value. Fiber digestibility does not decline as quickly as plants mature, but it is still an important measure of corn silage quality.

Corn silage is unique in that it is half forage and half grain. Neutral detergent fiber (NDF) digestibility is the primary metric of corn silage quality from a forage perspective, whereas starch is the main source of energy on the grain side. In fact, Rick Grant with the Miner Agricultural Research Institute in Chazy, N.Y., notes approximately 65% of digested nutrients in corn silage come from starch and nonfiber components.

While both are necessary for a balanced ration, starch content and digestibility can interfere with fiber digestion in the rumen. Even if corn silage has high NDF digestibility, a diet with too much starch and/or starch that is highly digestible can lower rumen pH, making energy from fiber less available to cattle.

“As you begin to interpret the forage tests that will soon be arriving, will your cows see all the energy predicted from the feed analysis?” Grant questions. “Laboratory analysis measures the potential digestibility of a forage, but too much fermentable starch in the ration or other poor formulation approaches will limit the energy actually derived.”

To demonstrate this, Grant explains a study where researchers assessed how various sources of starch affected in vitro NDF digestion rates of alfalfa, bromegrass, and corn silage. Alfalfa had an ideal uNDF240 of about 7% per hour on a dry matter basis, but this changed under some conditions when starch was added to the equation.

“When we added a slow-fermenting starch source such as ground sorghum, the NDF digestion was unaffected,” Grant states. “When we added a fast-fermenting starch source such as finely ground corn meal — which we commonly feed to dairy cows — NDF digestion rate was reduced by about 16%.”

With a uNDF240 value of 7% or less in corn silage-based diets, the risk of low rumen pH rises when starch content in the ration is 24% to 25%.

“Some nutritionists have started monitoring the rumen fermentable starch to uNDF240 ratio as a marker of risk for milkfat depression. They may be onto something, and a ratio of 2.8 to 1 or higher may signal ideal conditions for low milkfat,” Grant asserts.

On the other hand, uNDF240 of 10% or more can dampen dry matter intake, which also negatively affects milk production. Grant advises against including additional sources of starch in dairy rations if corn silage already has adequate levels of rumen fermentable starch. This will ensure more energy is available to animals from the forage portion of corn silage, therefore enhancing feed efficiency.

“As we feed corn hybrids with higher NDF digestibility, more starch, and perhaps softer starch, understanding the contribution of starch to the energy value of corn silage and its interaction with rumen fiber digestion is critical,” Grant asserts.