Research Note

Low Alfalfa, High Corn?
Risk Management Angle

Cost efficiency on-farm continues to power the decades-long shift from alfalfa to corn silage in dairy rations. But as corn silage concentration increases in the diet so do certain risks. What are those risks? How to manage them?

“Probably the biggest risks in high corn silage diets,” says Ohio State’s Dr. Bill Weiss are ruminal acidosis, mineral imbalances, and metabolizable protein deficits.”

Acidosis: Corn silage is high in starch (25-40%), which increases the risk of ruminal acidosis unless the diet contains adequate neutral detergent fiber (NDF). Weiss notes that as the concentration of forage NDF (fNDF) in a diet increases, starch concentration can also increase — up to about 30% — without increasing the risk of acidosis.

Effect of replacing alfalfa with corn silage on dietary factors that affect the
maximum concentration of starch that should be fed.

However, on-farm variability of fNDF in both alfalfa and corn silage confounds things, Weiss says: “Whenever you have only one forage, variability in fNDF is usually greater than when you have two or more sources of fNDF. The risk becomes greater as fNDF concentration decreases, requiring incorporation of byproducts such as soyhulls and beet pulp.”

Mineral imbalances: Weiss also points out that the total or partial replacement of alfalfa with corn silage reduces the buffering capacity of the diet because alfalfa contains high concentrations of potassium and calcium. Alfalfa’s greater cation-anion difference (DCAD) correlates with higher dry matter intake, NDF digestibility, and milk fat concentration.

“This means supplemental source of cations are usually needed with corn silage diets.”

Protein: “Replacing alfalfa with corn silage significantly increases the need for metabolizable protein (MP) in the diet.”

Depending on production levels, stage of lactation, feed costs, and some other factors, Weiss says, crude protein (CP) in lactation diets generally ranges around 14-18%. However, alfalfa hay and silages average 20-22% crude protein (CP), whereas corn silage averages 7-8% CP.

“So, as corn silage replaces alfalfa in diets, inclusion rates of protein supplements must increase and the choice of which supplements to use can have a major effect on production, feed costs, and efficiency.”

In well-balanced diets, Weiss adds, the efficiency of converting CP to MP would typically rank as follows:

• Diets with mostly corn silage, soyhulls, and soy protein sources greater than
• Diets with mostly corn silage, distillers grains, some soy protein sources greater than
• Diets with corn silage, alfalfa silage, low concentrations of distillers and soy protein sources

“Forage nutrient content and other characteristics change from year to year, so one of the best diet risk mitigation practices is to obtain nutrients from multiple sources.”

Questions?

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