Lactic Acid Conserves Metabolizable Energy during Fermentation of Silages?

Technical article written for American Farm Products

For submission to Progressive Dairyman

Joanne Knapp, Ph.D. PAS, Fox Hollow Consulting, LLC


There’s a perception in the dairy industry that lactic acid production during silage fermentation diminishes the Metabolizable Energy (ME) content of ensiled forages. Previous research does not support this perception. When lactic acid is formed during ensiling, virtually no dry matter or energy is lost. When cows consume lactic acid in their rations, it is 100% digestible. Lactic acid is easily absorbed across the reticulo-rumen wall into the portal bloodstream, where it is taken up by the liver and metabolized. Lactate conversion to glucose by the liver accounts for approximately 20% of total glucose production in lactating dairy cows. There is no loss of energy from lactic acid at any point in the process of digestion, absorption, or metabolism (Fig. 1).

In contrast, when cows consume fermentable carbohydrates such as sugars, starch, pectins, and NDF, on average, 25% of the Digestible Energy is lost in the rumen fermentation (Fig.1). Also, the ME content of starch, pectins, and NDF is affected by their digestibility. Sugars are assumed to be 100% digestible.

ndustry including CNCPS 6.1, CPM Dairy 3.0, and NRC 2001 based systems do not capture these differences, but rather assign lactic acid the same energy content as sugars and starch. Variations in lactic and acetic acid concentrations do not change the predicted ME content of silages and haylages in any of these feeding systems. On a practical basis, this discrepancy has little impact since it is small compared to the variation seen in starch and NDF digestibility, which does have a large impact on the predicted ME content of silages.

Why do ensiled forages have lower ME contents relative to fresh forages? There are four factors:

  1. Reduced dry matter digestibility, particularly starch, NDF, and in legume haylages, crude protein
  2. Oxidation of silage nutrients by molds and yeasts (aerobic fermentation)
  3. Loss of soluble sugars and silage acids (lactic, acetic, and butyric) in effluent
  4. Formation of acetic acid or ethanol from lactic acid (heterolactic fermentation)

Factors #2 – #4 also result in dry matter losses. Most of the dry matter losses are from the most digestible nutrients in the feed, the soluble sugars inside the plant cells. Nutrient oxidation by molds and yeasts (#2) is a “hidden” loss in that it’s invisible and very hard to measure. The nutrients simply disappear into thin air. Heating in the silage during the first two weeks and at feedout is an indirect indicator of this oxidative loss. Effluent losses (#3) are hard to measure, but they are often visible and easily avoided with proper dry matter content. Dry matter losses due to acetic acid formation can be estimated as half (2:1) of the acetic acid content. For example, if acetic acid concentration is 2%, then 1% of the dry matter and energy was lost.

You’ve all heard the mantra for making good silage:

  • Harvest forages at appropriate stage of maturity and moisture level
  • Pack thoroughly and quickly cover silage piles to eliminate air and reduce the potential for aerobic fermentation by molds and yeasts
  • Use silage inoculants to speed up and ensure the formation of lactic acid, which will drop pH, and inhibit molds and yeasts
  • Practice good face management and TMR mixing

For more than two decades, we’ve known that these are all important factors in reducing dry matter and energy losses in silages and enhancing nutrient utilization by dairy cows. The challenge is getting all the steps done right!