Technical article written for submission to Feedstuffs
Joanne Knapp, Ph.D. PAS, Fox Hollow Consulting, LLC
During silage fermentation, simple sugars are converted mostly to lactic acid. How does this process impact the energy content of the silage for feeding dairy cattle? We’ll review lactate metabolism and then answer this question on a theoretical and a practical basis.
Rumen lactic acid concentration is a balance between inputs (feed lactic acid and lactic acid from carbohydrate fermentation) and outputs (absorption across the reticulo-rumen epithelium and consumption by lactic acid utilizing bacteria (LAB); Figure 1). Lactic acid concentration in the rumen ranges from 3 to 6 mM and can climb up to 12 mM at times post-feeding (Khorasani and Kennelly, 2001; Khorasani et al., 2001). In comparison, total VFA concentration ranges from 100 to 130 mM in high-producing dairy cows. Lactic acid consumed in ensiled feeds can be the major source of rumen lactic acid in dairy cows consuming diets containing 40 to 60% forage, with 50% of the forage coming from silages or haylages. Lactic acid intake is substantially greater than lactic acid produced in microbial fermentation of feed carbohydrates (sugars, starches, pectins, hemicellulose, cellulose, etc.) under normal feeding and rumen conditions in dairy cattle (Fig. 1; Slyter, 1976). Lactic acid absorption by the reticulo-rumen epithelium and appearance in the portal blood flow is the major outflow; uptake and metabolism by LAB such as Selenomonas ruminantium and Megasphaera elsdenii is minor under normal rumen conditions (Fig. 1). Under feeding conditions in dairy nutrition that ensure good rumen function, the capacity for lactate absorption exceeds lactic acid production and rumen lactic acid concentration does not increase. In rumen acidosis, lactic acid production by microbial fermentation exceeds the absorption capacity, and concentration rises, dropping rumen pH and causing further disturbances in rumen function.
Lactate from either silage or rumen fermentation can be a mixture of L- and D-isomers. Both isomers are metabolized by rumen microbes and mammalian tissues (Harmon et al., 1984; Ewaschuk et al., 2005). The liver extracts most of the lactate delivered in the portal blood (Fig. 1), as well as the lactate produced by other tissues arriving via the hepatic artery (Reynolds et al., 1988; Kristensen et al., 2007). After uptake by the liver, lactate equilibrates with pyruvate, which is used to produce glucose, oxidized in the TCA (tricarboxylic acid) cycle to produce ATP, or converted to non-essential amino acids (AA) such as alanine. Lactate is an important substrate for gluconeogenesis, accounting for approximately 20% of liver glucose output in dairy cattle (Reynolds et al., 1988; Reynolds et al., 2003).
small intestine as well as the total tract digestibility. If starch is fermented by microbes, energy will be lost to the animal (Fig. 2). If starch is digested to glucose in the small intestine, it will provide nearly the same amount of ME as lactate on a pound-for-pound basis. While theoretical differences exist in the ME value of starch depending on whether it’s absorbed as VFA or glucose, research has not been able to detect differences in animal performance based on these alternative metabolic pathways (Nocek & Tamminga, 1991; Huntington et al., 2003). Total tract NDF digestibilities range from 35 to 60%; all of this is via fermentation to VFA (Fig. 2). No differences would be expected between lactate and VFA in the efficiency of conversion of ME to NEl.
Most of the silage acids in ensiled forages are derived from easily fermented sugars inside the plant cells. If the sugars are fermented via the homolactic pathway, 100% of the energy is retained in lactic acid. If the sugars are fermented via the heterolactic pathway with formation of acetic acid or ethanol, energy and dry matter are lost. Additional energy may be lost from the silage if the acids volatize off or leach out in the effluent. However, we can see that conversion of plant sugars to lactic acid in the silage fermentation with consumption of the lactic acid and absorption by the cow could provide more energy than retaining the sugars intact in the silage and then losing energy during the rumen fermentation.
How is lactic acid utilization handled in current ration formulation software? In CNCPS 6.1 and its commercial versions (AMTS and NDS), lactic acid is explicity described as the CHO A2 pool. Interchanging it for sugar or starch has non-significant impacts on energy content of the silage and no impact on the available energy in the ration. In CPMDairy 3.0, lactic acid is part of the CHO A1 silage acid pool, and varying its content with corresponding changes in sugars or starch has no impact on the energy content of the silage or the ration. In NRC 2001, lactic acid is implicitly modeled as part of the total digestible NFC (non-fiber carbohydrate) pool, with the same DE (Digestible Energy), ME, and NEl as the NFC. Accordingly, varying lactic acid content with sugar or starch will not change the energy content of the silage significantly. Thus, none of these models captures the theoretically higher energy in lactic acid as compared to fermentable carbohydrates. On a practical basis, this discrepancy has little impact since it is small compared to the variation seen in starch and NDF digestibility.
In summary:
- On a molar basis, lactic acid has half the energy of glucose, but on a weight basis (grams or lbs.), it has the same energy.
- Ruminant animals lose, on average, 25% of the energy in feed carbohydrates during the rumen fermentation.
- On a theoretical basis, lactate provides 33% more Metabolizable Energy than fermentable carbohydrates.
- Current ration formulation software packages do not account for the difference in ME content between lactic acid and fermentable carbohydrates; however, the underestimation of lactic acid’s energy content is not likely to have a significant impact on total ME availability.
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