Research Note

Isoacids: Potential & Limits

Supplementing branched-chain volatile fatty acids (BCVFA) — aka “isoacids” — can improve fiber digestibility, microbial protein production, and either milk production or production efficiency.

“Milk production efficiency responses of 5% to 10% have been recorded,” says Ohio State’s Dr. Jeffrey Firkins. “However,” he adds, “optimal results appear to depend on multiple factors.”

The earliest application of isoacids in dairy diets dates back to the 1980s, with the idea that a BCVFA supplement would improve efficiency of nutrient use, including crude protein, in a wide range of applications.

“Today,” Firkins says, “the more biologically correct interpretation appears to be that isoacids can improve the efficiency of microbial protein synthesis (EMPS) to convert RDP into microbial protein and decrease the wastage of RDP.

“And now, with a better idea of when isoacids work, they can be restricted in usage to the types of dietary conditions in which supplementation works best.”

Firkins notes that nutritionists are trying to formulate diets for RDP to meet microbial protein needs, but there likely is no RDP requirement: “NASEM (2021) capped RDP at 12%. Increasing RDP without a change in rumen-degraded carbohydrate should increase the growth of bacteria that can use AA for fuel.”

Research is showing that supplementary isoacids are not likely to be as effective with only limited nitrogen (N) available to rumen microbes: “The proteolytic bacteria producing amino acids (AA) and ammonia may use those products for themselves rather than releasing AA and ammonia for usage by the cellulolytic bacteria largely responsible for NDFD (neutral detergent fiber digestion).”

A deficiency of “nitrogenous growth factors” for cellulolytic bacteria could depress both NDFD and dry matter intake (DMI). By contrast, Firkins says, greater NDFD should increase the amount of microbial protein along with improving EMPS, thanks partly to a more “balanced consortium” of ruminal bacteria.

However, Firkins adds, both research and practical experience with isoacid supplementation in a wide variety of diets points to “sweet spots” with more highly degradable fiber sources, both forages and fibrous byproducts.

Recent studies also have highlighted differences in isoacid effects in multiparous versus primiparous cows. For multiparous, improved fiber digestibility can increase milk fat production, while in primiparous cows, the acetate from improved fiber degradability may divert to body weight gain.

In addition, Firkins notes, there’s growing interest in positive effects of BCVFA on the rumen epithelium and potentially on lower gut health.

“Isoacid supplementation offers significant advantages in the ‘right’ diets, without negative effects in the ‘wrong’ diets. Cost payback results from supplementation not being spread over the times with lower response.” 

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