Research Note
Growing Pigs:
Lower Lys: CP Ratio?
Modern low protein, amino acid fortified diets for growing pigs offer high performance with lower cost, less nitrogen excretion, and reduced environmental impact.
However, recent research may point to limits on the use of “synthetic” or feed-grade amino acids to replace “intact” protein sources, such as soybean meal.
“Extensive research shows that we can sustain or improve pig performance by incorporating feed-grade essential amino acids, starting with lysine,” says Kansas State’s Dr. Mike Tokach. “But studies also show that high amounts can compromise performance.”
“We don’t yet know exactly why performance declines,” Tokach says, “although it appears that in very high AA diets, pigs may experience a deficit of non-essential amino acids resulting from less nitrogen in the diet.”
While the NRC (2012) establishes a minimum dietary nitrogen (N) concentration in addition to essential AA, pigs fed low crude protein (CP) appear to need more N for endogenous synthesis of non-essential AA, which supports protein synthesis.
Tokach noted previous research showing that non-protein nitrogen in the form of ammonium phosphate can serve as a nitrogen source in diets deficient in non-essential amino acids. He then highlighted a recent study of low protein, amino acid fortified diets to determine whether nitrogen was the limiting factor to maintain feed efficiency in growing pigs.
The Kansas State researchers pointed out the minimum nitrogen requirement from NRC (2012) suggests that a SID (standardized ileal digestibility) Lys:CP ratio of 6.5 is optimal for pigs weighing 25 to 55 lbs. and would be lower in older pigs because of the increased proportion of their requirements that are for maintenance.
The K-State researchers found that for pigs 25-50 lbs. the SID Lys:CP ratio of 6.5 was more like an upper boundary. A lower ratio of 6.0 still sustained growth and feed efficiency. Moreover, for diets formulated above the 6.5 ratio, adding a non-protein nitrogen source (di-ammonium phosphate) or non-essential AA (glycine) to lower the ratio to 6.5 improved feed efficiency and CP digestibility.
“The use of dietary feed-grade amino acids benefits the swine industry in many ways,” Tokach adds, “including by reducing feed costs, nitrogen excretion, and environmental impact. Building on decades of research, we’re still learning their optimal use.”
And, despite ongoing market variability, long-term trends suggest continuing competitive prices for feed-grade lysine, methionine, threonine, tryptophan, valine, and others with swine diet applications.
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