Sulfur amino acid utilization by growing steers

Area of Science:

• Animal Nutrition
• Ruminant Physiology
• Amino Acid Metabolism

Background:

• Sulfur amino acids, particularly methionine and cysteine, are essential for protein synthesis and other metabolic functions in growing animals.
• Quantifying the precise requirements for these amino acids is vital for optimizing animal growth, feed efficiency, and overall health in beef cattle.

Purpose of the Study:

• To determine the methionine requirement of growing steers when adequate cysteine is available.
• To evaluate the efficiency of transsulfuration pathway in converting methionine to cysteine in steers.
• To assess the potential sparing effect of cysteine on methionine requirements.

Main Methods:

• Two experiments were conducted using growing steers with abomasal and ruminal infusions to control nutrient supply.
• Experiment 1 assessed methionine requirements with excess cysteine using break-point analysis for nitrogen retention.
• Experiment 2 evaluated transsulfuration efficiency by infusing different levels of methionine and cysteine.

Main Results:

• The total methionine requirement was estimated at 7.9 g/d when excess cysteine was supplied.
• Nitrogen retention increased with methionine supplementation but not cysteine supplementation in steers with limited sulfur amino acids.
• The total methionine requirement was estimated near 8.4 g/d when cysteine supply was limited, and supplemental cysteine did not spare methionine.

Conclusions:

• Growing steers require approximately 7.9-8.4 g/d of methionine, depending on cysteine availability.
• The lack of a cysteine sparing effect suggests significant non-protein functions of methionine in steers.
• These findings contribute to a better understanding of sulfur amino acid metabolism and requirements in ruminants.