Influence of dietary methionine concentration on growth and nitrogen balance in weanling Quarter Horses.

The research examined the effect of varying levels of dietary methionine (a type of amino acid) on the growth and nitrogen balance of weanling Quarter Horse. The study found no significant impact on growth measurements, nitrogen balance or urinary urea or ammonia, but did detect a quadratic response in plasma urea nitrogen.

Experimental Design

• The study involved 24 Quarter Horse weanlings that were around 120 days old.
• The horses were divided into four groups of six. Each group was assigned a different diet for 56 days which varied in its concentration of the amino acid methionine.
• Horses were individually fed twice daily with concentrates at 1.75% body weight.
• The four diet treatments consisted of a basal diet (0.20% methionine), basal diet + 0.03% methionine (0.23%), basal diet + 0.07% methionine (0.27%), and basal diet + 0.11% methionine (0.31%).
• Coastal bermudagrass hay was also provided to the horses at 0.75% of their body weight.

Data Collection

• Measurements of growth, body weight, rump fat, and plasma were taken every seven days during the study period.
• The final four days of the trial included a total collection of urine and feces from each horse.
• Samples were then analysed for nitrogen content; nitrogen balance was then calculated.
• Urine samples were also analysed for urea and ammonia concentrations, while plasma samples were analysed for urea concentration. Grain, hay, and fecal samples were analysed for their nutrient composition.

Results

• There were no discernable effects on growth measurements, nitrogen balance, or urinary urea or ammonia based on the methionine concentration of the diet.
• Intake of two other amino acids, Lysine and Threonine, did not differ among the treatment groups. However, as expected, methionine intake increased with greater inclusion of methionine in the diet.
• Plasma urea nitrogen showed a quadratic response to treatment, with a peak observed at the 0.27% methionine concentration.
• Given the lack of significant findings, the authors suggest further studies are necessary, ensuring methionine as the only dietary variable, to better understand the dietary requirements of methionine in weanling horses.