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The plasma free amino acid dose response technique: a proposed approach for determining lysine bioavailability of ruminally-protected lysine products
While most companies provide estimates of lysine (Lys) bioavailability for their products, values are often obtained with different techniques and there are little to no data comparing efficacy and cost-effectiveness of different products. Our objective is to propose that the plasma free amino acid (AA) dose-response technique be accepted as the standardized method for evaluating rumen protected Lys (RP-Lys) products, because its animal-derived estimates of bioavailability are obtained under conditions similar to commercial use. Bioavailability estimates are calculated by dividing the slope of the regression line relating changes in plasma Lys to increased feeding of RP-Lys divided by the slope of the Lys infusion regression line. Eleven dose-response Latin square trials using 66 lactating, ruminally-cannulated multiparous Holstein cows (days in milk = 60-315; milk yield = 25-62 kg/d at the start of the study) were conducted. Abomasally infused and fed amounts of Lys ranged from 0 to 84 g/d and periods from 4 to 21 d. The RP-Lys products were mixed with 1 kg of total mixed ration, placed in tubs, and fed 30 min before each of the 3 daily feedings. Product not consumed within 20 min was delivered via the ruminal cannula. Diets were formulated (NRC, 2001) to be adequate in metabolizable protein-Met but varied in predicted metabolizable protein-Lys (5.04-6.81%). One to 4 blood samples were taken from the tail vein for 1 to 3 d in each period. Basal plasma Lys concentrations ranged from 2.14 to 5.62 % of total AA without infusions, from 2.33 to 5.67% of total AA for fed RP-Lys, and from 2.94 to 8.62% of total AA with infusions. Results corroborate previous research which showed a positive linear relationship between both infused and fed RP-Lys and plasma Lys concentrations. Regression analysis (by trial and cow) indicated the 2 factors that most affected the magnitude of the response slopes, and hence the technique’s precision, were the cows basal level of plasma Lys (i.e., intercept, the lower the better) and the cows plasma Lys response to infused Lys (measured by the magnitude of their response to the highest level of infused Lys). Stage of lactation and milk production did not affect plasma Lys response. It is concluded that the plasma free AA dose response technique is sensitive to increasing amounts of absorbed Lys and therefore is an appropriate technique for evaluating RP-Lys supplements.
Keywords: Technique, Bioavailability, Lysine