1a. Objectives (from AD-416)
The objective of the proposed research is to determine the bioavailability of the essential amino acid lysine in a commercial product thought to protect the amino acid from rumen catabolism and to determine if feeding this product to lactating dairy cows will increase yield of milk and milk components, improve N efficiency, and reduce urinary N excretion.
1b. Approach (from AD-416)
Two trials will be conducted with lactating dairy cows. Trial 1 will be a 2-phase study to assess lysine bioavailability from a particular product in lactating cows by comparing the plasma lysine response curve obtained when feeding the product to that occurring with abomasal infusion of known amounts of lysine. In Phase 1, lactating dairy cows with permanent ruminal cannulae will be fed a standard diet that exceeds their requirement for absorbed lysine (by supplementing with high levels of soybean meal). Five doses (0, 20, 40, 60 and 80 g/d) of L-lysine-HCl will abomasally infused via rumen cannula to generate a linear plasma lysine response curve. Design of Phase 1 will be a 5 x 5 Latin square with 3-d periods. Blood plasma will be sampled over the last 24 h of each infusion period to account for any diurnal variation in blood lysine concentration; plasma lysine will be quantified by ion-exchange chromatography. During Phase 2, the same dairy cows fed the same standard diet will be supplemented with the same 5 different doses of L-lysine, but in the form of the experimental product. Phase 2 will also be a 5 x 5 Latin square design, but with 7-d periods. Blood samples will be collected and analyzed as described for Phase 1. Trial 2 will be a lactation study conducted using diets based on alfalfa and corn silages, high moisture and ground-shelled corn, and supplemented with soybean meal and distillers dried grains. The tentative plan is to feed a 2 x 2 x 2 arrangement of diets: 2 levels of crude protein (15.0 and 17%), with and without supplementation of rumen-protected methionine, and with and without the rumen-protected lysine product studied in Trial 1. This trial will an incomplete 8x8 Latin square with 4, 4-week periods (total 16 weeks). Sixty-four cows in early or mid-lactation will be blocked into 8 squares by parity and days-in-milk. Cows will be housed at the U.S. Dairy Forage Center research farm at Prairie du Sac. Within squares, cows will be randomly assigned to treatment sequences; cows in each square will get only 4 of the 8 diets but all 8 diets will be replicated an equal number of times over all 8 squares. Ration and weighback samples will be collected daily to determine the amount and composition of the ration actually consumed. Milk samples will be taken mid-week at both milkings during weeks 3 and 4 of each experimental period and analyzed for fat, protein, lactose, SNF and milk urea N. Cows will be weighed on three consecutive days at the start and at the end of each period. Blood samples and spot fecal and urine samples will be collected at the end of every 4-week period. Blood plasma will be deproteinized and analyzed for urea and free amino acids. Internal markers in urine (creatinine) and feces (indigestible ADF) will be used to estimate urinary excretion of urea N and total N, apparent nutrient digestibility, and fecal N excretion. Net N balance will be computed from N intake, milk protein yield and estimated urinary and fecal N excretion. Experimental data will be analyzed using the mixed procedures of SAS.
3. Progress Report
This project was initiated in May 2009. Trial 1 was conducted from November 2009 through January 2010; blood plasma lysine analyses were completed in April 2010. Statistical analysis of results from phase 1 indicated that, although plasma lysine responses to abomasal lysine infusions varied among animals, the approach yielded a satisfactory standard regression curve to assess lysine absorption. However, results from the feeding phase 2 were unsatisfactory, mainly because the plasma lysine concentrations on the 0-dose treatment were high; mean lysine concentrations were 60 micromolar on the 0-dose during infusion phase 1 versus 81 micromolar on the 0-dose during feeding phase 2. The high natural level of lysine made it difficult to determine increases in lysine due to feeding the protected lysine. Nevertheless, one of the two preparations of rumen-protected lysine gave rise to significant increases in plasma lysine, indicating substantial lysine absorption when the material was fed to dairy cows. However, the problem just described prevented quantification of lysine absorption. Feeding of a second lysine preparation did not significantly alter plasma lysine concentrations, suggesting ineffective rumen-protection or poor intestinal release of the lysine in that material. A report of these findings was sent to the cooperator in July 2010. After a site visit from the cooperator plus e-mail interchanges, it was decided to reassess lysine bioavailability in the two materials using a different approach in a new Trial 1 and to delay Trial 2 to a later time.