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 is related to the following objective and sub-objective of the parent project: Objective 1. Improve accuracy, reproducibility, and ease of measuring/estimating feed digestibility; sub-objective 1.B.: Develop and evaluate in vitro methods for assessment of ruminal degradability of dietary proteins. This project was initiated in May 2009. Two bioavailability trials were conducted, one in winter 2009-2010 and a second in fall 2010. Results from both studies suggested that measurement of blood lysine concentration does not accurately quantify lysine absorption from rumen-protected lysine formulations. In both bioavailability trials, the blood samples collected at many different times from individual cows were combined (to make one per cow) to avoid the need to do large numbers of the slow and costly lysine analyses. After a site visit from the cooperator in February 2011, it was decided to test a new analysis method that is more rapid and less costly so lysine can be measured in every blood sample collected from each individual cow in both experiments. If successful, development of this method will allow evaluation of diurnal variation of blood lysine concentration and possible identification of an optimal time at which to estimate lysine absorption from the rumen-protected materials. Despite the uncertain results from the bioavailability trials, a feeding study is proceeding to test effectiveness of feeding rumen-protected lysine in the lactating dairy cow. This study began in June 2011. The site visit mentioned above, plus e-mail interchanges, were used to keep the cooperator informed of the study’s progress.