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. The project was initiated in May 2009. Lysine is an essential amino acid that is required by all higher animals to synthesize protein. Sometimes dairy cows do not receive sufficient lysine from the usual sources: microbial formation in the rumen plus dietary protein that escapes breakdown in the cow’s rumen. Under these circumstances, supplementing the diet with rumen-protected lysine will improve yield of milk and milk protein. There are a large number of commercial, rumen-protected lysine products that claim to be effective for protecting lysine from breakdown in the rumen while still allowing the lysine to be absorbed into the blood so the cow can use that lysine to form milk protein. Results from two earlier studies suggested that measurement of blood lysine concentration did not accurately indicate how much lysine from a source of rumen-protected lysine was actually absorbed into the cow’s blood. Results were still ambiguous after complete analysis of lysine in all blood samples that were collected throughout the day from all cows used in both experiments. Because of this ambiguity, a feeding trial was conducted with lactating cows fed diets supplied with minimal lysine (so lysine was the limiting nutritional factor). Results from the feeding study showed that inclusion of the experimental form of rumen-protected lysine in the diet did not improve yield of milk or milk protein. This indicated that this source of rumen-protected lysine was not effective in delivering lysine that could be absorbed into the blood of the cow, resulting in increased milk or milk protein yield.