Rumen-protected lysine in dairy cows, bioavailability and effects

The first trial's objective was assessing bioavailability of an experimental formulation (EEL) for a commercial rumen protected (RP) Lys product (ASL). Abomasal infusion was used to develop a response curve relating bioavailability to plasma Lys concentration. Infusion linearly increased plasma Lys, but did not affect milk production or composition. Feeding ASL or EEL had no consistent effects and EEL bioavailability was not better than ASL. The second experiment was an iteration, with improvements learned from the first trial, aimed at determining ASL bioavailability. Plasma Lys increased with infusion in 7 out of 10 cows. Half of the cows responded to feeding ASL with increased plasma Lys. No effects of treatments, neither in milk yield nor composition, were observed. Estimated bioavailability was 28% by dose-response method and 22% by slope-ratio approach. The third trial tested production effects and N use efficiency of supplementing a low (14.5%) CP diet, containing soybean meal (SBM) and distillers grains (DDGS), with RP Met (RPM) and RP Lys (RPL); a positive control (16.8% CP) was included. High CP diet showed higher intake, milk yield and MUN, but lowest apparent N efficiency. No RPM main effects were observed; however, RPM*RPL interaction for milk yield was significant because RPM increased 1.2 kg milk/d but milk was not altered by RPM+RPL. Feeding RPM, but not RPL, increased milk yield/DMI on low CP diets containing SBM plus DDGS. The fourth trial aimed to determine if RPL supplementation improved nutritive value of RUP from corn protein. Diets contained alfalfa and corn silage; half or all CP from solvent SBM (SSBM) and expeller SBM (ESBM) were replaced by CP from corn (corn gluten meal and DDGS), while keeping RDP, RUP and metabolizable protein approximately constant. One all corn CP diet was RPL supplemented (20 g/d absorbed Lys). Intake was highest on SSBM+ESBM diet, intermediate on blended diet and lowest on the corn diets. Milk, milk protein yield, and feed efficiency were lowest in all corn CP diet without RPL; RPL supplementation increased 1.1 kg milk/d and 50 g protein/d. Dilution of SBM RUP with corn protein did not reduce milk yield.