Impacts of Balancing Feedlot Diets for Effective Energy and Predicted Amino Acid Requirement on Plasma Lysine Levels and Finishing Steer Performance.

The objective was to evaluate differences in steer finishing performance and plasma lysine levels when fed diets balanced for predicted lysine requirement to effective energy (EE) ratio using rumen-protected lysine. We hypothesized that finishing steers consuming diets optimized for these parameters would have improved feed efficiency, profitability (defined as when value of gain exceeded cost of gain), and increased plasma lysine concentration. Crossbred steers (n=120; 451 ± 32 kg) were used in a completely randomized design and stratified by BW and breed type. Animals were sorted into pens of 6 and fed between 112-179 d. Diets were balanced to meet EE requirement and be adequate for non-lysine AAs. Treatments included a lysine-limiting control that contained no rumen-protected products (NEGCON); a lysine-sufficient control (POSCON) that contained rumen-protected soybean meal; and treatments that contained encapsulated lysine at 50% (AJ50), 100% (AJ100) and 150% (AJ150) of the amount of product needed to meet lysine requirement. On d 151 steers were weighed on 2 consecutive days every 14 d and designated to be slaughtered when they were no longer profitable. Jugular blood was collected at 4, 8 and 12 h post-feeding from ruminally cannulated crossbred steers consuming NEGCON, AJ100 and AJ150 in a 3x3 Latin Square metabolism study. Data were analyzed using the LIFETEST procedure of SAS to test profitability and the GLM procedure to test for treatment differences among FBW, ADG, DMI, G:F, and plasma lysine concentration. Steer was used as experimental unit. Steers remained profitable for a greater time (days; P < 0.01) for NEGCON (165.6 ± 2.2), POSCON (163.8 ± 2.5) and AJ100 (165.6 ± 2.9) than AJ150 (153.9 ± 1.2). Additionally, AJ100 profitability tended to be greater (P < 0.07) than AJ50 (156.8 ± 1.5 days). Feed efficiency differed across treatments with G:F greater (P < 0.02) for POSCON and AJ100 (0.125 and 0.121) than AJ50 and AJ150 (0.108 and 0.103), but no treatment differed from NEGCON (0.110). Steer FBW, ADG or DMI did not differ. For cannulated steers, 8h plasma lysine (µg/mL) was greater (P < 0.02) in AJ150 than NEGCON and AJ100 (12.28 vs. 8.70 and 10.29 ± 1.05), but did not differ at 4h or 12h. Finishing steers became less profitable sooner when under or over-supplemented encapsulated lysine. Feed efficiency increased with use of rumen-protected products in diets formulated to meet lysine requirement. Finally, the highest supplemented level of encapsulated lysine resulted in greater plasma lysine concentration 8h post-feeding.