Effects of acidity and silage type on lysine retention among two lipid-coated ruminally protected lysine products

Abstract Text: Milk protein secretion among cattle consuming corn-based diets can be limited by metabolizable Lys. Thus, ruminally protected amino acids (RPAA) are commonly included in cattle diets to increase metabolizable Lys intake. Previous data suggest that Lys associated with lipid-coated RPAA is reduced by greater diet moisture content. Effects of silage type and acidity on Lys associated with lipid-coated RPAA products (EB and EC) were evaluated over time. Crystalline Lys and Lys mixed with lipid in amounts equal to either EB or EC served as negative controls. Controls and each lipid-coated RPAA (4 g) were placed in mesh bags (10 × 20 cm; pore size = 50 mm) and mixed with either alfalfa silage or corn silage at 2 different levels of acidity (pH = 4.6 or pH = 6.6) for 0, 6, 12 or 24 h. Silage pH was modified by mixing with 10% (wt/wt) NaOH. After removal, mesh bags were rinsed with cold water and contents of each bag were lyophilized. Lipid associated Lys was subsequently determined by analyzing total free Lys content after removal of triacylglycerols and free fatty acids with hexane:methanol. Dissociation kinetics were calculated using the nonlinear procedure of SAS and data were analyzed as a completely randomized design. As expected the proportion of Lys that dissociated after initial contact (ID) from the negative controls was complete (100 ± 2.5%) and indicated that Lys analyses reflected measures of lipid-associated Lys. The ID from RPAA was less (P = 0.02) for EB (20.4 ± 2.8%) than EC (31.3 ± 2.8%). Additionally, ID increased as acidity increased when RPAA was mixed with corn silage; however, ID was not affected by acidity when mixed with alfalfa silage (silage × pH ≤ 0.01). Amounts of Lys that slowly dissociated (SD) from RPAA during silage incubation was greater (P = 0.01) for EB (59.6 ± 4.8%) than for EC (38.4 ± 4.8%). Furthermore, SD increased with greater acidity in corn silage, but did not differ among alfalfa silage with either greater or lesser acidity (silage × pH = 0.01). Rate of Lys dissociation was not different among either RPAA (P = 0.40; 10.9 ± 1.9% per h) and was not affected by acidity (P = 0.73) or silage type (P = 0.62). Lipid associated Lys remaining at 24 h for EB (20.1 ± 5.7%) was not different (P = 0.24) compared to EC (30.3 ± 5.7%).

Keywords: Amino acid, Cattle, Lysine