Duodenal supply and small-intestinal digestibility of amino acids from pulp silage fed to lactating dairy cows

This study aimed to assess the supply and small-intestinal digestibility (SID) of amino acids (AA) from grass silage (GS) or pulp silage from a biorefinery pressed once (1P) or twice (2P) of two developmental stages (Dev), i.e., early (E) and late (L; 35 and 44 d of regrowth, respectively). Six lactating multiparous Holstein cows with ruminal, duodenal, and ileal cannulas (energy-corrected milk 30.7 ± 3.7 kg/d, dry matter intake (DMI) 20.6 ± 2.6 kg/d, and days-in-milk 176 ± 93 d) were enrolled in the study. The experimental design was a 64 incomplete Latin square with four, 21-day periods and a 23 factorial arrangement of treatments. The two pulps were ensiled similar to GS resulting in three silages within each Dev: E-GS, E-1P, E-2P, L-GS, L-1P, and L-2P. Treatments were fed as TMR including (DM-basis) an experimental silage (65.0%), soybean meal (6.82%), rolled wheat (26.4%), and mineral mix (1.82%). A total of 12 samples (covering every 2 h of a 24-h day) were collected from duodenal and ileal digesta, and one rumen fluid sample for microbial pellet to determine purine and AA composition. Data were analysed with the ‘lmer’ function in R, with Dev and processing, their interaction, and period as fixed and cow as random effects. Silage nutrient composition, total AA content, and N-fractions have been previously reported in Hansen et al. (2023). Duodenal flow of microbial AA was lower (P ≤ 0.05) for 1P compared with GS (49.6 vs. 56.0 g/kg DMI), regardless of Dev. Similarly, of individual AA, Arg, Gly, Leu, Pro, Ser, and Val from microbial AA (g/kg DMI) were 13.1 to 14.5% lower (P ≤ 0.05) for 1P vs. GS. There was an interaction (P ≤ 0.04) between Dev and processing for duodenal flow of feed AA (g/kg DMI), except for Cys and Gly, being greatest (P ≤ 0.05) for E-2P followed by E-1P, L-2P, L-1P and E-GS, and was lowest for L-GS. There was no difference in SID for total AA across treatments (average 77.9%; SEM = 1.11), and SID varied between individual AA [average across treatments: 65% (SEM = 4.0) for Cys and 85% (SEM = 2.7) for Gly]. However, SID increased by 2.3 and 2.6 %-units (P ≤ 0.05) with 2P vs. GS for Glu and Lys, increased with 1P and 2P for Met and Pro (5.9 and 3.0 %-units, respectively), and tended to (P < 0.08) increase for Ala and Ile. The greater duodenal flow, and improved SID, of pulp silages may stem from the greater availability of fibre-bound AA in the small intestine. Based on data from this experiment, SID varies notably between individual AA, and the supply of digestible AA from perennial ryegrass was improved when fed as pulp silage.