Supplementation of isoacids to lactating dairy cows fed low- or high-forage diets: Effects on performance, digestibility, and milk fatty acid profile.

Our objective was to determine the effects of isoacids (ISO) on the lactation performance, digestibility, and milk fatty acids (FA) profile of Holstein cows fed 2 forage NDF levels (FL). The study lasted 10 wk (including 2 wk for covariate) using a randomized complete block design. Sixty-four mid-lactating Holstein cows (662 ± 71 kg BW, 119 ± 51 DIM, 2 ± 0.9 parity [±SD]) were blocked by parity, DIM, and prior milk yield (MY) for multiparous cows or genetic merit for primiparous cows, and randomly assigned to 1 of the 4 diets (n = 16). Diets were arranged as a 2 × 2 factorial, with 2 FL containing 21% forage (HF) and 17% forage NDF (LF) without (WIA) or with ISO supplementation (IA; 7.85 mmol/kg of DM and 3.44 mmol/kg of DM for isobutyrate and 2-methylbutyrate, respectively). Diets were balanced for similar NE (1.58 Mcal/kg of DM), CP (16.0%), and total NDF (27.2%). Feed intake and MY were recorded daily. Nutrient digestibility for each cow was determined using indigestible NDF as a marker, and fecal samples were collected at 8-time points (4-h intervals between samples). Individual cow milk samples composited over a 10-wk period were analyzed using GC for FA profile. The statistical model included FL, ISO, and FL × ISO as fixed effects and block as a random effect (lme4 in R). The ISO did not affect DMI, and LF cows had greater DMI than HF cows (27.8 vs. 26.0 kg/d). However, ISO increased MY (34.7 vs. 37.2 kg/d) and ECM (41.9 vs. 39.0 kg/d) by 7% in cows fed the HF but not in those fed the LF diet, suggesting a FL × ISO interaction. Interestingly, ISO increased ADG (0.4 kg/d) but decreased MUN by 9% only in LF diet as indicated by the FL × ISO interaction. Additionally, ISO increased DM, OM, NDF, and CP digestibility by 10% to 24% in HF, but not in LF (FL × ISO). As expected, ISO increased milk odd-chain FA profiles in the IA groups irrespective of FL; for example, the IA had greater C15:0 (1.87 vs. 1.54 g/100g FA) and a tendency to be greater C17:0 levels (0.86 vs. 0.76 g/100g FA) compared with WIA groups. Overall, ISO improved MY and nutrient digestibility in cows fed the HF diets, whereas it increased ADG and decreased MUN in cows fed the LF diet. Additionally, ISO increased milk odd-chain FA (C15:0 and C17:0) regardless of FL.