The study objective was to investigate the impact of monensin and a fermentation extract derived from Bacillus licheniformis- derived fermentation extracts (BLFE) when fed alone or in combination on production performance and feed efficiency in lactating dairy cows. Sixty Holstein cows matched for DIM (108 ± 35) and parity were assigned to 4 treatments formulated to provide 2 doses of monensin (0 and 17.6 mg/kg DM) and 2 doses of BLFE (0 and 166 mg/kg DM) in a 2 × 2 factorial arrangement. Monensin and BLFE doses were top-dressed on a TMR without added monensin. Cows were allowed to adapt to treatments for 21 d before measuring DMI, milk yield, milk composition, and BW responses to treatments over the next 42 d. Rumen fluid VFA, apparent total-tract digestibility (ATTD) of nutrients, and blood AA concentrations were determined in the last week. The effects of treatments were analyzed, accounting for baseline measurements, using mixed-effects models. Neither monensin nor BLFE modified DMI, which averaged 30.8 kg/d across treatments. At this DMI level, the actual monensin and BLFE doses consumed by cows were 14 and 145 mg/kg of DM, respectively. The BLFE fed alone increased milk yield:DMI (1.19 to 1.26) and NEL:DMI (1.60 to 1.83). It tended to increase milk yield and FCM and ECM yields, DMI and milk protein yields, and CP intake. Supplementing BLFE with monensin attenuated those responses. Monensin fed alone increased BW gain (0.44 kg/d) and tended to increase the component-corrected milk yield: DMI, but supplementing it with BLFE negated those effects. Regardless of the presence or absence of monensin in the diet, BLFE increased milk protein content (3.45% to 3.52%), while monensin enhanced milk fat yield (95 g/d) and component-corrected milk yields (4.7% to 5.1%). Milk fat improvements from monensin were significant in multiparous cows but negligible in primiparous cows. In contrast, the feed and protein efficiency enhancements of BLFE were more pronounced in primiparous than in multiparous cows. Supplementing BLFE did not modify ruminal or fecal VFA profiles when supplemented alone or with monensin. Monensin tended to decrease the molar percentage of butyrate in both rumen fluid and feces, while molar percentages of acetate, propionate, and their ratio remained unmodified. Supplementing BLFE or monensin alone did not affect ATTD of CP, but supplementing both together increased ATTD of CP compared with supplementing BLFE alone (59.2% to 62.1%). The BLFE supplementation alone increased blood EAA concentrations (18%), whereas adding monensin to the diet containing BLFE reversed that increase. The poor alignment between blood AA and ATTD in CP suggests potential effects of treatment on gut microbiota metabolism or postabsorptive AA clearance. Under current feeding conditions, data show that BLFE and monensin distinctly increased milk protein and fat, respectively, and each supplement improved feed efficiency when fed alone. The potential antagonism observed when fed together suggests the need for further research to investigate interactions between monensin and BLFE at varying doses under different feeding conditions.
Rigert et al. (Sun,) studied this question.