Monensin D

Although performance benefits of monensin have been extensively studied in finishing cattle, growing cattle, and dairy cows, considerably less published work is available evaluating response to monensin supplementation in cow-calf production systems. This meta-analysis investigated the impacts of monensin on performance of beef cows and developing replacement heifers. The replacement heifer analysis was conducted using data from 18 different peer-reviewed publications and experiment station reports. The mature cow analysis included 21 different publications and experiment station reports. The metaphor package (version 2.4-0; Viechtbauer, 2010) for R (version 4.0.3; www.r-project.org) was used to determine the overall effect size of monensin compared to a negative control. Each study's n, means, and SEM or P value was used to calculate the mean difference and estimate of within study variance for responses of interest. In replacement heifers, monensin treatment increased (P < 0.01); average daily gain (+0.03 ± 0.008 kg/d), feed efficiency (+0.013 ± 0.008 gain:feed), and percentage cycling before the breeding season (+15.9 ± 5.13%); while decreasing (P < 0.01): dry matter intake (0.293 ± 0.081 kg), and age at puberty (-8.9 ± 1.48 d). Six studies reporting ad libitum forage intake for mature cows showed decreased (P = 0.008) DMI by 0.85 ± 0.32 kg/d. Six studies reported milk yield and revealed an increase (P = 0.01) of 0.39 ± 0.15 kg/d when cows were supplemented with monensin. Monensin supplementation resulted in a reduction (P = 0.02) in days to first estrus by 18 ± 8.2 d and percentage of cows exhibiting estrus prior to the breeding season was increased by 19 ± 8% (P = 0.03). There were no differences in artificial insemination pregnancy nor total pregnancy for either the heifer or mature cow data sets. This analysis indicates potential for use of monensin in heifer development and beef cow production systems. Further research is needed to elucidate the effects on reproductive efficiency, DMI, milk production, weight, and body composition change.

Previous studies have demonstrated that the effects of monensin on methanogenesis and ruminal fermentation in ruminants were time-dependent. To elucidate the underlying mechanism, we investigated the ruminal bacterial community successions during the adaptation to monensin supplementation and subsequent withdrawal in goats. The experiment included a baseline period of 20 days followed by a treatment period of 55 days with 32 mg monensin/d and a washout period of 15 days. Monensin supplementation reduced the α diversity and changed the structure of ruminal microflora. The α diversity was gradually restored during adaption, but the structure was still reshaped. The temporal dynamics of 261 treatment- and/or time-associated ruminal bacteria displayed six patterns, with two as monensin-sensitive and four as monensin-resistant. The monensin sensitivity and resistance of microbes do not follow a clear dichotomy between Gram-positive and Gram-negative cell types. Moreover, the temporal dynamic patterns of different bacterial species within the same genus or family also displayed variation. Of note, the relative abundance of the total ruminal cellulolytic bacteria gradually increased following monensin treatment, and that of the total amylolytic bacteria were increased by monensin, independent of the duration. In conclusion, under the pressure of monensin, the ruminal ecosystem was reshaped through a series of succession processes, and the carbohydrate-degrading bacteria presented a higher level of adaptability.

Controversy has existed as to whether monensin will provide equal or differential benefits in a higher-energy, lower-roughage close-up diet versus a higher-roughage, lower-energy diet. Our objective was to determine the rumen effects of a controlled-energy, high-fiber diet balanced to meet but not greatly exceed energy requirements during the dry period or a traditional 2-group approach of higher-energy close-up diet. The effects of added monensin in each diet type were determined. Multiparous Holstein cows (n = 17) were fitted surgically with ruminal cannulas. During the first 4 wk of the dry period, all cows were fed a controlled-energy, high-fiber diet (CE) as a total mixed ration for ad libitum intake. During the last 3 wk before calving, half of the cows were switched to a higher-energy, close-up diet until calving (CU), whereas the other half continued to receive the CE diet. Within each dietary group, half of the cows received monensin (MON) supplementation in the diet (24.2 g/t of total dry matter) and half did not (CON). After calving, all cows received the same lactation diet containing monensin (15.4 g/t of dietary dry matter). At 14 d prepartum, dry matter intake was not different across treatments. The weight of rumen contents was greater for cows fed CE. Rumen liquid dilution rate, solids passage rate, pH, total volatile fatty acid (VFA) concentrations, molar proportions of acetate and propionate, and papillae length did not differ among diets. Butyrate percentage tended to be greater for cows fed CE. Postpartum, dry matter intake, mass of rumen contents, solids passage rate, pH, total VFA concentration, molar percentages of propionate and butyrate, and papillae length did not differ among treatments. Liquid dilution rate (16.6, 10.7, 16.0, and 18.2%/h for CE + CON, CE + MON, CU + CON, and CU + MON, respectively) was affected by a diet × monensin interaction. Cows on the CE + CON diet had a greater ruminal proportion of acetate than did cows fed CU + CON, whereas cows fed monensin on either diet were intermediate (diet × monensin interaction). Addition of MON to the CU diet decreased the proportion of propionate (diet × monensin interaction). Cows fed CE had greater mass of rumen contents before parturtition but the high inclusion of wheat straw in the CE diet did not negatively affect rumen papillae length. Monensin inclusion differentially affected liquid passage rate and VFA concentrations.