341 Meta-analysis of the Performance Responses of Replacement Heifers and Beef Cows to Monensin

Performance benefits of monensin have been extensively studied in finishing and stocker cattle, but considerably less published work is available evaluating response to monensin supplementation in cow-calf production systems. Feed additives are more difficult to study in cow-calf production systems due to unstable diet characteristics and cow physiological state throughout the production cycle. This meta-analysis investigated the impacts of monensin on performance of extensively raised beef cow-calf and developing replacement heifers. The replacement heifer analysis was conducted with a maximum of 48 treatment means in 18 experiments. The mature cow analysis included 21 publications and 26 mean comparisons. 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. For replacement heifers, 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%) were increased (P < 0.01), while dry matter intake (-4.3%) and age at puberty (-8.9 ± 1.48 d) were decreased (P < 0.01). Six studies reporting ad libitum forage intake for mature cows showed that monensin decreased (P = 0.008) DMI by 0.85 ± 0.322 kg/day. Six studies showed monesin increased (P = 0.01) milk yield 0.39 ± 0.15 kg/day by mature cows in early lactation. There were no differences in artificial insemination pregnancy nor total pregnancy for either the heifer or mature cow data sets. This analysis also indicates potential for use of monensin in beef cow production systems, but further research is needed to elucidate the effects on DMI and milk production in beef cows.