Beneficial Effects of Feeding Ionophores on Feedlot Cattle Performance and Health

By Jerad Jaborek

What are ionophores? How do they work?

Ionophores are carboxylic polyether compounds approved for use in beef cattle feed for improved weight gain, feed efficiency, and/or the prevention and control of coccidiosis. Ionophores are a classification of antibiotics. However, ionophores are not used in human medicine, and therefore, are not regulated by the veterinary feed directive (VFD). In the United States there are three ionophores licensed for use: monensin (Rumensin, Elanco; Monovet, Huvepharma), lasalocid (Bovatec, Zoetis) and laidlomycin propionate (Cattlyst, Zoetis).

These ionophores are produced naturally by different Streptomyces bacteria to aid in the transport of different cations (ions with a positive charge) across the bacterial cell membranes. For some bacteria, generally gram-positive bacteria, ionophores that bind to their cell membrane disrupt the membrane potential and energy production of the bacterium, and therefore, reduces their viability in the rumen environment. Therefore, ionophores cause a shift in the rumen microbiome population. Similar to ionophore-sensitive bacteria, ionophores can cause the same disruption to the protozoan parasites (Eimeria bovis and Eimeri zuernii) that colonize the intestinal lining and are responsible for causing coccidiosis. On this basis, monensin and lasalocid are also classified as coccidiostats.

The shift in the rumen microbiome increases the amount of lactic acid utilizing bacteria and reduces the amount of lactic acid producing bacteria and methane producing bacteria. This shift in rumen volatile fatty acid (VFA) production leads to the increased the production of propionate produced by the microorganisms in the rumen and improves overall energetic efficiency for the ruminant. Additionally, the shift in the microbiome reduces the amount of hyper-ammonia producing (HAP) bacteria that aid in protein digestion in the rumen. This reduction of HAP bacteria reduces the contribution of microbial protein production and increases the amount of bypass protein (i.e., rumen undegradable protein; RUP) leaving the rumen to be digested in the small intestine. This can change the ratio of microbial protein versus dietary RUP, more specifically the amino acids, being absorbed by the animal.

Ionophore Effects on Cattle Feedlot Performance

Ionophore use in ruminant feed has been approved since 1975. Multiple reviews and meta-analyses have been conducted over the years for ionophores approved in the United States. The results will be summarized here to show their impact on cattle feedlot performance over time.

Goodrich and others summarized the effects of monensin in 1984 using data from 228 feeding trials and 11,274 head of cattle. On average, monensin offered a slight improvement in average daily gain (ADG) 1.6% (2.40 vs. 2.42 lb./day), and larger improvements in feed efficiency (7.5%) by reducing dry matter feed intake (DMI) by 6.4% (19.1 vs. 17.0 lb./day).

In a 2012 Journal of Animal Science publication, Duffield and others conducted a meta-analysis (i.e., summary) on the inclusion of monensin in growing and finishing cattle diets from 64 qualifying scientific manuscripts and research reports (169 research trials) dated back to 1972. Feeding monensin reduced DMI 3.1% (-0.59 lb.), increased ADG 2.5% (0.06 lb./day) and improved feed efficiency 6.4% (-0.53 lb. feed/lb. gain).