Paul Beck on Feeding Ionophores to Beef Cows

Weekly, Oklahoma State University Extension Beef Cattle Nutrition Specialist Paul Beck offers his expertise on the beef cattle industry. This is part of the weekly series known as the “Cow Calf Corner,” published electronically by Dr. Derrell Peel, Mark Johnson, and Beck. Today, he discusses feeding ionophores to beef cows.

Forage and supplemental feed costs have the most influence on profitability of cow-calf operations. Feed and pasture have been estimated to be 68% of variable costs and 47% of total costs of maintaining a cowherd. An economic analysis by Kansas State University showed that the top 1/3 producers in profitability had 39% lower feed and pasture costs than low-profit producers.

Ionophores are a class of antibiotics that shift rumen microbial populations to increase energetic efficiency and reduce the production of waste molecules such as methane. These feed additives are used extensively in the stocker and finishing production segments of the beef industry but their use of ionophores in the cow-calf industry has lagged. The ionophore monensin (marketed as Rumensin) was approved for use in replacement beef and dairy heifers in 1983 and for beef cows in 1988. In finishing cattle ionophores decrease feed intake by 5% while increasing gains by 1.5% and feed efficiency by 7%.

For developing replacement heifers, similar to growing and finishing cattle, monensin increased average daily gain, decreased feed intake, and improved feed efficiency. The reproductive effects were impressive, showing decreased age at first puberty and the percentage of heifers cycling before the breeding season without affecting weight at puberty. Even though early puberty and cyclic activity of heifers is often positively correlated with earlier conception, this analysis did not find differences in first service or AI pregnancy rates or total pregnancy rates for heifers.

For mature cows, bodyweight and body condition score changes were not affected by feeding monensin. Monensin significantly decreased forage intake and increased milk production in a limited number of experiments. Monensin also decreased the days to estrus post-calving and increased the number of cows showing estrus before breeding. As with replacement heifers, there was no difference in pregnancy in this analysis.

This data supports using monensin to help push anestrous cattle to start cycling before breeding that would otherwise be late breeders or open. Calculation of energetics from feed intake and performance data show feeding monensin increased dietary energy by 4.7%. The combination of reduced feed intake, increased milk production, and earlier estrus activity has the potential to have long-term positive impacts on cowherd efficiency.

This research was published in the journal Translational Animal Science in 2022 and is available at https://doi.org/10.1093/tas/txac086.

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