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Monensin studied in cattle grazing low-quality forageMonensin studied in cattle grazing low-quality forage

March 31, 2016

7 Min Read
Monensin studied in cattle grazing low-quality forage

*Dr. Joao Vendramini is an associate professor of pasture and forage management at the University of Florida Range Cattle Research & Education Center in Ona, Fla.

WARM-season grasses are the main forages and feed resources used in beef cattle production in tropical and subtropical areas.

In extensive grazing systems, limited fertilization and fixed and continuous stocking rates have resulted in forages with highly variable herbage mass and nutritive value, which may negatively affect animal performance.

Concentrate supplementation is the management practice most commonly used to increase the productivity of beef cattle on pasture during periods of shortage of forage quantity and/or quality. The benefits of concentrate supplementation on the performance of beef cattle grazing warm-season grasses have been well reported in the literature. However, producers are often limited in their ability to use this management practice due to increased prices of concentrate supplements.

Feed additives that enhance animal performance through increased growth rate and/or feed conversion in clinically healthy and nutritionally normal animals are termed growth promoters. Ionophores have been used as a growth promoter in the livestock industry for decades, and numerous studies have presented positive responses of these growth promoters on ruminants receiving diets with high levels of concentrate.

However, the effectiveness of these growth promoters for beef cattle grazing low-quality forages on extensive grazing systems has not been reported consistently. Monensin is the most used ionophore in the feeding industry, but there are other ionophore and non-ionophore products on the market.

Although the mechanisms have not been completely elucidated, the main effects of monensin in ruminants are: (1) a shift in production of volatile fatty acids, (2) a change in feed intake and digestibility, (3) altered gas production and (4) increased protein use efficiency.



Few published studies in the literature have reported on the effects of monensin on the performance and forage intake of beef cattle grazing warm-season grass pastures. Recently published studies showed that monensin had limited effects on cattle consuming low-quality forage with reduced amounts of concentrate supplement.

Vendramini et al. (2015) tested the effects of feeding 200 mg of monensin per day to beef heifers with a starting bodyweight of 750 lb. that were grazing bahiagrass (Paspalum notatum) pastures in Florida at two stocking rates: 0.6 or 1.0 heifer per acre. Heifers received 1 lb. of a daily concentrate supplement containing 14% crude protein (CP) and 78% total digestible nutrients.

Pastures grazed with greater stocking rates had lesser overall herbage mass (2,500 lb. versus 3,100 lb. of dry matter per acre) with similar forage CP (8.5%) and digestibility (49.7%). Heifers grazing pastures at greater stocking rates had decreased average daily gains (ADG) in August.

However, there was no effect of monensin supplementation on ADG (mean = 1.0 lb. per day), blood glucose (mean = 74.5 mg/dL), insulin (73 micro-international units per milliliter) and insulin-like growth factor-1 (IGF-1; mean = 68.2 nanograms [ng] per milliliter).

The same heifers were moved to a drylot and were observed to have similar total dry matter intake (2.1% of bodyweight) and forage dry matter intake (2.0% of bodyweight) for both the control group and heifers receiving monensin.

Linneen et al. (2015) tested the effects of monensin supplementation on mature cows receiving low-quality forage and observed no difference in cow bodyweight or body condition score change during the experimental period.

Despite of the lack of response in animal performance, Vendramini et al. (2015) observed an increase in ruminal propionate concentration (from 16 mol to 19 mol per 100 mol) in fistulated steers receiving increasing levels of monensin (from 0 mg to 375 mg per day) on low-quality forage diets. The magnitude of the increase in propionate was not sufficient to increase levels of glucose, insulin and IGF-1 in the blood. In addition, forage intake, rumen pH, ammonia and concentrations of acetic, isobutyric and butyric acid were not affected by the treatments.

Although monensin did not affect dry matter intake in those trials, there are reports in the literature showing that monensin may affect forage dry matter intake. Walker et al. (1980) indicated that dry matter intake levels may be reduced by 5-10% when beef cows are supplemented with 200-300 mg of monensin per day, while Randel and Rouquette (1976) reported that 200 mg of monensin per day reduced dry matter intake of lactating beef cows by 12.4%; however, monensin did not affect milk production and composition.

Feed efficiency data are rarely available because of the inherent difficulty in measuring the feed intake of grazing animals. Some trials may show no change in ADG yet decreased pasture intake; however, either the forage intake was not measured, or the scientific methods did not have the precision to detect small variations in forage intake. In this case, the benefit of feeding monensin may be realized only if the stocking rate is increased.

Conversely, it has been observed that cattle grazing warm-season forages and receiving greater levels of concentrate usually have a more consistent and positive response to monensin.

Vendramini et al. (2016) tested the combination of two supplementation levels — 1% or 2% of bodyweight — with or without monensin on 376 lb. heifers grazing bahiagrass pastures during the spring. The pastures had sufficient herbage mass for ad libitum intake (3,700 lb. of dry matter per acre) with adequate CP (13.8%) but limited digestibility (48%). The concentrate supplement (17% CP and 78% total digestible nutrients) was fed daily.

There was no significant interaction between concentrate levels and monensin. However, heifers receiving the 2% supplementation level had greater ADG (2.1 lb. versus 1.8 lb. per day), blood glucose (76.3 mg versus 66.4 mg/dL) and IGF-1 (192 ng versus 173 ng/mL) than those on the 1% level, and heifers receiving monensin had greater ADG (2.2 lb. versus 1.8 lb. per day) and blood glucose concentrations (78 mg versus 70 mg/dL) than the control heifers. In addition, heifers receiving monensin had a decreased coccidia count (0.1 versus 0.7 log transform count) than the control.

Similarly, Martins et al. (2014) observed that young calves (200 lb. bodyweight) grazing low-quality bahiagrass pastures (6% CP and 37% digestibility) and supplemented at 2% of bodyweight had increased ADG and decreased coccidia infestation when 200 mg of monensin per day was added to the supplement.


The Bottom Line

Cattle grazing low-quality forages with limited or no concentrate supplementation have decreased intake of non-structural carbohydrates, which may decrease the precursors of propionate in the rumen and the potential benefit of monensin. Conversely, when greater levels of concentrate are offered to ruminants grazing low-quality forage pastures, monensin has been effective at improving the performance of beef cattle.

Further research is necessary to elucidate the effects of monensin on low-quality forage intake and to identify the potential benefits of using monensin on beef cattle grazing low-quality forages in extensive grazing systems.



Martins, P.G.M.D.A., J.D. Arthington, G. Caputti, O.F.R. Cunha, A.C.J. Pereira and J.M.B. Vendramini. 2014. Effects of rumensin and fenbendazole administration on measures of gastrointestinal parasite load and performance of early weaned beef calves. In: Proceedings of Southern Association of Agricultural Scientists/American Society of Animal Science annual meeting, Dallas, Texas.

Randel, R.D., and F.M. Rouquette Jr. 1976. Effect of monensin on lactation in beef cows. J. Anim. Sci. 43:232.

Vendramini, J.M.B., V.C. Gomes, F.A. Kuhawara, R.F. Cooke and J.M.D. Sanchez. 2016. Supplementation levels and monensin effects on performance of early weaned calves grazing bahiagrass pastures. In: Proceedings of Southern Association of Agricultural Scientists/American Society of Animal Science annual meeting, San Antonio, Texas.

Vendramini, J.M.B., J.D. Sanchez, R.F. Cooke, A.D. Aguiar, P. Moriel, W.L. da Silva, O.F.R. Cunha, P.D.S. Ferreira and A.C. Pereira. 2015. Stocking rate and monensin supplementation effects on performance of beef cattle consuming warm-season grasses. J. Anim. Sci. 93:3682-3689.

Walker P.M., B.A. Weichenthal and G.F. Cmarik. 1980. Efficacy of monensin for beef cows. J. Anim. Sci. 51:532.

Volume:88 Issue:04

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