Use of monensin on beef cattle grazing low-quality forages

May, 2016
Joao Vendramini, UF/IFAS Range Cattle REC, Ona

Warm-season grasses are the main forages and feed resources for beef cattle production in Florida. 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 impact animal performance negatively. In addition, freezing events decrease nutritive value of warm-season grasses grazed during the winter. Concentrate supplementation is the most used management practice to increase the productivity of beef cattle on pasture during periods of limited 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 the use of this management practice due to increased prices of concentrate supplements. Therefore, it is important to maximize the efficiency of supplements in the periods of limited forage.

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 has been used as a growth promoter in the livestock industry for decades and numerous studies have presented positive responses for these growth promoters on ruminants receiving diets with high levels of concentrate. However, the effectiveness of these growth promoters on beef cattle grazing low-quality forages on extensive grazing systems is not consistently reported. Monensin is the most used ionophores in the feeding industry, although there are other ionophore and nonionophore products available in the market. Although the mechanisms are not completely clear, the main effects of monensin on ruminants are: 1) Shift in production of volatile fatty acids in the rumen, 2) Change feed intake and digestibility, 3) Alter gas production, and 4) Increase protein use efficiency.

There are few published studies in the literature reporting the effects of monensin on performance and forage intake of beef cattle grazing warm-season grass pastures in extensive grazing systems. A recent study conducted at the UF/IFAS Range Cattle Research and Education Center, Ona, FL tested the effects of monensin supplementation (200 mg/d) on 2-year old beef heifers (750 lb BW) grazing bahiagrass (Paspalum notatum) pastures at two stocking rates, 0.6 or 1.0 heifers/acre. Heifers received 1 lb of a concentrate supplement (14% CP and 78% TDN) daily. Pastures grazed with greater stocking rates had lesser overall herbage mass (2,500 vs. 3,100 lb DM/acre) with similar forage CP (8.5%) and digestibility (49.7%), and heifers grazing pastures at greater stocking rates had decreased average daily gain. However, there was no effect of monensin supplementation on average daily gain (1.0 lb/d), blood glucose, and insulin at either stocking rate. The same heifers were moved to a drylot and similar total DM intake (2.1% BW) and forage DM intake (2.0% BW) were observed between the control and the heifers receiving monensin. In Oklahoma, Linneen et al. (2015) tested the effects of monensin supplementation on mature cows receiving low-quality forage and observed that there was no difference in cow BW and BCS change during the experimental period.

Despite of the lack of response in animal performance, research conducted at Ona observed there was an improvement in ruminal conditions (greater propionate concentration) in fistulated steers receiving increasing levels of monensin (from 0 to 375 mg/d) on low-quality forage diets. The magnitude of the increase in propionate was not sufficient to increase levels of blood glucose and insulin, and increase animal performance. In addition, forage intake, rumen pH and ammonia were not affected by the treatments. Although monensin did not affect dry matter intake in the recent trials at Ona, 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 to 300 mg of monensin/d and Randel and Rouquette (1976) reported that 200 mg monensin/d reduced dry matter intake of lactating beef cows by 12.4%; however, monensin did not affect milk production or composition. Feed efficiency data is rarely available in additives research on pasture because of the inherent difficulty in measurements of feed intake in grazing animals. Therefore, there may be trials with no change in average daily gain and decreased pasture intake; however, the forage intake was not measured or the scientific methods do not have precision to detect small variations in forage intake. In this case, the benefit of feeding monensin may be realized only if stocking rate is increased.

Conversely, it has been observed that calves grazing warm-season forages and receiving greater levels of concentrate usually have more consistent and positive response to monensin. Research was conducted at Ona to test the combination of two supplementation levels (1 or 2% body weight) with or without monensin on young heifers (376 lb body weight) grazing bahiagrass pastures during the spring. The pastures had sufficient herbage mass for ad libitum intake (3,700 lb DM/acre) with adequate CP (13.8%) but limited digestibility (48%). The concentrate supplement (17% CP and 78% TDN) was fed daily. Heifers receiving monensin had greater average daily gain (2.2 vs 1.8 lb/d) than heifers not receiving monensin at 1 and 2% BW supplementation. In addition, heifers receiving monensin had decreased coccidia count (0.1 vs. 0.7 log transf. count) than control. Similarly, a previous study conducted at Ona observed that young calves (200 lb BW) grazing low-quality bahiagrass pastures (6% CP and 37% digestibility) and supplemented with 2% BW had increased average daily gain and decreased coccidia infestation when monensin (200 mg/d) was added to the supplement. It was observed that young calves usually have greater coccidian count and monensin may be an important feed additive to decrease coccidiosis.

In conclusion, it is unlike that cattle grazing low-quality forages with limited or no concentrate supplementation will improve performance with monensin supplementation. Conversely, when greater levels of concentrate are offered to calves grazing low-quality forage pastures, monensin has been effective to improve performance and decrease coccidia count of beef cattle. Further research is necessary to identify potential benefits of using monensin on beef cattle grazing low-quality forages in extensive grazing systems

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