THE FLORIDA CATTLEMAN AND LIVESTOCK JOURNAL
Use of ionophores in grazing systems
Warm-season grasses are the main forages and feed resources for beef cattle production in Florida. If managed properly, warm-season grasses have excellent production and acceptable nutritive value during the spring, summer, and early fall. However, the production and nutritive value of warm-season grasses are limited during the winter. Concentrate supplementation has been the most used management practice 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 performance of beef cattle grazing warm-season grasses is well known; however, producers are often limited to the use of this management practice due to increased prices of concentrate supplements. It is estimated that winter feeding (conserved forage and concentrate) may comprise 35 to 47% of the annual cost of cow-calf operations.
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. Growth promoters can be defined as substances, other than dietary nutrients, that increase growth rate and/or feed efficiency in healthy animals.
Monensin is an ionophore 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. The main expected benefits of monensin on ruminants are: 1) Shift in production of volatile fatty acids, 2) Change feed intake and digestibility, 3) Alter gas production, and 4) Increase protein use efficiency. However, the effectiveness of the use of monensin on beef cattle grazing warm-season forages on extensive grazing systems is not consistently reported.
It has been observed that cattle grazing warm-season forages and receiving greater levels of concentrate usually has more consistent and positive response to ionophores. Research conducted at the UF/IFAS Range Cattle Research and Education Center tested the effects of adding 20 ppm of monensin to the supplement of early weaned calves grazing dormant bahiagrass in the winter and receiving 2% BW supplementation. The addition of monensin in the supplement resulted in significant increase in ADG from 1.7 to 2.0 lb/d. In addition, calves receiving monensin had 76% reduction in the incidence of coccidia. There was no difference in forage mass, implying that forage intake was likely similar among treatments. The calves were moved to a drylot and maintained in the same treatment for 30 d to evaluate the effect of monensin on forage intake. There was no effect of monensin on forage dry matter intake (0.7% BW) or total DM intake (2.6% BW). It is likely that the ionophore increased the efficiency of converting the supplement to proprionic acid and increased average daily gain of the calves. In the same experiment, a group of early weaned calves were allocated to graze annual ryegrass with 1% BW supplementation, with or without 20 ppm of monensin. Conversely to the results of the calves grazing bahiagrass with greater supplementation levels, there was no difference in average daily gain between treatments. It is likely that the calves grazing annual ryegrass had lesser supplementation and substrate to propionic acid production in the rumen. It seems clear that monensin can improve the efficiency of utilization of concentrate supplements to cattle grazing warm-season grasses; however, the effect of monensin on cattle receiving limited or no supplementation has been variable.
A different study with replacement heifers was conducted at the UF/IFAS Range Cattle Research and Education Center to test the effects of monensin (200 mg/d) to beef heifers grazing bahiagrass pastures at two stocking rates, 0.7 or 1.1 heifers/acre for 2 years in Florida. Heifers received 1 lb of a concentrate supplement daily. The objective of the study was to verify the effectiveness of monensin in grazing animals with limited supplementation and forage quantity. Pastures grazed with greater stocking rates had lesser herbage mass (2,300 vs. 2,800 lb/acre), however, there was no effect of stocking rates or monensin on forage CP (8.5%) and digestibility (49.7%). Pastures with greater stocking rate had lesser ADG (0.8 vs. 1.0 lb/d); however, there was no effect of monensin supplementation on ADG (mean = 0.9 lb/d). In addition, forage intake total DM intake (2.1% BW) or forage DM intake (2.0% BW) was similar between treatments. The combinations of environmental factors and forage characteristics may have limited the potential positive effects of the monensin.
It has been hypothesized that increasing monensin levels may have positive effect on forage digestibility and performance of cattle receiving warm-season forages with limited concentrate levels. A recent study at the UF/IFAS Range Cattle Research and Education Center evaluated the effects of increasing levels of monensin on rumen fistulated animals receiving bermudagrass hay and 1 lb concentrate/d. There was an increase in propionic acid concentration acid in the rumen and a trend to decrease acetic acid; however, rumen pH, ammonia, isobutyric, and butyric acid concentrations were not affects by monensin. In addition, there was no effect on dry matter intake, 2.1 % BW.
There is an opportunity to use ionophores to increase the efficiency of supplementation in cow-calf systems, primarily in programs with greater concentrate supplementation levels, such as early weaned calves and heifer development. However, there is still limited information about the effects of ionophore on beef cattle grazing warm-season pastures with limited supplementation levels. Moreover, the potential positive effect of the ionophore may be conditional to forage quantity and nutritive value. Further research will be conducted at the UF/IFAS Range Cattle Research and Center to identify grazing management practices to improve the efficiency of using ionophores and performance of cattle grazing warm-season forages.
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