David Bransby, Brian Gamble, Tony Dawkins, Ellis Burgess, and Randall Rawls

Treating stockers with the Ivomec® sustained release (SR) Bolus for worm control can increase weight gain in beef cattle an average of 44 pounds per head compared to the pour-on formulation, according to results of AAES grazing studies conducted at several locations in Alabama.

Beef animal production losses caused by gastrointestinal parasites (mainly nematodes, or worms) in the United States are estimated at more than $400 million annually. Beef cattle are most vulnerable to these parasites in the first six to 12 months after weaning, commonly known as the stocker phase. Prior to weaning some immunity to worms seems to be transferred via the milk from brood cows to their calves, and by 12 months after weaning most animals have developed considerable immunity themselves. Therefore, it is during the stocker phase that there are large economic benefits from deworming.

Many deworming drugs or anthelmintics are available to reduce worms in cattle, and ivermectin, or Ivomec®, is one of the most effective. This product first became available in an injectable formulation, and later as a pour-on. However, both methods of drug administration required animals to be treated twice: once at the start of grazing, and again about 42 days later. In addition, drug levels in the blood are high immediately after administration, but drop substantially after about four weeks. This often results in low levels of infection developing within six to eight weeks after the second treatment.

Figure 2.The Ivomec® SR bolus.

To overcome these problems the Ivomec SR Bolus was developed. This product is a capsule (one inch in diameter and 3.5 inches long) that releases ivermectin into the rumen continuously for 135 days (Figure 2).

It is administered by securing the animal in a neck-clamp and releasing the bolus into the animal's throat with a bolus gun (see Figure 1 above). In a 1994 AAES grazing experiment, the Ivomec SR Bolus achieved exceptional worm control, keeping stockers almost completely free of worms for 168 days.

Following approval by the Food and Drug Administration, the Ivomec SR Bolus became commercially available at the beginning of 1997. However, the product costs $9 per animal, compared to $5-7 per animal for the injectable and pour-on formulations, but little information was available to verify whether this higher cost would be more than offset by greater animal weight gains.

To address this issue, four grazing experiments were conducted by the AAES in 1997. At the Sand Mountain (Crossville) and Tennessee Valley (Belle Mina) substations stockers grazed tall fescue pastures with both high and low levels of fungus infection and with no supplement for 196 and 168 days, respectively. At the Upper Coastal Plain (Winfield) and Wiregrass (Headland) substations cattle grazed bermudagrass for 140 days. In the Wiregrass experiment animals received a broiler litter/corn supplement, but no supplement was provided in the Upper Coastal Plain experiment. In all the experiments, half the animals received one Ivomec SR Bolus, and the other half were treated with Ivomec pour-on when grazing started, and again 56 days later. A total of 208 animals were used in the four experiments.

In the tall fescue experiments the SR Bolus provided an average of 33.5 pounds of extra weight gain, when compared to the pour-on (see Table 1). This response was not different for high- and low-fungus infected pastures. If the extra weight gain is valued at 70 cents per pound, the additional income from use of the SR Bolus instead of the pour-on was $23.45 per animal.

Cattle treated with the bolus at the Upper Coastal Plain Station gained almost double the weight gained by animals treated with the pour-on (see Table 2). The advantage for the bolus at the Wiregrass Station was lower, possibly because animals were supplemented, thus reducing the amount of grazing and reinfection with worm larvae.

Results from all four experiments indicated that the additional gains provided by the SR Bolus more than offset the additional cost of this product when compared to the pour-on. Even if weight gain was valued at 35 cents per pound, which is a common price for contract grazing, it would still have paid to use the bolus instead of the pour-on. Consequently, with the added advantage of only one administration of the product at turnout, the SR bolus offers an attractive economic opportunity for stocker producers. However, it is not recommended for brood cows because it is retained in the rumen, thus leading to an excessive accumulation of the product if used repeatedly over multiple years.