Years of research and practical experience show that implants work, both in stocker cattle and in the feedyard. Kansas State University Extension beef specialist Chris Reinhardt, PhD, says results of hundreds of implant studies on stocker cattle show 15 to 20 percent increases in average daily gain across a range of nutritional levels.

On the downside, trials also have shown that some implant programs, in some situations, can reduce the percentage of cattle grading Choice or better, a factor that becomes more important as producers target premium markets for their cattle. But as scientists learn more about the interactions between implants, animal physiology and environmental factors, producers become better able to tailor their implant strategies for an optimum balance of efficiency and carcass value.

Citing South Dakota State University animal scientist Robbie Pritchard, Reinhardt says cattle prioritize nutrients for use in a specific order: first for maintenance, then for lean tissue growth, then for depositing marbling fat and finally for depositing external fat.

When nutrients, particularly energy, are abundant, cattle have adequate energy to deposit external fat. Implants increase the nutrient demand for lean tissue deposition, and in this environment will have little effect on marbling, which has a higher deposition priority than external fat, Reinhardt says.  

On the other hand, when nutrients are in short supply, such as in a drought-stressed winter pasture, stocker cattle will channel most of the available nutrients toward maintenance and muscle growth, and less toward marbling. Reinhardt points out that implants provide a fairly consistent percentage improvement in gains, compared with no implant, across the full range of forage conditions. So, implanted cattle on a lower plain of nutrition could have higher nutrient demand when supply already is limited. In this case, too much growth promotion could draw nutrients away from marbling and toward lean deposition.

Reinhardt points out that unlike external fat, marbling fat deposition cannot compensate later when nutrition improves. Cattle that cannot meet nutritional demands for depositing marbling while on grass can later gain weight and external fat in the feedyard, but some degree of marbling potential, at a given fat endpoint, is lost.

Reinhardt points out that stocker cattle, depending on the location and time of year, often face circumstances in which forage conditions are marginal or, at best, unpredictable. During August in the Northern Plains, he says, or July in fescue country, or nearly anytime in the Southern Plains, poor pasture conditions are potentially no more than a few days or weeks away. 

As nutrition improves, so does the degree to which cattle respond to different implants. Reinhardt says that in a comparison of 33 individual stocker implant studies, with base rate of gain ranging from 0.62 to 2.2 pounds per day, researchers compared the effects of implants of different dosage and composition on increase in daily gains. Overall, as the base rate of gain increased from 1 pound to 2 pounds per day, indicating better forage quality, the additional gains from implanting also increased, meaning response to implants increases with increasing nutrient supply. The degree of that response, however, varies between implants. In these trials, at higher nutrition levels, increases in average daily gain ranged from 0.12 pound for a zeranol implant to 0.29 pound for estradiol.

So when forage quality and rates of gain are high, an implant that generates a greater response can be desirable. But when nutrients are limited, the extra growth promotion could have a negative effect on future marbling potential. Stocker operators, Reinhardt says, should consider using a relatively mild initial implant when forage quality and quantity are limited, particularly if they participate in a market that rewards quality grade.