Stephen Simpkins, Wendell McElhenney, Hardin Rahe, Dwight Wolfe, Donald Mulvaney, and Sarah Lino

Growth-promoting implants have been used for years as an economical way to enhance cattle gain. Their use is widely accepted, too, especially since extensive research has shown that meat and animal products from implanted animals are as safe and acceptable to consumers as comparable products derived from non-implanted steers.

Implants that result in maximal growth and feed efficiency of steers contain a combination of the sex hormones androgen and estrogen. For more than 40 years, implants utilizing synthetically produced sex steroids and analogs have been used to enhance the rate and efficiency of growth, to improve lean growth, and to augment carcass characteristics to the extent that the value of implanted cattle is increased.

Producers are concerned, however, that these growth-promoting implants, especially those containing trenbolone acetate, may be associated with decreased marbling. Is such the case with Synovex-Plus®? A team of AAES researchers decided to find out.

Synovex-Plus® is an implant that contains trenbolone acetate (androgen characteristics) and estradiol benzoate (a form of estrogen), a combination that has proved effective in enhancing the rate and efficiency of gain in feedlot steers.

Although the effects of Synovex-Plus® on growth and feed efficiency are well known, the mode of action and the effect on carcass characteristics, especially marbling, have not been fully understood. The AAES researchers initiated their trial to evaluate differences between calves implanted with Synovex-Plus® and calves receiving no implant for growth, feed efficiency, carcass characteristics, and IGF-1 concentration. IGF-1 is a hormone essential for growth.

For the study, 10 purebred Angus steers and two purebred Herefords were randomly assigned to either the Synovex-Plus® implant group or the no-implant control group. Body weight and blood samples were obtained at the initiation of the experiment. Additional blood samples were taken at days 28, 56, and 70. Serum was removed from the blood samples on the day of collection and was frozen until the end of the experiment, when analyses would be conducted to determine IGF-1 concentration. Steers were weighed bi-weekly through day 56. Individual feed consumption was facilitated by using Calan® gates, a system which allows only the animal assigned to a feed trough to open the gate.

While no differences in total amount of feed consumed were detected between the implant group and the control group during the 56 days of recorded feeding, the implanted steers gained 50.4 pounds more weight than the controls (Table 1). The combination of slower growth and similar feed consumption resulted in control calves requiring more feed per unit gain compared to the implanted calves. In fact, the implanted group showed a 10.2% improvement in feed efficiency and a 19.6% increase in average daily gain.

Table 1. Means for Feed Consumption, Calf Growth, and Feed Efficiency by Treatment Groupa
	Treatment
Item	Control	Implant
Feed consumed, kg	722.3	729.0
Total gain, kg	89.3	112.2
Average daily gain, kg/db	1.6	2.0
Feed/gainb	8.2	6.6
aImplant group received a Synovex-Plus® implant that contains 28 mg estradiol benzoate and 200 mg trenbolone acetate.
bProbability of obtaining a treatment difference at least this large when no treatment differences exist is less than 5%.

Despite those impressive showings, however, if the quality of the end-product in implanted calves were to be severely reduced, the economic advantages of the implants would be offset with discounts at the market. Because growth-promoting implants generally enhance protein deposition and diminish fat deposits, decreased marbling is a concern when the implants are used. Obviously some implants are more detrimental to marbling than others. In general, the more effective the implant is for increasing growth and feed efficiency, the more detrimental it is for marbling.

In this study, except for the percentage of kidney, pelvic, and heart fat, no differences were found on measures taken on the carcasses (Table 2).

Table 2. Means for Carcass Characteristics by Treatment Groupa
	Treatment
Item	Control	Implant
Carcass weight, kg	312.3	327.7
Backfat, cm	1.3	1.2
Ribeye area, cm2	71.2	74.3
Yield grade, units	3.4	3.1
Boneless trimmed retail cuts, %	49.1	49.7
Kidney, pelvic, and heart fat, %	2.4	1.9
Marblingb	535.8	534.2
aImplant group received a Synovex-Plus® implant that contains 28 mg estradiol benzoate and 200 mg trenbolone acetate.
bMarbling scores were given numerical values such that slight00=400, small00=500, etc.

As for changes in serum concentrations of IGF-1 by treatment group (see figure), the IGF-1 concentration in the serum of implanted steers increased across time, while serum IGF-1 concentration in the control steers decreased across time. This suggests that IGF-1, the essential growth hormone, was related to the additional gain of the implanted steers. The reason for this linear decrease in serum IGF-1 concentration in control calves is unclear, based on the carcass composition data (Table 2).

Serum concentrations of IGF-1 (ng/ml) in steers implanted with Synovex-Plus and non-implanted steers.

For example, it does not appear that the effect can be attributed to the control animals approaching their physiological maturity, because the control and implant animals exhibited similar degrees of fatness.

The magnitude of the improvements in body weight gain and improvement in feed conversion efficiency due to implants obtained in this study provides further evidence supporting the use of anabolic agents as growth enhancers, particularly since this study also showed no detrimental effects on carcass quality.

This study indicates that the beneficial effects of Synovex-Plus® in cattle may be mediated at least in part by alteration of the somatotropic-IGF-1 axis. In other words, implants alter growth hormone secretion, causing an increase in production of IGF-1. The net effect could be an increase in protein synthesis and protein gain, which translates into greater, more efficient live weight gain and profit for the cattle producer.

As the mechanism of action of growth-promoting implants becomes better understood, that could lead to the development and production of implants that are more effective for the cattleman.