General aspects of the effectiveness of anabolic agents in increasing protein production in farm animals, in particular in bull calves.

The role of farm animals as converters of feed protein into protein edible for humans has often been criticized for the lack of efficiency of this process. In a series of 192 N-balance studies with veal calves it was found, however, that these animals convert at an early age even more than 70 percent of the feed protein into body protein. The deterioration of the N-conversion, so undesirable from a point of view of food production, is found when the animals grow older. With increasing age the conversion ratio went down to approximately 40-50 percent in our studies. This could only be explained by assuming a poorer efficiency of the intermediate metabolic processes with increasing age. For an increase of the production of edible protein from the available feed sources in the world, it is obviously of great importance to maintain the favourable conversion ratio of young farm animals for the longest possible time. The object of this paper is to consider whether and to what extent a positive effect on the efficiency of the protein formation can be exerted by the application of anabolic agents using the veal calf as a model. In the category of artificial estrogens we found DES to be effective in this respect. In the category of natural hormones testosterone and progesterone did not exert anabolic effects. The implantation of 17beta-estradiol improved the conversion of feed protein into body protein to approximately the same extent as DES. A combination of estradiol with testosterone tended to be more effective than estradiol alone. The more recently isolated agents zeranol and trenbolone did not lead to significant improvements in N-retention. A combination of trenbolone with estradiol improved N-retention better than any of the other treatments. The quantitative importance of the phenomena discussed for protein production is illustrated as follows. With the most effective treatment studied, the percentage of feed protein converted into body protein was increased during an experimental period of 38 days, from 39 percent in the control group to 58 percent in the treated group. There is no doubt that a further intensification of the research in this field is recommendable in order to create and study active agents combinining a high degree of efficacy with the absence of undesirable side effects for man and animals.