420 ANIMAL BIOCHEMISTRY 



bovine or sheep rations unless they can be purchased at a lower price 

 than plant proteins. The superior biological value of animal proteins 

 constitutes no advantage for ruminants since the microorganisms of 

 the latter animals supply the essential amino acids. 



ANTIBIOTICS AND VITAMINS 



The discovery in 1949 that antibiotics added to the ration can 

 increase the rate of growth of normal chicks and young pigs opened 

 up new vistas in livestock feeding. Since that time numerous experi- 

 ments have been conducted with a variety of antibiotics. The anti- 

 biotics most commonly used include aureomycin, terramycin, stabilized 

 penicillin, and bacitracin. Some of the other antibiotics have little 

 or no effect. The cause of the growth-stimulating effect of antibiotics 

 is not entirely clear. Such chemicals may act through the selective 

 reduction of harmful organisms in the digestive tract. Some of the 

 earlier experiments support this concept since animals housed under 

 less sanitary conditions showed the greatest benefit from antibiotic 

 administration. Chicks grown under sterile conditions and those fed 

 antibiotics have thinner intestinal linings. This could result in more 

 complete absorption of nutrients from the small intestine and thus 

 provide far more rapid growth. Some experimental evidence suggests 

 that a stimulation of the pituitary gland may result from antibiotic 

 consumption. Thus, increased growth could result from an increased 

 synthesis of the growth hormone (page 319). 



The antibiotic feed supplements on the market generally also supply 

 a mixture of vitamins since they are prepared from fermentation 

 broths. Vitamin B12 (page 345), also called animal protein factor 

 (APF), has recently been shown to stimulate growth of poultry and 

 swine. This vitamin and other B vitamins may play roles in the 

 beneficial effects obtained with antibiotic supplements. Although the 

 physiology of the stimulation is not completely understood, such 

 antibiotic-vitamin B^o supplements have become very important factors 

 in the development of feeds producing rapid and efficient growth. 



Antibiotic-vitamin supplements increase the rate of gain of young 

 calves but only to an age of about 6 months. No beneficial effects 

 were observed in older calves or in beef and dairy cattle. 



