are more likely to give a response. However,an experiment recently completed at Belts- 
ville indicated that zinc bacitracin gave as good a growth rate and feed conversion as 
spiramycin, an antibiotic which had not been previously used, Results of antibiotic feeding 
at the University of Wisconsin, covering a period from 1951 to 1954 and 1955 to 1959, 
were given at the 1960 meeting of the Informal Poultry Nutrition Conference, Although 
there was extreme variability in the results of both periods, the general trend showed 
about the same response for both periods. 
The results of studies on the effect of antibiotics on laying hen performance have 
been more controversial than those with growing chicks. Some work at Beltsville has 
furnished a possible explanation for these contradictory reports. Our findings indicate 
that when a hen is laying below her genetic capacity because of stress conditions, or 
some subclinical infection, antibiotics may improve production, On the other hand, when 
a healthy bird is producing at or near her genetic capacity, antibiotic feeding is of little 
value. Another report at the 1960 Informal Poultry Nutrition Conference from Ohio State 
University, indicated that antibiotics were most effective during cold weather. Con- 
versely, studies at our Glendale Station have shown that high-level antibiotic feeding, 
50 to 100 gms. of chlortetracycline per ton of feed, consistently improved egg production 
during the extremely hot summer months encountered in that area. These reports are 
not necessarily contradictory but rather emphasize the fact that antibiotics are more 
effective under stress conditions. 
The majority of reports on the inclusion of arsenicals in the diets of growing poultry 
show increased growth rate and in some cases increased efficiency of feed utilization, 
As in the case of antibiotics, the responses to arsenicals are variable depending upon 
environment and the type of diet being supplemented. Early work at Beltsville indicated 
that the arsenical effect differed somewhat from that of antibiotics since occasionally 
the two effects were additive. 
Perhaps the most comprehensive study on the effect of arsenicals on growth rate is 
that of Milligan, et al (9) involving 13 different experiments and almost 10,000 broiler 
chicks, They found that when practical diets contained low levels of antibiotics it was 
economically advantageous to supplement with arsonic acid. 
The effect of arsenicals onegg productionis less clear cut, since some investigations 
indicate no effect and others only a small increase. A 3-year study at Beltsville failed to 
show any influence of arsanilic acid onegg production, whereas the work at the Glendale 
Station indicated that arsenicals are of some value in supporting production in hot 
climates. 
Both arsonic and arsanilic acids have been fed at levels as high as 335 gms. per ton 
of feed without harmful effect. At higher levels toxicity was observed. Arsanilic acid at 
0.03-percent dietary level results in increased clotting time of blood. 
Reports by Texas workers in 1957 (7) indicate that a low level molybdenum supple- 
mentation (0.13 to 0.3 p.p.m.) improved chick growth on a practical diet. All other reports 
on the effect of molybdenum have been essentially negative. However, because of the 
very slight cost and on the possibility that it may be of some value, many commercial 
feeds are supplemented with this mineral. 
At levels of 300 to 500 p.p.m. of molybdenum, toxicity was exhibited by a severe 
growth depression and 6,000 p.p.m. resulted in an 80-percent growth depression over 
the control. Feeding 0.3 p.p.m. supplementary molybdenum resulted ina substantial 
increase of the element in chicken blood and liver. Levels of 500 p.p.m. were toxic for 
the laying hen and resulted in molybdenum content in the egg of 12 p.p.m. Work at Belts- 
ville has demonstrated an interrelationship between molybdenum and sulfate and molyb- 
denum and copper since these two materials counteractto some extent the growth depres- 
sion caused by feeding 300 and 500 p.p.m. of molybdenum. 
45 
