DAIRY CATTLE 
The two main groups of feed additives that have attracted research interest and have 
been given serious consideration in practical application todairy cattle are the hyperthy- 
roid-producing compounds and the antibiotics. 
Iodinated protein or thyroprotein, as well as synthetic 1-thyroxine, when fed to milk- 
ing cows, produces an increase in milk production. A few tests have been conducted with 
beef cows nursing calves. Results have not been impressive. Recent work on dairy cows 
has been summarized in Publication 714 of the National Research Council (11). The state- 
ment is made that ‘If thyroprotein is to be used inthe stimulation of milk production, its 
feeding should begin about two months after lactation starts. It should continue for not 
longer than three to four months and should be discontinued during the last two or three 
months of the lactation period.’’ Milk yields within the 3 to 4 month period may be in- 
creased 10 to 25 percent. To achieve this and to avoid excessive loss in weight in the 
cows, feed intake must be added as well as improved management. Feeding of the mate- 
rial late in lactation is not generally advised because of possible effects on viability of 
calves and on subsequent lactation, 
Many investigations have been conducted on the use of various antibiotics in the 
rearing of dairy calves. While there have been contradictions in results, the main body 
of evidence shows first a benefit in reduced incidence of diarrhea or scours and sec- 
ondly, a stimulation in rate of growth. The period of use has been studied extensively. 
In general, the greatest benefit appears to be derived from feeding during early life up 
to possibly 4 months. In experiments at Beltsville, the use of chlortetracycline during 
the first 90 days of life has been most beneficial in reduced incidence of scours. Any 
increase in rate of growth compared to controls during this period is not evident in 
weights of calves by the time they reach the age of 6 months or a year. 
SWINE 
As already mentioned, the thyrogenic compounds attracted the attention of animal 
nutritionists during the 1940s. In the case of swine, nearly all of the early research con- 
cerned with the feed additives treated in this report was devoted to the two groups of 
compounds that either enhance or retard the activity of the thyroid glands. lodinated 
protein was the principal enhancing substance and thiouracil the inhibitory one tested. 
In growing pigs, the iodinated protein had little effect on growth rate, feedefficiency, 
and carcass characteristics. Such effect as observed was usually found in the first month 
or two following weaning. Recently, the protein compound along with triiodothyronine has 
been tried in lactating sows. In one study, iodinated protein improved growth of the suck- 
ling pigs. 
In the work with thiouracil and related compounds, it was found that the more prom- 
ising use of the drug was in the last month prior to marketing. As is evident in Table 3, 
even this application did not prove beneficial, the growth rate being depressed, the feed 
saving of little consequence, and carcass quality unimproved. In a study at Beltsville (5) 
on the interaction of temperature and thiouracil feeding upon feeding and carcass char- 
acteristics it was observed that a 30° to 40°F. temperature differential modified the 
effect of the thiouracil. At approximately 50°F., the drug increased rate of gain, de- 
creased backfat thickness, and increased protein content over that of controls. Ata 
higher temperature approaching or at 90°F. the thiouracil tended to retard rate of 
growth while carcass fatness was not affected significantly. This work illustrates the 
complexity of the problem and difficulties in practical applications. 
The use of other hormones of the sex group has not yielded the favorable results 
with swine that have been found in beef cattle. Stilbestrol, estradiol, progesterone, 
testosterone, and others have been tried both orally and by injection or implant. As 
shown in Table 3, effects on rate of gain have been nil and feed savings have been small. 
The reasons have not been apparent. Some years ago a study was carried out at Belts- 
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