DIETHYLSTILBESTROL (STILBESTROL) AND RELATED ESTROGENS IN 
CATTLE, SHEEP, SWINE, AND POULTRY 
Absorption 
The rate of absorption of diethylstilbestrol (stilbestrol) from pellets implanted in 
cattle has not been extensively studied. Hammond and Day (16) reported absorption data 
from diethylstilbestrol and Hexestrol tablets which were calculated from the difference 
in average weight of implanted and recovered tablets. Rates of absorption from tablets 
of 15 mg. to 1,000 mg. were rather constant and centered about 0.30 percent per day. 
Cowie and Flux (6) reported an absorption of 0.38 mg. per day (0.38 percent) for a 100 
mg. tablet of stilbestrol implanted subcutaneously. When 15 of these pellets were im- 
planted the amount absorbed increased to 4.97 mg. per day, but the rate of absorption 
was not considerably different, being 0.33 percent per day. More recently, Hale, et al. (15) 
studies the rate of absorption of 12 mg. stilbestrol pellets in ears of steers. Pellet 
residues were removed 28, 56, 84, and 112 days after implantation and the rate of 
absorption was found to follow the rate of a first order reaction. The half-life of the 
best pellet of the three studied was 73 days. The amount of stilbestrol absorbed daily 
was 0.073 mg. 
The rate or absorption of stilbestrol pellets in poultry has received more attention. 
Absorption rate is greatest early in the treatment period and falls off with time, perhaps 
due to a decrease in surface area or encapsulation of the pellet. In one experiment Lorenz 
(19) reported an absorption of 3 mg./week from a 25 mg. stilbestrol pellet implanted 
subcutaneously or a rate of 1.64 percent per day. A later report (20) showed an average 
absorption of 2 mg./week from 20 to 25 mg. pellets, or a rate of 1.5 percent per day. 
This absorption rate fell to 0.75 percent per day after 4 weeks (21). Jones and Deatherage 
(17), using 15 mg. pellets, found an average of 3.97 mg. of unabsorbed stilbestrol re- 
maining after 4 weeks of implantation. This absorption rate calculates as 2.6 percent 
per day. Stob, et al. (31) found an absorption of about 3 mg. per week from 12 mg. © 
stilbestrol pellets during the first 2 weeks (absorption rate of 3.7 percent per day) of 
treatment. After 4 weeks only about half of the 12 mg. pellets were absorbed, and cal- 
culations for absorption would indicate 1.5 mg. per week or 1.8 percent per day instead 
of the higher early rate. 
Vondell (41) also reported that about 1/2 to 1/3 of the original pellet was present 
after 6 weeks of stilbestrol treatment. The pellets were imbedded in tissue at this time. 
Recently, Umberger, et al. (39) recovered unabsorbed pellets after 5 weeks implantation 
of a 12 mg. pellet. The average residue weighed 6.3 mg. indicating an average absorption 
rate for 5 weeks of 1.4 percent perday. A comparison of the absorption rates of stilbestrol 
pellets in cattle (ca 0.40 percent per day) with those in poultry (ca 1.5 to 2.0 percent per 
day) indicates a rate four to five times greater in poultry. 
The obvious hazard in the use of impiants is that (detectable amounts of) residual 
estrogens may be present in the tissues of the treated animal. Studies on stilbestrol 
residues in meats and tissues, particularly with the use of oral administration of them, 
has shown that stilbestrol must be discontinued at least 48 hours before slaughter to 
provide a safe interval for clearance from the body. This is clearly not practical with 
stilbestrol implants. 
In a recent study using tritium labeled hexestrol injected subcutaneously into lambs 
and goats, Glascock (11) found a very rapid absorption from the site of injection, Only a 
few percent of the original activity could be determined after a few hours. 
The rate of absorption of orally administered stilbestrol, which is by far the most 
common route of administration, is difficult to assess quantitatively. Rapid excretion 
into the feces and urine has been noted by Mitchell, Neumann, and Draper (26), who found 
the appearance of radioactivity in the urine in the 0 to 6 hour interval, and in the feces in 
the 6 to 12 hour interval after the oral administration of tritium labeled stilbestrol. This 
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