During the same period, various scientists were appraising the effect of low level 
feeding of DDT. They determined that consistent effects upon rats could be produced with 
rather low levels of DDT in feed. Many workers, including those of USDA, ARS at Kerr- 
ville and Beltsville, established that DDT and other chlorinated hydrocarbons were stored 
in the fat and excreted in milk of treated livestock, and were not likely to be destroyed 
in cooking (21), whether exposure was by spraying or from ingestion, depending upon level 
of exposure, species, sex, and condition of the animals (3,7,9,10,14,15,16,18,19,22,25,26, 
27,28,29, 30, 31,35,37,39,40,41,42,43,45,46,47,50,54,55,56,57,58,60,65,67,85,87,88,114,118). 
As data and pressure accumulated against residues of chlorinated hydrocarbons in 
foods, it was more and more apparent that compounds that were not stored in tissues or 
excreted in milk were essential. Industrial and governmental laboratories fell to the task. 
Soon, safer compounds appeared, with virtually no problems of storage in animal tissues, 
(27), and occurring in extremely low amounts in milk (27,50), eliminating them from 
use, but promising the discovery of such materials that would not appear in milk. 
There has been virtually no evidence to indicate that the residues of insecticides on 
feed and forage following recommended use have poisoned livestock. This is due, of 
course, to careful preliminary study and evaluation of the toxicological hazards and to 
proper recommendations by industry and government. Table 1 illustrates the levels of 
the compounds which have been fed to livestock for extended periods without evidence 
of poisoning. 
While poisoning has not been observed in animals consuming insecticides as residue 
on feeds, the consumption has usually led to storage in their tissues or excretion in their 
milk, or both. The storage of the insecticides in tissues has never seemed to have a 
harmful effect upon livestock; rather, it is the amount of residue on the feed which pro- 
duced the residue that determines the appearance of poisoning. 
We may dispense with residues in animal tissues as being of no general significance 
to livestock health, and do so in clear conscience, but we are then confronted with their 
significance to people consuming animal products, for the level of residues in the tissues 
establishes the dietary level for people, and the level of intake for people is significant. 
Our interest, then, in the appearance and disappearance of pesticide residues in 
tissues of livestock is one of concern for the welfare of the consumer. The significance 
of these residues in human foods has been and will be discussed by experts in that field. 
As the analytical methods became more and more sensitive, experiments had to be 
repeated to prove or revise earlier conclusions concerning the amounts of residues pres- 
ent. 
The analytical chemists were momentarily foiled by the animals in at least two 
cases. Aldrin was found to be converted by animals to its epoxide, dieldrin (6) and hep- 
tachlor to its epoxide (33,34,35,41). Older methods, specific for aldrin and for heptachlor, 
had shown absence of residues of these compounds, but their epoxides were shown to be 
present in appreciable quantities. Chlordane also was reported stored as a metabolite 
(3242 
It is interesting to note that while hundreds of tolerances have been established for 
fruits, vegetables, and feeds for a number of insecticides, only four compounds have 
tolerances in meat, these being DDT, methoxychlor, toxaphene, and malathion. DDT may 
be present at 7 p.p.m. and methoxychlor at 3 p.p.m. in fat of cattle, sheep or hogs. Toxa- 
phene may be present at 7 p.p.m. in fat of cattle, goats, and sheep. Malathion may be 
present at 4 p.p.m. in fat of cattle, hogs, and poultry. Insecticides may not be present in 
milk, although official tolerances of zero have been set only for malathion and meth- 
oxychlor. The tolerance is also zero for malathion in eggs. 
These tolerances are indeed few compared to the total number of insecticides to 
which livestock are exposed. 
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