A manufacturer, in complying with regulations, may determine the maximum residues 
occurring in meat or milk after proposed use, show these quantities to be without hazard, 
and petition for a tolerance at that level. He might also establish the quantities remaining 
after a reasonable period of time, say 30 days, petition for a tolerance at that level, and 
then specify a 30-day delay between treatment and slaughter if the tolerance is granted. 
, Alternatively, he can determine the time required forthe residue to completely disappear, 
add a few days for safety, and make his recommendation include this time interval. 
When insecticides are not present, no tolerance is required. 
Compounds that do not store and are not excreted in milk are, therefore, at a pre- 
mium and would command much of the market if available. Very few can qualify without 
the protection of an interval of time between application and harvest or consumption. 
The waiting period is not useful when producing dairy cattle are concerned. Since 
complete absence of residues in milk is the rule, direct application to the cattle is vir- 
tually ruled out for all except dusts of methoxychlor since the other materials available 
at this time are present in milk after treatment. The milk at that time must be destroyed 
or enter commerce illegally. Neither alternative is desirable. The waiting period can 
be used in feed or pasture work by keeping cattle off the crop or delaying harvest until 
the residues on the crop have disappeared. 
Generally, these restrictions do not apply to dairy cattle not in production, but even 
in this case, some residues could appear at the onset of lactation if appreciable residues 
existed in the fat depots of the animals" body at that time. 
Acute Toxicity 
DDT, TDE, and methoxychlor were found to be virtually nontoxic for livestock and 
poultry when applied as sprays and of very low toxicity when given to them in feed or 
as medications (1,9,10,11,14,17,28,37,39,42,45,54,55,56,57,68,71,73,79,86,88,90,98,104, 
£935115,120). 
Chlordane, benzene hexachloride, toxaphene, strobane, dieldrin, aldrin, heptachlor, 
isodrin, endrin, and many other chlorinated hydrocarbons, and some organic phosphorus 
compounds were shown to be hazardous for livestock and poultry unless reasonable 
precautions were observed (2,4,8,15,17,24,36,44,49,62,66,69,74,78,83,84,86,89,90,96,97, 
98,99,100,101,104,105,106,109,110,119,121). Early inour toxicological studies at Kerrville, 
we found that dairy calves 1 to 2 weeks of age were the most susceptible of all the farm 
animals, and that chickens were almost equally so. We also recognized that resistance to 
poisoning increased as the animals grew to maturity. Unfortunately, the passage of time 
brought chemicals showing equivalent toxicity for young and old animals. Summaries of 
the results of our toxicological studies are given in Tables 2 and 3. 
We utilized this knowledge by selecting the young dairy calf as our standard test 
subject, representing the most susceptible species and age. The data derived from these 
studies formed a basis for safe recommendations for use of the chemicals. Although we 
desired the widest possible margin of safety between use dosage and minimum toxic 
doses, we decided that a twofold margin for young calves could be utilized on all other 
livestock if reasonable precautions were observed. (In other words, the recommended 
dose for livestock in general should not exceed one-half the minimum toxic dose for the 
young calves). After more than 12 years of using this criterion, we still feel confident 
of its usefulness, but have learned that there are exceptions and that factors other than 
dosage are equally important. 
Effect of Formulation 
Because most insecticides were not soluble in water, formulations were needed that 
would disperse or dilute them sufficiently for safe use. In general, simple solutions in 
139 
