So looking at the MSDS for Grazon herbicide I first find it is NOT highly toxic and has pretty high LD50's for all components straight out of the bottle.
Acute toxicity
Acute oral toxicity
Low toxicity if swallowed. Small amounts swallowed incidentally as a result of normal handling operations are not likely to cause injury; however, swallowing larger amounts may cause injury.
As product:
LD50, Rat, female, 2,598 mg/kg
Acute dermal toxicity
Prolonged skin contact is unlikely to result in absorption of harmful amounts.
As product:
LD50, Rabbit, male and female, > 5,000 mg/kg
Acute inhalation toxicity
Prolonged excessive exposure to mist may cause adverse effects. Based on the available data, respiratory irritation was not observed.
Maximum attainable concentration.
LC50, Rat, male and female, 4 Hour, dust/mist, > 1.38 mg/l
Independent study does recommend:
“KEEP LIVESTOCK OUT of treated areas for at least 7 days and until foliage of any poisonous weeds such as ragwort has died and become unpalatable”
A Cornell news release on their research showed that the correct application is the key and use on dense growth prolonged the half life.
E X T O X N E T
A Pesticide Information Project of Cooperative Extension Offices of Cornell University, Michigan State University, Oregon State University, and University of California at Davis. Major support and funding was provided by the USDA/Extension Service/National Agricultural Pesticide Impact Assessment Program.
ENVIRONMENTAL FATE
In heavy clay soil, picloram has a half life of slightly over two months. However, when more organic material is present, the half life of the compound nearly doubles. Breakdown by soil microorganisms occurs slowly, resulting in the formation of carbon dioxide (CO2) and the release of a chloride ion (
5). The compound is mobile and relatively persistent in soil and can therefore leach to groundwater. Picloram has been detected in the groundwater of seven states (
10).
In water, the action of sunlight is an important mechanism leading to the breakdown of the product. Herbicide levels in farm ponds which were 1 ppm at the time of spraying reached 10 ppb in 100 days primarily due to dilution and the action of sunlight. The movement of picloram in runoff after heavy rainfall may occur.
Picloram is readily absorbed by plant roots, less so by the foliage, and is readily translocated throughout plants. It remains stable and intact in plants.
http://pmep.cce.cornell.edu/profiles/extoxnet/metiram-propoxur/picloram-ext.html
*** So looking at Grazon in particular if you are finding compost to be contaminated it would seem one of the following might be true.
1. the application rate used was too high
2. the plant growth was too dense ( the field needed mowed or brush hogged before application)
3. the animals were allowed to graze too soon (Under normal conditions and proper application they should NOT graze the area within a two month period after application)
4. The application area had standing water or is poorly drained having heavy clay soils that prevented proper Photodegradation of the chemicals.
5. The other possibility is there is another contaminant like paraquat.
https://www.epa.gov/pesticide-worker-safety/paraquat-dichloride-one-sip-can-kill