What combination of volts to amps in cattle prod type devices will kill pernicious weeds such as thistles?

No combination of volts to amps in cattle prod type devices kills pernicious weeds such as thistles. Volts and amps are linked by a mathematical principle, known as Ohm's law, that states that V=IR, where V is voltage, I is current (amps) and R is resistance. In general, biological systems -- especially human -- are most dramatically affected by amperage. Any cattle prod type device that delivers sufficient amperage to fry weeds will be as dangerous to the user as to the property.