Calotropis weed growing in abandoned fields, still cattle or goats never browse on this plant, why?

Because this plant produces highly poisonous cardiac glycosides.

Option 1 is correct . Because this plant produces highly poisonous cardiac glycosides. 

Many plants produce and store chemicals that make the herbivore sick when they are eaten, inhibit feeding or digestion, disrupt its reproduction or even kill it. The weed Calotropis growing in abandoned fields. The plant produces highly poisonous cardiac glycosides and that is why you never see any cattle or goats browsing on this plant. Thorns are the most common morphological means of defence in Acacia, Cactus.

A wide variety of chemical substances that we extract from plants on a commercial scale (nicotine, caffeine, quinine, strychnine, opium, etc.,) are produced by them actually as defences against grazers and browsers.