https://www.sciencedirect.com/science/article/abs/pii/S2211912416300013

86% of the global livestock feed intake in dry matter consists of feed materials that are not currently edible for humans

But it also makes an important contribution to food security through the provision of high-quality protein and a variety of micronutrients – e.g. vitamin A, vitamin B-12, riboflavin, calcium, iron and zinc – that can be locally difficult to obtain in adequate quantities from plant-source foods alone

We already make enough food to feed the planet multiple times over, the issue isn't how much we've got, it's how to get it to people. Distribution is the issue.

But no, 1/3rd is not grown for livestock, this isn't true at all.