Only Male Livestocks get nourishment from grass

[Neptjunoue]

While males can survive on eating grass alone, or at least last much longer, females die off fast as if there as no food. If you witness abandoned civilization, you may see that only males survive.

[AnnaC]

I think that's because pregnant females require so much food. If the cows could stay segregated and had the same amount of grazing space I think they'd last the same amount of time.

[DatOneGuy]

Pregnant or not, the females don't last. You can have males last forever penned up in one place they'll eat the grass until it's all gone, females on the other hand, not so much.