To sit one must be able to operate one's hind quarters separately to one's forward appendages, or so I'd assert. A dog can do that. A human can do that. A frog can do that. But a cow cannot. That'd be an argument for lying down.
However, the cow is rather attentive, with a raised head, and lying down often implies a wish to doze or sleep. A cat takes this same posture as a form of more settled sitting, in that a cat shows a "resting on belly, head up" posture and a "lying on side, head down" posture. I think you could argue that the former is "sitting" and the latter is "lying". Using that as one's premise, you could argue that the cow is sitting.
I believe the latter argument in this case. The cow is sitting.