Kinski breastfeeding the baby -- was it real?

There's a scene where there all in the field having lunch and a younger girl brings a crying baby to Tess to feed.

The camera stays fixed as Kinski opens her blouse, bares her breast and puts what looks like an actual baby to her and the baby seems to begin suckling her nipple.

It was just too real looking and responsive to have looked like a prop baby. Just after it begins suckling, the camera angle changes, hiding the baby's face and it appears to be more like an actual prop she's holding.

Is that possible? Would an 18 year old Kinski actually agree to let someone else's baby suckle on her? What mother is going to provide her baby to be used like this and have it suckle on a stranger's nipple?