I wonder is it possible to render to fields with mantra?
I remember that that option was available in old versions of mantra but I don't see anything related to fields now.

Just a guess, you probably have to render every 0.5 frame then do the interlacing afterwards in cops.

Mark Elendt has a workaround for field render in Houdini 8. Good for even field dominance, somewhat more complicated for odd field dominance.
Fields in mantra may be restored in Houdini 9
Contact Angel in support@sidefx

It is possible in more detail about Mark Elendt's method?

There is a choice of 2 cmd files that must be placed in the Pre-Include box of the Render tab of your Camera object.
In the attached example hip, the cmd file is placed in your /tmp folder.
For odd field dominance, the .cmd is..
# Specification of fields
# Renders the odd field first
set frame = $arg1
set delta = `round($frame*2)%2-.5`
set y0 = `$delta/576-0.5`
set y1 = `$delta/576+1.5`
ray_window 0 1 $y0 $y1
echo $delta
echo $y0
echo $y1
……………………….
For even field dominance
# Specification of fields
# Renders the even field first
set frame = $arg1
set delta = `round($frame*2+1)%2-.5`
set y0 = `$delta/576-0.5`
set y1 = `$delta/576+1.5`
ray_window 0 1 $y0 $y1
echo $delta
echo $y0
echo $y1
………………………
However you will need to test this out properly, because for a good result in PAL, against the logic. I used the even field script with an even field dominance fieldmerge COP to get the correct result.
I am not able to explain why this is so

Thanks, cmhking! Happy New year!

Somebody tried such variant?
In COP it is created network:
render> fieldmerge> ROP File Output
Adjustments render OP:
Tab Image, point Interlacing: Black Interlaced - Odd First (or Even First)
Adjustments fieldmerge OP:
To establish a flag “ Detect Field from Input ”

Look a file