According to DIEC2, it is dejeccions (pl) that has the sense 'faeces'.
Dejecció sg. is 1. 'abatiment o decaïment moral' ="dejection", 2. 'evacuació de materials, especialment de matèries fecals' = "defecation"

Interesting. The GDLC gives it in the singular:

[...]
2 Conjunt de matèries fecals evacuades. Dejecció biliosa.

To be fair, mind you, the article I was reading had it in the plural 'dejeccions ramaderes' and a search for 'dejecció ramadera' vs 'dejeccions ramaderes' certainly supports this. I'll change it.

While we're on the subject, any ideas how to translate 'purí'? According to the GDLC it means 'fem líquid' which, in turn, it defines as 'part líquida dels fems, constituïda pels orins i la descomposició de les parts sòlides, emprada com a adob, rica en nitrogen i en potassi'. Would we just call this 'liquid waste' in English?

purí: "slurry" is what they used to call it in "The Archers", though "slurry" has a wider range of senses, e.g. coal slurry.