It depends on what the goal is - maximize biomass production, or scavenge nutrients.

A couple of years back, I was on a vegetable farm.  They had started planting cover crop mixtures, mostly because they wanted - needed - to increase soil O.M.  On some of the cover crops, they had applied composted manure. On other fields, they had applied nothing.  You could see the cover crops were hurting where no compost was applied.  Looking at those fields, I thought that applying some N would have gone a long ways to increasing their biomass.