Nitrogen - milk contains roughly about 5% nitrogen as part of its protein content. Human urine, on the other hand, contains (roughly, it varies on time of day and the particular human) about 15% nitrogen - I leave you to draw your own conclusions as to which will cause more green growth, though the most effective and cleanest use for the latter is adding it to the compost heap occasionally.
Regardless of milk's nitrogen content, I wouldn't describe it as a 'good' fertiliser because it's not balanced.
UPDATED ANSWER (in response to both comments)
Yes, I am serious about urine on the compost heap. The first one in the morning contains the most nitrogen, so collect that and use it, if your heap is a bit short on nitrogen giving materials (such as green leaves). I used it for years, used to collect it from my kids, saved money on compost activators, but I didn't use it every day, probably about once a week during spring and early summer particularly.
Stranded Pirate: I know that human urine does contain nitrogen, not only from the scientific point of view, but from experience, and not the one I'm mentioning above. I had a client some years ago, an older couple, and she was puzzled as to why the grass nearest the french windows, just beyond the path outside, was lush, long and green in February and March when the rest of the lawn wasn't. I took the husband aside and he confessed - his arthritis made it difficult to get upstairs, so when his wife went to bed, he'd stay up and watch tv, and if he needed to, he used to nip outside to pee instead, without her knowledge. I never told her, just suggested he might want to spread it around a bit more.