Horse manure is a great fertilizer as long as it is aged. I use cow manure and it too does very well acting like a slow release fertilizer.

I fertilize at all times of the year with no ill effects to my mature bamboos.

A younger or less well extablished plant may see some damage from any kind of fertilizer since it may be too much for a small plant to process.

Kt

If you have a lot of horse manure, you might as well put it to use. I just put about 1ft of grass clippings on my grove of phyllostachys parvifolia which is just starting to shoot, and appears to have several 1+ inchers, but ensuring they have an immediate organic source of nitrogen should give them a boost to max out what you can get for your shooting season.

I have used horse manure myself, but I prefer using fall leaves, grass clippings and sometimes even weeds and small branches since any organic matter will break down. I've put so much stuff down that it is starting to create raised mounds on my bamboo groves.